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Avian distribution patterns along a Sonoran Desert bajada
Journal ofArid Enznronments (1984) 7,59-74
Avian distribution patterns along a Sonoran Desert bajada
Stephen B. Vander Wall & James A. MacMahon* Accepted 11 March 1983 Avianpopulation density, foragingbehavior, nest site selectionand other aspects of vegetationuse were studied at four points alonga vegetationgradient at Organ Pipe Cactus National Monument, Arizona, U.S.A., to elucidate causesof species distributions. The vegetationgradient, a result of edaphic change along a bajada (coalesced alluvial fans), ranged from a diverse subtree-large cacti-shrub community on gravellysoilsat the top of the bajadato a simple lowshrub community on fine sandy loam at the bottom of the bajada, Twenty-three species of birds bred in the diverse vegetation at the top of the bajada. Proceeding down the bajada, bird speciesdropped out of the community as vegetationarchitecture and composition became more simple, with only two species breeding all along the gradient. Most species distributions were determined by requirements for specific vegetation components for nesting and foraging. Competition and abiotic factors seem to have played only a minor role, if any, in determining distributions. Causes for speciesdistributions along this bajada are similar to those affecting avian distribution along vegetation gradients on temperate mountain slopes but differ from those along tropical elevationalgradients.
Introduction Vegetation gradients of various types have become important objects of study for the analysis of avian community structure, including changes over time (seres) (Odum, 1950; Johnston & Odum, 1956; Karr, 1968; Smith & MacMahon, 1981) and in space (e.g. elevational gradients). Temperate and tropical mountain slopes have been used to evaluate the relative importance of physical conditions, vegetation attributes, and competition in determining species distributional limits along such gradients (Terborgh, 1971; Diamond, 1973; Terborgh & Weske, 1975; Able & Noon, 1976; Noon & Able, 1978; Pearson & Ralph, 1978; Noon, 1981). Recently, ordination techniques have been used to examine avian habitat requirements by rearranging mosaics of sites into artificial gradients based on habitat qualities (James 1971; Anderson & Shugart 1974; Whitmore, 1975, 1977; Rotenberry, 1978). Effects of broad geographic gradients of vegetation change on avian species richness and diversity have also been studied (Cook, 1969; Tramer, 1974; Rabenold, 1979; Short, 1979). Sonoran Desert bajadas (coalesced alluvial fans) are excellent field laboratories for studying the response of organisms to vegetation gradients, since edaphic changes along bajadas result in marked vegetational changes (Yang & Lowe, 1956; Klikoff, 1967; Phillips & MacMahon, 1978). The gravelly soils of the upper bajada typically support a diverse plant community including various subtrees, columnar cacti, chollas and shrubs. Down the bajada, soils become finer, subtrees and large cacti gradually drop from the plant community, and the shrub stratum becomes less diverse (Yang & Lowe, 1956; Phillips & • Department of Biology and Ecology Center, UMC 53, Utah State University, Logan, Utah 84322 U.S.A. 0140-1963/84/010059 + 16 $03.00/0
cD
1984 Academic Press Inc. (London) Limited
60
S. B. VANDER WALL &
J. A. MACMAHON
MacMahon, 1978). By comparing bird communities on sites differing in specific vegetation attributes, factors determining species distribution patterns can be elucidated. Both the relative shortness (usually < 10 km) and gradual slope (= 1'8 per cent, Phillips & MacMahon, 1978)of baiadas facilitate the study of avian distribution patterns, because all vegetation types occur within the same climatic regime and all members of the local avifauna can reach all points along the bajada. In this study, we examined bird communities of a series of bajada sites differing in vegetation physiognomy and plant species composition. At each site, avian community composition and species foraging and nesting requirements were determined. Site comparisons yield data we use to address probable causes of the bird distributional limits along the vegetation gradient. Study areas Four study sites differing in vegetation structure (physiognomy and foliage profile) were selected at Organ Pipe Cactus National Monument (OPCNM), Arizona, U.S.A. At each site, three replicate 10 ha plots of relatively homogeneous vegetation were established. All sites were> 3 km from permanent water to avoid the influence of water availability on bird community structure (MacArthur, 1964; Karr, 1968; Austin, 1970); major washes and riparian habitats were avoided. All sites were located on a baiada radiating from the Ajo Mountains. Four physiognomic groups of perennial plants occurred on the bajada, (I) Subtreessingle stemmed non-succulent plants typically 2-6 m tall at maturity'(e.g, 'paloverde, Gercidium microphyllum; ironwood, Olneya tesota; acacia, Prosopis glandulosa. (2) Shrubsnon-succulent plants typically < 2 m tall at maturity (e.g. creosotebush, Larrea tridentata: bursage, Ambrosia deltoides; brittlebush, Enceliafarinosa) plus sapling subtrees < 1 m tall. (3) Large cacti-chain (Opuntiafulgida) and buckhorn (0. acanthocarpa) cholla > 1 m tall and saguaro (Gerus giganteus) and organ pipe cactus (C. thurberi) .> 3 m tall. (4) Small cacti-all other species of cacti (e.g. hedgehog cactus, Echinocereus engelmannii; barrel cactus F erocactus wislizenii), cholla < 1 m tall and columnar cacti < 3 m tall. Characteristics of the perennial vegetation, location, and physical nature of each site are summarized below. The Lukeville site, situated on the upper portion of a baiada, is located 4-4 km north of Lukeville, Arizona, and 0'1 km east of Arizona Highway 85 (31°55'N, 112°48'W). The terrain is gently rolling with a mean elevation of 475 m. The soil is a very gravelly loam (Anonymous, 1972). The vegetation is composed of numerous subtrees (X ± S.D. = 127'6 ± 34'9 per ha) and large cacti (53'5 ± 23'9 per ha). Shrubs are abundant (5536'0 ± 650'4 per ha) and small cacti are common (104'7 ± 34'8 per ha). TheAjo site is in Aio Valley, 21·5 kmnorth ofOPCNM headquarters and Or l km west of Highway 85 (32°7'N, 112°46'W). The terrain is slightly rolling to flat (elevation 549 m) with a gravelly sandy loam soil. The vegetation is dominated by numerous subtrees (74·9 ± 20-4 per ha) and shrubs (3727 ::!: 818·0 per ha). Large cacti are virtually absent and small cacti are infrequent. The Kuakatch site is just north of Kuakatch Wash, 5·0 km west of OPCNM's north entrance (32°12'N, 112°47'W). The site is nearly flat and levelat 518 m elevation. The soil is fine sandy loam. The plant community is dominated by shrubs (1870,8 ± 191·3 per ha), but subtrees are conspicuous though infrequent (4.5 ± 2·1 per ha). Again, large cacti are virtually absent and small cacti are infrequent. Sonoyta, the lowest site along the baiada is located in Sonoyta Valley, 3·3 km south-east of Lukeville, just north and east of Gachado Well (31°52'N, 112°47'W). The area is a flat nearly level plain (elevation 428 m) with very fine sandy loam and slightly saline soil (Anonymous, 1972). Shrubs dominate the vegetation (2660'1 ± 580'6 per ha). Subtrees and large cacti are absent, and several species of small cacti occur uncommonly. Precipitation during the winter rainy season (November-March) in 1977-8 and 1978-9
AVIAN DISTRIBUTIONPATTERNS
61
was 58 and 110 mm, (1'9 and 3'6 times the mean precipitation of the previous 10 winter rainy seasons), respectively (National Oceanic and Atmospheric Administration, 196779). Abundant winter rain, probably the most important abiotic factor affecting avian breeding populations in Sonoran Desert ecosystems, suggests that species populations, distributions and vegetation uses were similar in both field seasons.
Methods We censused birds using the spot-map method (Kendeigh, 1944). All plots were censused in March and April 1978 except Kuakatch which was censused in March and April 1979. Due to the low, sparse nature of the vegetation and the low number and density of bird species, five censuses were deemed sufficient to sample breeding birds populations. All censuses were initiated within 10 min of sunrise and completed in - 2 h. Owls were censused at night by playing tape recorded vocalizations of Elf (Micrathene whitneyi), Screech (Otus asio), and Ferruginous (Glaucidium brasilianum) owls on each plot. Birds encountered during censuses were separated into three groups. (1) Breeding residents (based on territorial pairs or nests); (2) breeding visitors (species known to breed in the vicinity but not on the study site); and (3) migrants and winter visitors (reported in Vander Wall, 1980). Breeding bird densities were determined using the maximum number of pairs observed. Breeding visitors were divided into two groups. (1) Birds observed visiting the site two or fewer times (designated 'v'); and (2) birds observed visiting the site three or more times and which may have a significant influence on the bird community (assigned a density of 0'5 for each pair). We collected foraging behavior data on 11 breeding species. The remaining 12 species were either uncommon or difficult to observe. For them, published accounts of foraging behavior and diet were used to determine foraging requirements. Our foraging data were gathered between 0830 and 1130 h (MST). Foraging technique, foraging substrate, foraging height, perch substrate and plant species were recorded at approximately 15 s intervals using a metronome (Wiens, Martin etal., 1970). The metronome was not used for the Ashthroated Flycatcher (Myiarchus cinerascens) and Curve-billed Thrasher (Toxostoma curvirostre); rather actual foraging maneuvers were recorded. We classified foraging techniques as gleaning, probing and hawking. We defined foraging and perching substrates as: twig « 1 em diameter); branch (~1 but <4 cm); large branch (~4 but < 10 cm); trunk (~1O cm); perennial foliage; annual foliage (stems, leaves and flowers of annuals); flowers (including buds); ground; and air. For bird species that occurred on two or more sites along the vegetation gradient, we made separate foraging observations (usually 100 to 300 observations per species) on each site. For infrequently observed species, observations from all sites were combined. We also located nest sites of each species and for each recorded plant species, plant height and nest structure. We determined density and volume of perennial plants by counting and measuring (height, greatest diameter, least diameter) individuals on 10 circular subplots (radii = 10 m) on each 10 ha plot. Foliage volume was calculated using the formula: Foliage volume =0/31'1' (height x greatest diameter x least diameter) Foliage height profiles were determined using the technique of Karr (1971). Foliage height diversity was calculated based on the vegetation layers: 0-75 em, 75-150 em, and> 150 cm. Results
Community structure along the Bajada Bird species richness and pair density decreased dramatically from upper to lower bajada sites (Table 1). Number of breeding species per 10 ha decreased from Lukeville
Gambel's Quail Callipeplagambelii White-winged Dove Zenaida asiatica Mourning Dove Zenaida macroura Roadrunner Geococcyx californianus ElfOwl M icrathene uihuneyi Lesser Nighthawk Chordeiles acutipennis Costa's Hummingbird Calypte costae Gilded Flicker Colaptes auratus chrysoides Gila Woodpecker Melanerpes uropygialis Ladder-backed Woodpecker Picoides scalaris Ash-throated Flycatcher M yiarchuscinerascens Verdin Auriparusjlaviceps
Species
2 0'5 1 1
3 1
3 3
1
0·5 1
0'5 2
v 2 3
2
v 2'5 0·5 3 5
1
2
3
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2'5 2'5
0'5
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0'5
1
1
1
1
v
v
v
v v
v
2
0'5
1
1
i-
v
v
1
0·5
3
Sonoyta
1
1
1
1
1
v
2
1
2
0·5
0'5
1
1
3
Kuakatch
1
2
Ajo
0'5
0·5
1
1
1
0'5
1 0'5
1
2'5
2
1'5
0'5
1
1
2
-3
1
I
Lukeville
1
3
Table 1. Densityofbreeding birds (pair/1O ha) alonga vegetation gradient at OrganPipe CactusNational Monument. Infrequent visitors (two or fewer visits) aredesignated with a v. Frequent visitors (three ormorevisits)areassigned a density of0·5 perpair. If twoormore pairsoffrequent visitors werepresent, it is indicated with an asterix
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Cactus Wren Campylorhynchus brunneicapillus Mockingbird M imuspolyglottos Curve-billed Thrasher Toxostomacurvirostre Black-tailed Gnatcatcher Polioptila melanura Phainopepla Phainopepla nitens Loggerhead Shrike Lanius ludooicianus Lucy's Warbler Vennivora luciae Scott's Oriole Icterusparisorum Brown-headed Cowbird Molothrus ater House Finch Carpodacusmexicanus Black-throated Sparrow Amphispiza bilineata Total breeding pairs Total breeding species Bird species diversity 2
0'5 6 30'5 18 2'66
0'5 6 31 16 2'43
3 36 21 2'90
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4 24'5 18 2'69
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11
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13
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S. B. VANDERWALL & J. A. MACMAHON
64
(X= 18'3), Ajo (14'7), Kuakatch (8'3), to the Sonoyta site (2'3). Similarly, mean pair density also decreased: Lukeville (32'5), Ajo (21'7), Kuakatch (11'5), and Sonoyta (2'8). Bird species diversity was significantly correlated with foliage height diversity (r= 0'93, N= 12, P
Ell Owl Gilded Flicker Gila Woodrecker Thrasher
Curv.~bill.d
White·wina.d Dove Costo's
Hummlnsblrd
Verdin
LUCy'S Warbler Ash.throottd Flycatcher
Cactus Wren Mockinablrd
Block-toiled GnoI co Ie her Pholnogeplo Mournl"?
Dov.
Block-throoled
sparrow
Figure 1. Distribution patterns (horizontal bars) of breeding birds along a Sonoran Desert bajada at Organ Pipe Cactus National Monument. Vegetation typical of each study site is depicted. Some uncommon bird species are not included because their distribution patterns are insufficiently known. Note that species are added to the community with increasing vegetation complexity but none are lost.
Species widely-distributed along the vegetation gradient either had similar population densities where they occurred (e.g. Gambel's Quail, Callipepla gambelii; Mourning Dove, Zenaida macroura) or had densities that declined with decreasing vegetation complexity (e.g. Ash-throated Flycatcher; Black-throated Sparrow, Amphispiza bilineata) (Table 1). Those species which were most dominant, i.e. had high relative abundances in habitats where they had their greatest relative abundance, were significantly more widely distri-
65
AVIAN DISTRIBUTION PATTERNS 1·5
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Dominonce (Ofo)
Figure 2. Relationship between habitat breadth (W) (McNaughton & Wolf, 1970) and dominance (relative abundance of each species in the habitat where it makes its greatest contribution) for 23 species along a Sonoran Desert bajada.
buted along the habitat gradient (r= 0'55, N = 23, P< 0'01; Fig. 2); a pattern similar to that reported by McNaughton & Wolf (1970) and Able & Noon (1976). Only two species, Mourning Dove and Black-throated Sparrow, were found breeding all along the vegetation gradient. The other 21 species had their lower distributional limits at various points along the bajada, Below, we examine foraging behavior and vegetationuse of these species to identify resources and other environmental variables that limit species distributions.
Nest sites Proportion of the avian community requiring different types of nests (e.g. cavities, open cups, etc.) changed along the bajada; cavities were used less frequently and open cups were used more frequently as vegetation structure became simpler (Table 2). Physiognomic categories of plants used for cavity, open cup and covered nests also changed along the gradient. On the Lukeville site, 95'7 per cent of nest cavities were placed in large cacti; only subtrees were used on the Ajo and Kuakatch sites. Subtrees and large cacti were used most frequently for open cup nests on the upper bajada (Lukeville, 81 per cent; Ajo, 62 per cent) and shrubs and ground most frequently on the lower bajada sites (Kuakatch, 53 per cent; Sonoyta, 100 per cent). Covered nests were never found in shrubs. These trends in nest site selection are due to both the absence of species that require specific physiognomic components of the vegetation for nesting at some sites and to changes in the types of nest sites selected by certain species as a result of changes in vegetation structure. Table 2. Proportion of the community (pairs/1O ha) using different types of nest sites along the vegetation gradients
Lukeville
Ajo
Kuakatch
Sonoyta
Nest type
Density
%
Density
%
Density
%
Density
%
Cavity Covered Open Cup No Nesr] Total
8'2 5'1 18'7 0'5 32'5
25'2 16'0 57'5 1'5
4'3 2'7 14'4 0'3 21'7
19'8 12'4 66'4 1'3
1'0 1'7 8'8 0'0 11'5
8'7 14'8 76'5 0'0
0'1 * 0'3 2'4 0'0 2'8
3'8 10'7 85'7 0'0
* Bred off site.
t Lesser Nighthawk and Brown-headed Cowbird.
66
s. B. VANDERWALL&J. A. MACMAHON
Several species appeared to be limited in distribution along the gradient by a lack of suitable nest sites. Elf Owls (six nests), Gila Woodpeckers (Melanerpes uropygialis, 11 nests), and Gilded Flickers (Colaptes auratus chrysoides, three nests) were cavity nesting species which nested exclusively in saguaros and hence were able to breed only on the Lukeville site. Gila Woodpeckers and Gilded Flickers which visited the Ajo site bred in large isolated saguaros> 100m away from the study plots. These three species prefer to nest high above the ground (usually >5 m; Hensley, 1954; Ligon, 1968; Vander Wall, 1980) and consequently subtrees do not provide suitable nest sites since trunks in which a cavity can be excavated seldom exist above 2 or 3 m high. Three other cavity nesting species, Ladder-backed Woodpecker (Picoides scalaris), Ashthroated Flycatcher and Lucy's Warbler (Vennivora luciae), had more general nest site requirements. For example, Ash-throated Flycatchers nested primarily in saguaro cavities (four of five nests) 7 to 11 m high on the Lukeville site, but on the Ajo and Kuakatch sites they used subtree cavities (six nests) I'Cr2'3 m high. Absence of breeding Ash-throated Flycatchers on the Sonoyta site may be due to nest site limitations. Shrub communities were apparently suitable habitats for foraging (e.g, over 90 per cent of the Ash-throated Flycatcher foraging observations on the Kuakatch site were on shrubs or annual foliage) but lacked suitable cavities for nesting. Lucy's Warblers did not appear to be limited in distribution along the bajada by availability of nest sites (see below) and too few Ladderbacked Woodpeckers bred along the gradient to accurately assess their distribution pattern. Cactus Wrens (Campylorhynchus brunneicapillus) and Curve-billed Thrashers nested almost exclusively in chollas at OPCNM (32 of 33 nests), and appeared to be limited in distribution by a lack of suitable nest sites on several plots. Two of the Ajo plots had very few chollas (none> 1 m tall) and no breeding pairs of wrens or thrashers (visiting pairs nested in chollas> 100 m away from the study plot). On the third Ajo plot there was only one mature chain cholla which a pair of Cactus Wrens used for a nest site. On the Kuakatch plots the few chollas present were small (0'75--1'25 m tall) and Curve-billed Thrashers, which usually nest in large (> 1'25 m) chollas, were not present. Most of the remaining species seemed not to be restricted in distribution by availability of suitable nest sites. Several species had general nest site requirements (e.g. Mourning Dove; Costa's Hummingbird, Calypte costae; Black-tailed Gnatcatcher, Polioptila melanura; House Finch, Carpodacus mexicanus; Black-throated Sparrow), nested on the ground (i.e. Lesser Nighthawk, Chordeiles acutipennis) or made no nest at all (i.e, Brownheaded Cowbird, Molothrus ater). Several other species (e.g. White-winged Dove, Zenaida asiatica; Roadrunner, Geococcyx californianus; Verdin, Auriparus jiaviceps; Mockingbird, Mimus polyglottos) appeared to have specific nest site requirements but insufficient data were available to determine if nest sites were likely to be a limiting factor.
Foraging sites The distribution of some species may be dependent on the availability of suitable foraging substrates. This possibility was examined by comparing the distribution and abundance of species with an availability index of its most important foraging substrates (e.g. foliage volume). Population densities of foliage-gleaning insectivores and the Ash-throated Flycatcher (a foliage-hawking insectivore) increased along the vegetation gradient and were significantly correlated to foliage volume (Verdin, r= 0'96, P
AVIAN DISTRIBUTION PATTERNS
67
10
'0 .c Q <,
8
~
I Sonoyto
'0 ~
III
'" 5
6
.~
u
'" .5 III
0' '" .2
4
<5
LL
2
I
II Kuokctch II II II
I
Folioge volume (1000 m 3 /bo)
Figure 3. The relationship between foliage volume and the density of foliagegleaning and hawking birds. Abbreviations of species names are: AtF-Ash-throated Flycatcher, BtG-Black-tailed Gnatcatcher, LW-Lucy's Warbler, SO-Scott's Oriole, V-Verdin.
(e.g. Murdock, Evans et aI., 1972, Southwood, Brown et aI., 1979, Hatley & MacMahon, 1980; however, see Folse, 1982). Thus, we hypothesize that low foliage volume (and thus low arthropod biomass) on the lower bajada sites may be a limiting resource for several insectivores. Lucy's Warblers and Phainopeplas tPhainopepla nitens) occurred primarily in the more densely vegetated washes, and, thus, their distributions largely coincided with the bajada's dendritic drainage pattern. These distributions may be related to greater foraging success in the lusher, more productive, habitat. Foraging site availability did not appear to limit the distribution of those species that nested exclusively in saguaro. Gilded Flickers foraged exclusively on the ground and foliage of annuals (N = 36). For Gila Woodpeckers, subtrees accounted for 89'0 per cent of the foraging substrates and large cacti only 10'3 per cent (N = 282). These data do not include use of cactus flowers which' occurs late inthe breeding season. Elf Owls are exclusively insectivorous, hawking insects from foliage, air and ground (Ligon, 1968). The density and distribution of these three species were not correlated with availability of suitable foraging substrates. For example, Gila Woodpecker density did not vary in proportion to subtree density, a relative measure of trunk and branch surface (G-test, P> 0'05). Ample foraging sites for these species appeared to be available on the Ajo site, but nest site limitations apparently prevented breeding. Availability of foraging substrates on the Aio and Sonoyta sites did not appear to limit the distributions of Curve-billed Thrasher and Cactus Wrens. Curve-billed Thrashers foraged on the ground 98'4 per cent of the time and Cactus Wrens were generalists, foraging on a variety of substrates, the most important of which were ground (20-35 per cent), branches (23-35 per cent) and perennial foliage (11-17 per cent). Two species, Black-throated Sparrows and Mourning Doves, regularly bred all along the gradient. For the ground-foraging, granivorous Mourning Dove, seeds (primarily those of annuals, Martin, Zim et aI., 1951) are probably readily available all along the bajada. For Black-throated Sparrow, shrubs and the annual-foliage and ground directly under shrubs account for 90 per cent or more of the foraging substrates on all sites and all foraging occurred within 100 ern of the ground. Density of Black-throated Sparrows de-
68
S. B. VANDER WALL&]. A. MACMAHON
creased from the top to the bottom of the baiada and was correlated with shrub foliage volume (r= 0'80, P
• • •
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Figure 4. The relationship between shrub foliage volume and the density of Black-throated Sparrows.
Song posts, observation posts and cover Mockingbirds appeared to be restricted in distribution by the availability of sufficient elevated perches which served as song posts, launch sites for short aerial displays and observation posts while foraging. The importance of elevated perches is evident when the population densities of the Kuakatch (2'7 pairs/IO ha) and the Sonoyta (no breeding pairs) sites are compared. These sites were similar in most respects but the Kuakatch site had elevated perches (subtrees) that were lacking at the Sonoyta site. Mockingbirds foraged primarily (96'2 per cent, N = 80) on the ground, annual foliage and shrubs. Density of Mockingbirds, unlike Black-throated Sparrows, was not correlated with foliage volume (P > 0'05). Elevated perches may also have been Important for both White-winged Doves which frequently use them as launch sites for courtship flights and Loggerhead Shrikes (Lanius ludovicianus) which use them for observation posts while foraging. Distribution of Gambel's Quail appeared to be influenced by the distribution of adequate cover (see Gullion, 1960). Gambel's Quail did not occur on the Sonoyta site where cover was sparse and it occurred only along washes on the Kuakatch and Ajo sites where interwash cover was sparse. On the Lukeville site, cover was more evenly distributed and Gambel's Quail was more widespread. This omnivorous ground-foraging species is not likely to be limited by suitable food resources on these more open sites.
AVIAN DISTRIBUTION PATTERNS
69
Discussion Avian distribution patterns along environmental gradients may be influenced by a suite of factors, subsumed under three major categories: (1) Abiotic factors; (2) biotic factors (exclusive of competition); and (3) competition. Abiotic factors (e.g. temperature, relative humidity, wind speed), which may vary greatly along montane elevational gradients, vary negligibly along the gradual slopes ofbajadas and probably have little direct effect on avian distribution. Edaphic conditions, thought to be responsible for the vegetation gradient, may affect bird species indirectlythrough vegetation components. Certain species may be restricted in distribution due to physiological and behavioral constraints imposed by the abiotic environment. House finches, for example, must nest within flight distance of a water source (Salt, 1952; Bartholomew & Cade, 1956) while other species seek cool microenvironments for nesting (Ricklefs & Hainsworth, 1969; Austin, 1974, 1976; Ohmart, 1975) or foraging (Calder, 1968; Ricklefs & Hainsworth, 1968). Because availability of cool microenvironments increases with increased vegetation structural complexity, the possibility of physiological constraints affecting species distribution patterns cannot be excluded. Raitt & Maze (1968) stated that physiologicallimitations may be more important in determining some species distribution patterns than availability of nest sites. However, the effects of the abiotic environment appear to be primarily indirect. As vegetation complexity increases, behavioral options open to birds to avoid physiological stress also increase. The biotic environment (exclusive of competition) appears to have had an important influence on distribution patterns. Of 21 species with lower distributional limits on the baiada, seven (Roadrunner, Lesser Nighthawk, Ladder-backed Woodpecker, Loggerhead Shrike, Scott's Oriole, Brown-headed Cowbird, House Finch) were uncommon and consequently their distributions are poorly known. Ofthe 14 remaining species, six (Elf Owl, Gilded Flicker, Gila Woodpecker, Ash-throated Flycatcher, Cactus Wren, Curve-billed Thrasher) appear to be limited in distribution by availability of suitable nest sites. Four species (Verdin, Black-tailed Gnatcatcher, Phainopepla, Lucy's Warbler) may be limited by availability of food resources. Mockingbirds appear to be limited by availability ofsong posts and Gambel's Quail by the presence of suitable cover. Distributions of critical resources for White-winged Doves and Costa's Hummingbirds are too poorly known to infer causes of their distributions. Thus lower distributional limits of at least 12 of 21 species appear to be influenced by biotic factors. Many bird species depend on specific physiognomic components of the vegetation. This dependence is most evident when important vegetation components are rare. For example, the Ajo site (30 ha) had only one large chain cholla. The only pair of Cactus Wr:ens that bred on this site, which in other respects appears to be good habitat for Cactus Wrens, nested in this chain cholla. The only chain cholla on the Sonoyta site attracted a pair ofVerdins that probably would not have nested without the cholla. Tomoff(1974) emphasized that 'local variation in the densities of nest site plants, i.e. cacti and spinescents, significantly modify the occurrence and densities of breeding birds'. Small changes in vegetation stratification appear to have a significant effect on the bird community. Between the Sonoyta and Kuakatch sites (Fig. 3), foliage insectivores increased 600 per cent while foliage volume increased only 17'1 per cent. The most obvious difference between these two sites, and the apparent cause of the striking difference in foliage insectivores, is that the Kuakatch site has a sparse subtree stratum (4'5 subtrees/ha) which the Sonoyta site lacks. These subtrees were important nest sites for the foliage insectivores (six of seven nests were in subtrees) but were infrequently used as foraging substrates (17'8 per cent of foraging observations). For the community as a whole, most species (63'7 per cent) which were added to the bird community along the vegetation gradient became established with the establishment of the subtree layer (i.e, between the Sonoyta and Ajo sites). Thus, as observed by Karr & Roth (1971) and Willson (1974), the presence of a subtree layer, even though very sparse, has an important influence on bird community structure.
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Species that could modify their nest site selection or foraging behavior had wide distributional ranges along the bajada, For example, Black-throated sparrows seldom nested in creosote bush on the upper bajada (one of 14 nests) but used it frequently on the lower bajada (four of six nests) where few suitable alternatives existed. Nest site shifts by Ashthroated Flycatchers and Mourning Doves were discussed above. On the lower baiada, Black-tailed Gnatcatchers foraged at lower heights and foraged more frequently on shrubs to conform to changes in vegetation composition and foliage height profile. These species exhibit a wider distributional range than species which had narrower foraging (e.g. Verdin) or nesting (e.g. Gila Woodpecker) requirements. Replacement of congeners or ecologically similar species along an environment gradient has frequently been invoked as evidence for competitive exclusion (Terborgh, 1971; Terborgh & Weske, 1975; Able & Noon, 1976; Noon, 1981). Along this vegetation gradient, there were no species replacements and thus competitive interactions of this sort are not evident. Several characteristics of the vegetation may account for the lack of species replacement. For the most part, composition ofthe perennial vegetation along the gradient follows the same pattern as the bird community, i.e. vegetation composition of each site is largely a subset of the vegetation on the next higher site (Vander Wall, 1980). This pattern contrasts with many seres and elevational gradients where marked turnover in vegetation composition is characteristic (op. cit.). In addition, the subtree canopy on the upper bajada never becomes closed, and hence does not alter the configuration of the lower vegetation stratum, an important factor influencing the reassortment of bird species in forest situations (Willson, 1974). Other observations, however, suggest that some species distribution patterns may be affected by competitive interactions. For example, Cactus Wrens and Curve-billed Thrashers both use cholla extensively for nesting. When chollas are few, as frequently occurs on the lower portion of the bajada, competition for suitable nest sites may determine which species has the broadest distribution. Aggressive interactions between Ash-throated Flycatchers and Phainopeplas may serve to reinforce the Phainopepla distribution pattern. Ash-throated Flycatchers, which conduct most of their foraging in interwash areas, aggressively attack other insect-hawking species (Austin & Russell, 1972)including Phainopeplas (pers. observ.). These aggressive interactions may indicate interference competition. Competition may also limit the distribution of foliage insectivores. Densities of the three most important members of this guild, Ash-throated Flycatchers, Black-tailed Gnatcatchers and Verdins, were significantly correlated with foliage volume (Fig. 3). On the Kuakatch site, where the foliage volume is greatly reduced and subtrees occur infrequently, only the flycatcher and gnatcatcher were present. The Verdin, which forages predominantly in subtrees (Vander Wall, 1980), may have been excluded from this site through preemption of food resources by the gnatcatcher and flycatcher. Until experiments examine ecological release of species following removal of potential competitors, the importance of competition as a causative factor in shaping distribution patterns remains uncertain. In addition, we cannot exclude the possibility that competition with distantly related taxa (e.g. ants, spiders, lizards, rodents) has affected avian distribution patterns (cf. Pulliam & Brand, 1975; Brown & Davidson, 1977; Wright, 1981). The relative importance of various factors as determinants of avian distributional limits is similar for this and Able & Noon's (1976) study, despite striking differences in the nature of the vegetation gradient, but differs from that found by Terborgh & Weske (1975) (Table 3). Abiotic factors in all three studies were thought to be oflittle importance despite the fact that physical conditions along two of the gradients have broad ranges. The low importance of abiotic factors may be due to the broad physiological tolerances of birds and the concomitant accommodation to local conditions. Competitive exclusion as a factor determining distributional limits appears to be only low to moderately important along temperate gradients (Table 3) in comparison with tropical gradients where Terborgh & Weske (1975) estimated that two-thirds of the species
71
AVIAN DISTRIBUTION PATTERNS
Table 3. Gradient characteristics along a Sonoran Desert bajada (this study), a temperate elevational gradient (Able & Noon, 1976) and a tropical elevational gradient (Terborgh & Weske, 1975) Factor Climate Type of gradient Habitat Gradient characteristics Bird species richness
This study Arid temperate Edaphic (soil moisture) Desert scrub Habitat gradation no ecotones Low
Able & Noon Moist temperate Elevational
Terborgh & Weske Moist tropical Elevational
Deciduous-coniferous Evergreen forest forest Habitat zonation Habitat zonation ecotones ecotones Very high Medium
Relative number of avian distributional limits determined by: Abiotic factors Low Low Competitive exclusion Low to moderate Low Biologicalfactors (excluding High High competitive exclusion)
Low High Low
were limited by competitive interactions. Noon (1981) suggested that 'much greater species diversity of tropical elevational gradients, relative to temperate ones, accounts for the pattern of intrahabitat spatial exclusion observed for these communities'. In addition, in arid and semi-arid regions climatic variability (e.g. annual precipitation) is great (Logan, 1968; MacMahon, 1980). Such climatic variability is thought to cause widely varying food resource levels resulting in an inequable environment for consumers (Wiens, 1974). Bird species abundance may seldom rise to the point where food resources become limiting (Rotenberry, 1980). Populations maintained in a state of dynamic non-equilibrium may conform to the predictions of competition theory only infrequently (e.g. during 'ecological crunches', Wiens, 1977). A similar explanation may hold for the northeastern United States (the area of Able & Noon's 1976study). In that region 'strong seasonality fosters increased summer diversity, owing to strong resource oscillations combined with winter limitation of consumers. Temporarily superabundant resources in summer relax limits to ecological overlap and enhance diversity' (Rabenold, 1979). In contrast, less variable tropical environments may be in a state of equilibrium and thus competitive interactions may be a more potent force in structuring communities. However, other possible scenarios can be sketched. Biological factors (excluding competition) are important in determining species distributionallimits along temperate vegetation gradients but relatively unimportant along tropical vegetation gradients (Table 3). Able & Noon (1976) found that over 50 per cent of species' distributional limits were at ecotones whereas birds along our Sonoran Desert bajada, where ecotones were poorly defined appear to respond more to specific physiognomic characteristics of the vegetation. This difference may be due primarily to scale; the relatively steep elevational gradients of Able & Noon (1976) show distinct habitat zonation with fairly narrow ecotones, a common characteristic of mountains (Beals, 1969), while the edaphic structure and gradual slopes of baiadas result in habitat gradation with no distinct ecotones. We thank Douglas Andersen, Robert Bayn, Peter Landres, Barry Noon, Kimberly Smith, Rick Vetter & Eric Zurcher for their help. This research was aided by a Grant-in-Aid of Research from Sigma Xi and NSF Grant DEB 78-05328.
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Able, K. P. & Noon, B. R. (1976). Avian community structure along elevational gradients in the northeastern United States. Oecologia, 26: 275-294. Anderson, S. H. & Shugart, H. H. (1974). Habitat selection of breeding birds in an east Tennessee deciduous forest. Ecology, 55: 828-837. Anonymous (1972). Soil Survey, a special report. Organ Pipe Cactus National Monument, Pima County, Arizona. Soil Conservation Service M7-L-22776, 65 pp. Austin, G. T. (1970). Breeding birds of desert riparian habitat in southern Nevada. Condor, 72: 431436. Austin, G. T. (1974). Nesting success of the Cactus Wren in relation to nest orientation. Condor, 76: 216-217. Austin, G. T. (1976). Behavioral adaptations of the Verdin to the desert. Auk, 93: 245-262. Austin, G. T. & Russell, S. M. (1972). Interspecific aggression of Ash-throated Flycatchers on Cassin's Sparrows. Condor, 74: 481. Bartholomew, G. A. & Cade, T. J. (1956). Water consumption of House Finches, Condor, 58: 406412. Beals, E. W. (1969). Vegetational changes along altitudinal gradients. Science, 165: 981-985. Brown, J. H. & Davidson, D. W. (1977). Competition between seed-eating rodents and ants in desert ecosystems. Science, 196: 880-882. Calder, W. A. (1968). The diurnal activity of the Roadrunner, Geococcyx califamicus. Condor, 70: 84-85. Cody, M. L. (1975). Towards a theory of continental species diversities: bird distributions over Mediterranean habitat gradients. In Cody, M. L. & Diamond, J. M. (Eds), Ecology andEvolution ofCommunities, pp. 214-257. Cambridge, Massachusetts: Belknap, 545 pp. Cook, R. E. (1969). Variation in species density in North American birds. Systematic Zoology, 18: 63-84. Diamond, J. M. (1973). Distributional ecology of New Guinea birds. Science, 179: 759-769. Emlen, J. T. (1972). Size and structure of a wintering avian community in southern Texas. Ecology, 53: 317-329. Folse, L. J., Jr. (1982). An analysis of avifauna-resource relationships on the Serengeti Plains. EcologicalMonographs, 52: 111-127. Gullion, G. W. (1960). Ecology of Gambel's Quail in Nevada and the arid southwest. Ecology, 41: 518-536. Hatley, C. L. & MacMahon, J. A. (1980). Spider community organization: seasonal variations and the role of vegetation architecture. Environmental Entomology, 9: 632-639. Hensley, M. M. (1954). Ecological relations of the breeding bird population of the desert biome of Arizona. Ecological Monographs, 24: 185-207. James, F. C. (1971). Ordinations of habitat relationships among breeding birds. Wilson Bulletin, 83: 215-236. Johnston, D. W. & Odum, E. P. (1956). Breeding bird populations in relation to plant succession on the Piedmont of Georgia. Ecology, 37: 50-62. Karr, J. R. (1968). Habitat and avian diversity on strip-mined land in east-central Illinois. Condor, 70,348-357. Karr, J. R. (1971). Structure of avian communities in selected Panama and Illinois habitats. Ecological Monographs, 41: 207-233. Karr, J. R. & Roth, R. R. (1971). Vegetation structure and avian diversity in several New World areas. American Naturalist, 105: 423-435. Kendeigh, S. C. (1944). Measurements of bird populations. Ecological Monographs, 14: 67-106. Klikoff, L. G. (1967). Moisture stress in a vegetational continuum in the Sonoran Desert. American Midland Naturalist, 77: 128-137. Ligon, J. D. (1968). The biology of the Elf Owl, Micrathene whitneyi. Miscellaneous Publications MuseumZoology, University ofMichigan No. 136,70 pp. Logan, R. F. (1968). Causes, climates and distributions of deserts. In Brown, G. W. (Ed.), Desert Biology, Vol. 1, pp. 21-50. New York: Academic Press. 635 pp. MacArthur, R. H. (1958). Population ecology of some warblers of northeastern coniferous forests. Ecology, 39: 599-619. MacArthur, R. H. (1964). Environmental factors affecting bird species diversity. American Naturalist, 98, 387-397.
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Terborgh, J. & Weske, J. S. (1975). The role of competition in the distribution of Andean birds. Ecology, 56: 562-576. Tomoff, C. S. (1974). Avian species diversity in desert scrub. Ecology, 55: 396-403. Tramer, E. J. (1974). On latitudinal gradients in avian diversity. Condor, 76: 123-130. Vander Wall, S. B. (1980). The structure of Sonoran Desert bird communities: effects of vegetation structure and precipitation. Ph.D. dissertation, Utah State University, Logan, Utah, 154 pp.
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Whitmore, R. C. (1975). Habitat ordination of passerine birds of the Virgin River Valley, Southwestern Utah. Wilson Bulletin, 87: 65-74. Whitmore, R. C. (1977). Habitat partitioning in a community of passerine birds. Wilson Bulletin, 87: 253-265. Wiens, J. A. (1974). Climatic instability and the 'ecological saturation' of bird communities in North American grasslands. Condor, 76: 385-400. Wiens, J. A. (1977). On competition and variable environments. American Scientist, 65: 590--597. Wiens, J. A., Martin, S. G., Holthaus, W. R. & Iwen, F. A. (1970). Metronome timing in behavioral ecology studies. Ecology, 51: 350--352. Willson, M. F. (1974). Avian community organization and habitat structure. Ecology, 55: 10171029. Wright, S. J. (1981). Extinction-mediated competition: the Anolis lizards and insectivorous birds of the West Indies. American Naturalist, 117: 181-192. Yang, T. W. & Lowe, C. H. (1956). Correlation of major vegetational climaxes with soil characteristics in the Sonoran Desert. Science, 123: 542.
Avian distribution patterns along a Sonoran Desert bajada
Stephen B. Vander Wall & James A. MacMahon* Accepted 11 March 1983 Avianpopulation density, foragingbehavior, nest site selectionand other aspects of vegetationuse were studied at four points alonga vegetationgradient at Organ Pipe Cactus National Monument, Arizona, U.S.A., to elucidate causesof species distributions. The vegetationgradient, a result of edaphic change along a bajada (coalesced alluvial fans), ranged from a diverse subtree-large cacti-shrub community on gravellysoilsat the top of the bajadato a simple lowshrub community on fine sandy loam at the bottom of the bajada, Twenty-three species of birds bred in the diverse vegetation at the top of the bajada. Proceeding down the bajada, bird speciesdropped out of the community as vegetationarchitecture and composition became more simple, with only two species breeding all along the gradient. Most species distributions were determined by requirements for specific vegetation components for nesting and foraging. Competition and abiotic factors seem to have played only a minor role, if any, in determining distributions. Causes for speciesdistributions along this bajada are similar to those affecting avian distribution along vegetation gradients on temperate mountain slopes but differ from those along tropical elevationalgradients.
Introduction Vegetation gradients of various types have become important objects of study for the analysis of avian community structure, including changes over time (seres) (Odum, 1950; Johnston & Odum, 1956; Karr, 1968; Smith & MacMahon, 1981) and in space (e.g. elevational gradients). Temperate and tropical mountain slopes have been used to evaluate the relative importance of physical conditions, vegetation attributes, and competition in determining species distributional limits along such gradients (Terborgh, 1971; Diamond, 1973; Terborgh & Weske, 1975; Able & Noon, 1976; Noon & Able, 1978; Pearson & Ralph, 1978; Noon, 1981). Recently, ordination techniques have been used to examine avian habitat requirements by rearranging mosaics of sites into artificial gradients based on habitat qualities (James 1971; Anderson & Shugart 1974; Whitmore, 1975, 1977; Rotenberry, 1978). Effects of broad geographic gradients of vegetation change on avian species richness and diversity have also been studied (Cook, 1969; Tramer, 1974; Rabenold, 1979; Short, 1979). Sonoran Desert bajadas (coalesced alluvial fans) are excellent field laboratories for studying the response of organisms to vegetation gradients, since edaphic changes along bajadas result in marked vegetational changes (Yang & Lowe, 1956; Klikoff, 1967; Phillips & MacMahon, 1978). The gravelly soils of the upper bajada typically support a diverse plant community including various subtrees, columnar cacti, chollas and shrubs. Down the bajada, soils become finer, subtrees and large cacti gradually drop from the plant community, and the shrub stratum becomes less diverse (Yang & Lowe, 1956; Phillips & • Department of Biology and Ecology Center, UMC 53, Utah State University, Logan, Utah 84322 U.S.A. 0140-1963/84/010059 + 16 $03.00/0
cD
1984 Academic Press Inc. (London) Limited
60
S. B. VANDER WALL &
J. A. MACMAHON
MacMahon, 1978). By comparing bird communities on sites differing in specific vegetation attributes, factors determining species distribution patterns can be elucidated. Both the relative shortness (usually < 10 km) and gradual slope (= 1'8 per cent, Phillips & MacMahon, 1978)of baiadas facilitate the study of avian distribution patterns, because all vegetation types occur within the same climatic regime and all members of the local avifauna can reach all points along the bajada. In this study, we examined bird communities of a series of bajada sites differing in vegetation physiognomy and plant species composition. At each site, avian community composition and species foraging and nesting requirements were determined. Site comparisons yield data we use to address probable causes of the bird distributional limits along the vegetation gradient. Study areas Four study sites differing in vegetation structure (physiognomy and foliage profile) were selected at Organ Pipe Cactus National Monument (OPCNM), Arizona, U.S.A. At each site, three replicate 10 ha plots of relatively homogeneous vegetation were established. All sites were> 3 km from permanent water to avoid the influence of water availability on bird community structure (MacArthur, 1964; Karr, 1968; Austin, 1970); major washes and riparian habitats were avoided. All sites were located on a baiada radiating from the Ajo Mountains. Four physiognomic groups of perennial plants occurred on the bajada, (I) Subtreessingle stemmed non-succulent plants typically 2-6 m tall at maturity'(e.g, 'paloverde, Gercidium microphyllum; ironwood, Olneya tesota; acacia, Prosopis glandulosa. (2) Shrubsnon-succulent plants typically < 2 m tall at maturity (e.g. creosotebush, Larrea tridentata: bursage, Ambrosia deltoides; brittlebush, Enceliafarinosa) plus sapling subtrees < 1 m tall. (3) Large cacti-chain (Opuntiafulgida) and buckhorn (0. acanthocarpa) cholla > 1 m tall and saguaro (Gerus giganteus) and organ pipe cactus (C. thurberi) .> 3 m tall. (4) Small cacti-all other species of cacti (e.g. hedgehog cactus, Echinocereus engelmannii; barrel cactus F erocactus wislizenii), cholla < 1 m tall and columnar cacti < 3 m tall. Characteristics of the perennial vegetation, location, and physical nature of each site are summarized below. The Lukeville site, situated on the upper portion of a baiada, is located 4-4 km north of Lukeville, Arizona, and 0'1 km east of Arizona Highway 85 (31°55'N, 112°48'W). The terrain is gently rolling with a mean elevation of 475 m. The soil is a very gravelly loam (Anonymous, 1972). The vegetation is composed of numerous subtrees (X ± S.D. = 127'6 ± 34'9 per ha) and large cacti (53'5 ± 23'9 per ha). Shrubs are abundant (5536'0 ± 650'4 per ha) and small cacti are common (104'7 ± 34'8 per ha). TheAjo site is in Aio Valley, 21·5 kmnorth ofOPCNM headquarters and Or l km west of Highway 85 (32°7'N, 112°46'W). The terrain is slightly rolling to flat (elevation 549 m) with a gravelly sandy loam soil. The vegetation is dominated by numerous subtrees (74·9 ± 20-4 per ha) and shrubs (3727 ::!: 818·0 per ha). Large cacti are virtually absent and small cacti are infrequent. The Kuakatch site is just north of Kuakatch Wash, 5·0 km west of OPCNM's north entrance (32°12'N, 112°47'W). The site is nearly flat and levelat 518 m elevation. The soil is fine sandy loam. The plant community is dominated by shrubs (1870,8 ± 191·3 per ha), but subtrees are conspicuous though infrequent (4.5 ± 2·1 per ha). Again, large cacti are virtually absent and small cacti are infrequent. Sonoyta, the lowest site along the baiada is located in Sonoyta Valley, 3·3 km south-east of Lukeville, just north and east of Gachado Well (31°52'N, 112°47'W). The area is a flat nearly level plain (elevation 428 m) with very fine sandy loam and slightly saline soil (Anonymous, 1972). Shrubs dominate the vegetation (2660'1 ± 580'6 per ha). Subtrees and large cacti are absent, and several species of small cacti occur uncommonly. Precipitation during the winter rainy season (November-March) in 1977-8 and 1978-9
AVIAN DISTRIBUTIONPATTERNS
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was 58 and 110 mm, (1'9 and 3'6 times the mean precipitation of the previous 10 winter rainy seasons), respectively (National Oceanic and Atmospheric Administration, 196779). Abundant winter rain, probably the most important abiotic factor affecting avian breeding populations in Sonoran Desert ecosystems, suggests that species populations, distributions and vegetation uses were similar in both field seasons.
Methods We censused birds using the spot-map method (Kendeigh, 1944). All plots were censused in March and April 1978 except Kuakatch which was censused in March and April 1979. Due to the low, sparse nature of the vegetation and the low number and density of bird species, five censuses were deemed sufficient to sample breeding birds populations. All censuses were initiated within 10 min of sunrise and completed in - 2 h. Owls were censused at night by playing tape recorded vocalizations of Elf (Micrathene whitneyi), Screech (Otus asio), and Ferruginous (Glaucidium brasilianum) owls on each plot. Birds encountered during censuses were separated into three groups. (1) Breeding residents (based on territorial pairs or nests); (2) breeding visitors (species known to breed in the vicinity but not on the study site); and (3) migrants and winter visitors (reported in Vander Wall, 1980). Breeding bird densities were determined using the maximum number of pairs observed. Breeding visitors were divided into two groups. (1) Birds observed visiting the site two or fewer times (designated 'v'); and (2) birds observed visiting the site three or more times and which may have a significant influence on the bird community (assigned a density of 0'5 for each pair). We collected foraging behavior data on 11 breeding species. The remaining 12 species were either uncommon or difficult to observe. For them, published accounts of foraging behavior and diet were used to determine foraging requirements. Our foraging data were gathered between 0830 and 1130 h (MST). Foraging technique, foraging substrate, foraging height, perch substrate and plant species were recorded at approximately 15 s intervals using a metronome (Wiens, Martin etal., 1970). The metronome was not used for the Ashthroated Flycatcher (Myiarchus cinerascens) and Curve-billed Thrasher (Toxostoma curvirostre); rather actual foraging maneuvers were recorded. We classified foraging techniques as gleaning, probing and hawking. We defined foraging and perching substrates as: twig « 1 em diameter); branch (~1 but <4 cm); large branch (~4 but < 10 cm); trunk (~1O cm); perennial foliage; annual foliage (stems, leaves and flowers of annuals); flowers (including buds); ground; and air. For bird species that occurred on two or more sites along the vegetation gradient, we made separate foraging observations (usually 100 to 300 observations per species) on each site. For infrequently observed species, observations from all sites were combined. We also located nest sites of each species and for each recorded plant species, plant height and nest structure. We determined density and volume of perennial plants by counting and measuring (height, greatest diameter, least diameter) individuals on 10 circular subplots (radii = 10 m) on each 10 ha plot. Foliage volume was calculated using the formula: Foliage volume =0/31'1' (height x greatest diameter x least diameter) Foliage height profiles were determined using the technique of Karr (1971). Foliage height diversity was calculated based on the vegetation layers: 0-75 em, 75-150 em, and> 150 cm. Results
Community structure along the Bajada Bird species richness and pair density decreased dramatically from upper to lower bajada sites (Table 1). Number of breeding species per 10 ha decreased from Lukeville
Gambel's Quail Callipeplagambelii White-winged Dove Zenaida asiatica Mourning Dove Zenaida macroura Roadrunner Geococcyx californianus ElfOwl M icrathene uihuneyi Lesser Nighthawk Chordeiles acutipennis Costa's Hummingbird Calypte costae Gilded Flicker Colaptes auratus chrysoides Gila Woodpecker Melanerpes uropygialis Ladder-backed Woodpecker Picoides scalaris Ash-throated Flycatcher M yiarchuscinerascens Verdin Auriparusjlaviceps
Species
2 0'5 1 1
3 1
3 3
1
0·5 1
0'5 2
v 2 3
2
v 2'5 0·5 3 5
1
2
3
3 2
2'5 2'5
0'5
1" ':'
v 0·5
0'5
1
1
1
1
v
v
v
v v
v
2
0'5
1
1
i-
v
v
1
0·5
3
Sonoyta
1
1
1
1
1
v
2
1
2
0·5
0'5
1
1
3
Kuakatch
1
2
Ajo
0'5
0·5
1
1
1
0'5
1 0'5
1
2'5
2
1'5
0'5
1
1
2
-3
1
I
Lukeville
1
3
Table 1. Densityofbreeding birds (pair/1O ha) alonga vegetation gradient at OrganPipe CactusNational Monument. Infrequent visitors (two or fewer visits) aredesignated with a v. Frequent visitors (three ormorevisits)areassigned a density of0·5 perpair. If twoormore pairsoffrequent visitors werepresent, it is indicated with an asterix
0"-
Z
:I: 0
;..
3:
(J
?" 3: ;..
':-'
Ro
rr-
;..
~
tT1 ;.::l
0
Z
<: ;..
?='
~
N
Cactus Wren Campylorhynchus brunneicapillus Mockingbird M imuspolyglottos Curve-billed Thrasher Toxostomacurvirostre Black-tailed Gnatcatcher Polioptila melanura Phainopepla Phainopepla nitens Loggerhead Shrike Lanius ludooicianus Lucy's Warbler Vennivora luciae Scott's Oriole Icterusparisorum Brown-headed Cowbird Molothrus ater House Finch Carpodacusmexicanus Black-throated Sparrow Amphispiza bilineata Total breeding pairs Total breeding species Bird species diversity 2
0'5 6 30'5 18 2'66
0'5 6 31 16 2'43
3 36 21 2'90
0'5
I
v
0'5
1
0'5
2"
z-
0'5 v
3
2'5
I
I
4
2
2
2
2
2
0'5
6 20 15 2'32
v
0'5
0'5
1'5
1
0'5
4 24'5 18 2'69
I
I
I';'
2'5
0'5
2
1
2'08
11
5 20'5
0'5
1'5
0'5
3
v
3
9 2'00 8 1'90
13
0'5
I
0'5
3
2
2'5 11
v
1'5
0'5
3
I
2'5 10'5 8 1'94
v
v
0'5
I
v
2
3
I 2 2 2'5 2 2 0'69 0'50
v
v
v
2 4 3 1'04
v
<
w
0"-
Z V>
::0
tT1
-l -l
;;..
'"
Z
0
-l
C
0;
::0
C U; -l
>= Z
;J>
S. B. VANDERWALL & J. A. MACMAHON
64
(X= 18'3), Ajo (14'7), Kuakatch (8'3), to the Sonoyta site (2'3). Similarly, mean pair density also decreased: Lukeville (32'5), Ajo (21'7), Kuakatch (11'5), and Sonoyta (2'8). Bird species diversity was significantly correlated with foliage height diversity (r= 0'93, N= 12, P
Ell Owl Gilded Flicker Gila Woodrecker Thrasher
Curv.~bill.d
White·wina.d Dove Costo's
Hummlnsblrd
Verdin
LUCy'S Warbler Ash.throottd Flycatcher
Cactus Wren Mockinablrd
Block-toiled GnoI co Ie her Pholnogeplo Mournl"?
Dov.
Block-throoled
sparrow
Figure 1. Distribution patterns (horizontal bars) of breeding birds along a Sonoran Desert bajada at Organ Pipe Cactus National Monument. Vegetation typical of each study site is depicted. Some uncommon bird species are not included because their distribution patterns are insufficiently known. Note that species are added to the community with increasing vegetation complexity but none are lost.
Species widely-distributed along the vegetation gradient either had similar population densities where they occurred (e.g. Gambel's Quail, Callipepla gambelii; Mourning Dove, Zenaida macroura) or had densities that declined with decreasing vegetation complexity (e.g. Ash-throated Flycatcher; Black-throated Sparrow, Amphispiza bilineata) (Table 1). Those species which were most dominant, i.e. had high relative abundances in habitats where they had their greatest relative abundance, were significantly more widely distri-
65
AVIAN DISTRIBUTION PATTERNS 1·5
• J::.
1·0
-0 0
Q>
.E
•
• ••
§
0·5
:0 0
~
•
I
.. •
•
•
Dominonce (Ofo)
Figure 2. Relationship between habitat breadth (W) (McNaughton & Wolf, 1970) and dominance (relative abundance of each species in the habitat where it makes its greatest contribution) for 23 species along a Sonoran Desert bajada.
buted along the habitat gradient (r= 0'55, N = 23, P< 0'01; Fig. 2); a pattern similar to that reported by McNaughton & Wolf (1970) and Able & Noon (1976). Only two species, Mourning Dove and Black-throated Sparrow, were found breeding all along the vegetation gradient. The other 21 species had their lower distributional limits at various points along the bajada, Below, we examine foraging behavior and vegetationuse of these species to identify resources and other environmental variables that limit species distributions.
Nest sites Proportion of the avian community requiring different types of nests (e.g. cavities, open cups, etc.) changed along the bajada; cavities were used less frequently and open cups were used more frequently as vegetation structure became simpler (Table 2). Physiognomic categories of plants used for cavity, open cup and covered nests also changed along the gradient. On the Lukeville site, 95'7 per cent of nest cavities were placed in large cacti; only subtrees were used on the Ajo and Kuakatch sites. Subtrees and large cacti were used most frequently for open cup nests on the upper bajada (Lukeville, 81 per cent; Ajo, 62 per cent) and shrubs and ground most frequently on the lower bajada sites (Kuakatch, 53 per cent; Sonoyta, 100 per cent). Covered nests were never found in shrubs. These trends in nest site selection are due to both the absence of species that require specific physiognomic components of the vegetation for nesting at some sites and to changes in the types of nest sites selected by certain species as a result of changes in vegetation structure. Table 2. Proportion of the community (pairs/1O ha) using different types of nest sites along the vegetation gradients
Lukeville
Ajo
Kuakatch
Sonoyta
Nest type
Density
%
Density
%
Density
%
Density
%
Cavity Covered Open Cup No Nesr] Total
8'2 5'1 18'7 0'5 32'5
25'2 16'0 57'5 1'5
4'3 2'7 14'4 0'3 21'7
19'8 12'4 66'4 1'3
1'0 1'7 8'8 0'0 11'5
8'7 14'8 76'5 0'0
0'1 * 0'3 2'4 0'0 2'8
3'8 10'7 85'7 0'0
* Bred off site.
t Lesser Nighthawk and Brown-headed Cowbird.
66
s. B. VANDERWALL&J. A. MACMAHON
Several species appeared to be limited in distribution along the gradient by a lack of suitable nest sites. Elf Owls (six nests), Gila Woodpeckers (Melanerpes uropygialis, 11 nests), and Gilded Flickers (Colaptes auratus chrysoides, three nests) were cavity nesting species which nested exclusively in saguaros and hence were able to breed only on the Lukeville site. Gila Woodpeckers and Gilded Flickers which visited the Ajo site bred in large isolated saguaros> 100m away from the study plots. These three species prefer to nest high above the ground (usually >5 m; Hensley, 1954; Ligon, 1968; Vander Wall, 1980) and consequently subtrees do not provide suitable nest sites since trunks in which a cavity can be excavated seldom exist above 2 or 3 m high. Three other cavity nesting species, Ladder-backed Woodpecker (Picoides scalaris), Ashthroated Flycatcher and Lucy's Warbler (Vennivora luciae), had more general nest site requirements. For example, Ash-throated Flycatchers nested primarily in saguaro cavities (four of five nests) 7 to 11 m high on the Lukeville site, but on the Ajo and Kuakatch sites they used subtree cavities (six nests) I'Cr2'3 m high. Absence of breeding Ash-throated Flycatchers on the Sonoyta site may be due to nest site limitations. Shrub communities were apparently suitable habitats for foraging (e.g, over 90 per cent of the Ash-throated Flycatcher foraging observations on the Kuakatch site were on shrubs or annual foliage) but lacked suitable cavities for nesting. Lucy's Warblers did not appear to be limited in distribution along the bajada by availability of nest sites (see below) and too few Ladderbacked Woodpeckers bred along the gradient to accurately assess their distribution pattern. Cactus Wrens (Campylorhynchus brunneicapillus) and Curve-billed Thrashers nested almost exclusively in chollas at OPCNM (32 of 33 nests), and appeared to be limited in distribution by a lack of suitable nest sites on several plots. Two of the Ajo plots had very few chollas (none> 1 m tall) and no breeding pairs of wrens or thrashers (visiting pairs nested in chollas> 100 m away from the study plot). On the third Ajo plot there was only one mature chain cholla which a pair of Cactus Wrens used for a nest site. On the Kuakatch plots the few chollas present were small (0'75--1'25 m tall) and Curve-billed Thrashers, which usually nest in large (> 1'25 m) chollas, were not present. Most of the remaining species seemed not to be restricted in distribution by availability of suitable nest sites. Several species had general nest site requirements (e.g. Mourning Dove; Costa's Hummingbird, Calypte costae; Black-tailed Gnatcatcher, Polioptila melanura; House Finch, Carpodacus mexicanus; Black-throated Sparrow), nested on the ground (i.e. Lesser Nighthawk, Chordeiles acutipennis) or made no nest at all (i.e, Brownheaded Cowbird, Molothrus ater). Several other species (e.g. White-winged Dove, Zenaida asiatica; Roadrunner, Geococcyx californianus; Verdin, Auriparus jiaviceps; Mockingbird, Mimus polyglottos) appeared to have specific nest site requirements but insufficient data were available to determine if nest sites were likely to be a limiting factor.
Foraging sites The distribution of some species may be dependent on the availability of suitable foraging substrates. This possibility was examined by comparing the distribution and abundance of species with an availability index of its most important foraging substrates (e.g. foliage volume). Population densities of foliage-gleaning insectivores and the Ash-throated Flycatcher (a foliage-hawking insectivore) increased along the vegetation gradient and were significantly correlated to foliage volume (Verdin, r= 0'96, P
AVIAN DISTRIBUTION PATTERNS
67
10
'0 .c Q <,
8
~
I Sonoyto
'0 ~
III
'" 5
6
.~
u
'" .5 III
0' '" .2
4
<5
LL
2
I
II Kuokctch II II II
I
Folioge volume (1000 m 3 /bo)
Figure 3. The relationship between foliage volume and the density of foliagegleaning and hawking birds. Abbreviations of species names are: AtF-Ash-throated Flycatcher, BtG-Black-tailed Gnatcatcher, LW-Lucy's Warbler, SO-Scott's Oriole, V-Verdin.
(e.g. Murdock, Evans et aI., 1972, Southwood, Brown et aI., 1979, Hatley & MacMahon, 1980; however, see Folse, 1982). Thus, we hypothesize that low foliage volume (and thus low arthropod biomass) on the lower bajada sites may be a limiting resource for several insectivores. Lucy's Warblers and Phainopeplas tPhainopepla nitens) occurred primarily in the more densely vegetated washes, and, thus, their distributions largely coincided with the bajada's dendritic drainage pattern. These distributions may be related to greater foraging success in the lusher, more productive, habitat. Foraging site availability did not appear to limit the distribution of those species that nested exclusively in saguaro. Gilded Flickers foraged exclusively on the ground and foliage of annuals (N = 36). For Gila Woodpeckers, subtrees accounted for 89'0 per cent of the foraging substrates and large cacti only 10'3 per cent (N = 282). These data do not include use of cactus flowers which' occurs late inthe breeding season. Elf Owls are exclusively insectivorous, hawking insects from foliage, air and ground (Ligon, 1968). The density and distribution of these three species were not correlated with availability of suitable foraging substrates. For example, Gila Woodpecker density did not vary in proportion to subtree density, a relative measure of trunk and branch surface (G-test, P> 0'05). Ample foraging sites for these species appeared to be available on the Ajo site, but nest site limitations apparently prevented breeding. Availability of foraging substrates on the Aio and Sonoyta sites did not appear to limit the distributions of Curve-billed Thrasher and Cactus Wrens. Curve-billed Thrashers foraged on the ground 98'4 per cent of the time and Cactus Wrens were generalists, foraging on a variety of substrates, the most important of which were ground (20-35 per cent), branches (23-35 per cent) and perennial foliage (11-17 per cent). Two species, Black-throated Sparrows and Mourning Doves, regularly bred all along the gradient. For the ground-foraging, granivorous Mourning Dove, seeds (primarily those of annuals, Martin, Zim et aI., 1951) are probably readily available all along the bajada. For Black-throated Sparrow, shrubs and the annual-foliage and ground directly under shrubs account for 90 per cent or more of the foraging substrates on all sites and all foraging occurred within 100 ern of the ground. Density of Black-throated Sparrows de-
68
S. B. VANDER WALL&]. A. MACMAHON
creased from the top to the bottom of the baiada and was correlated with shrub foliage volume (r= 0'80, P
• • •
6
'0 s: Q
<,
e
•
5
~
0 a.
II>
J
0
c:i
4
0'"
'Ii"
~
0
a.
II>
"0 Q)
0 £
•
3
.<:
I
I
.Jt:. U
0
iIi
2
o
.,"
0
\"
§'
s
c:i
l.-
~
"
•
,<;0
•
• 2 Shrub volume (I000m 3 fha)
Figure 4. The relationship between shrub foliage volume and the density of Black-throated Sparrows.
Song posts, observation posts and cover Mockingbirds appeared to be restricted in distribution by the availability of sufficient elevated perches which served as song posts, launch sites for short aerial displays and observation posts while foraging. The importance of elevated perches is evident when the population densities of the Kuakatch (2'7 pairs/IO ha) and the Sonoyta (no breeding pairs) sites are compared. These sites were similar in most respects but the Kuakatch site had elevated perches (subtrees) that were lacking at the Sonoyta site. Mockingbirds foraged primarily (96'2 per cent, N = 80) on the ground, annual foliage and shrubs. Density of Mockingbirds, unlike Black-throated Sparrows, was not correlated with foliage volume (P > 0'05). Elevated perches may also have been Important for both White-winged Doves which frequently use them as launch sites for courtship flights and Loggerhead Shrikes (Lanius ludovicianus) which use them for observation posts while foraging. Distribution of Gambel's Quail appeared to be influenced by the distribution of adequate cover (see Gullion, 1960). Gambel's Quail did not occur on the Sonoyta site where cover was sparse and it occurred only along washes on the Kuakatch and Ajo sites where interwash cover was sparse. On the Lukeville site, cover was more evenly distributed and Gambel's Quail was more widespread. This omnivorous ground-foraging species is not likely to be limited by suitable food resources on these more open sites.
AVIAN DISTRIBUTION PATTERNS
69
Discussion Avian distribution patterns along environmental gradients may be influenced by a suite of factors, subsumed under three major categories: (1) Abiotic factors; (2) biotic factors (exclusive of competition); and (3) competition. Abiotic factors (e.g. temperature, relative humidity, wind speed), which may vary greatly along montane elevational gradients, vary negligibly along the gradual slopes ofbajadas and probably have little direct effect on avian distribution. Edaphic conditions, thought to be responsible for the vegetation gradient, may affect bird species indirectlythrough vegetation components. Certain species may be restricted in distribution due to physiological and behavioral constraints imposed by the abiotic environment. House finches, for example, must nest within flight distance of a water source (Salt, 1952; Bartholomew & Cade, 1956) while other species seek cool microenvironments for nesting (Ricklefs & Hainsworth, 1969; Austin, 1974, 1976; Ohmart, 1975) or foraging (Calder, 1968; Ricklefs & Hainsworth, 1968). Because availability of cool microenvironments increases with increased vegetation structural complexity, the possibility of physiological constraints affecting species distribution patterns cannot be excluded. Raitt & Maze (1968) stated that physiologicallimitations may be more important in determining some species distribution patterns than availability of nest sites. However, the effects of the abiotic environment appear to be primarily indirect. As vegetation complexity increases, behavioral options open to birds to avoid physiological stress also increase. The biotic environment (exclusive of competition) appears to have had an important influence on distribution patterns. Of 21 species with lower distributional limits on the baiada, seven (Roadrunner, Lesser Nighthawk, Ladder-backed Woodpecker, Loggerhead Shrike, Scott's Oriole, Brown-headed Cowbird, House Finch) were uncommon and consequently their distributions are poorly known. Ofthe 14 remaining species, six (Elf Owl, Gilded Flicker, Gila Woodpecker, Ash-throated Flycatcher, Cactus Wren, Curve-billed Thrasher) appear to be limited in distribution by availability of suitable nest sites. Four species (Verdin, Black-tailed Gnatcatcher, Phainopepla, Lucy's Warbler) may be limited by availability of food resources. Mockingbirds appear to be limited by availability ofsong posts and Gambel's Quail by the presence of suitable cover. Distributions of critical resources for White-winged Doves and Costa's Hummingbirds are too poorly known to infer causes of their distributions. Thus lower distributional limits of at least 12 of 21 species appear to be influenced by biotic factors. Many bird species depend on specific physiognomic components of the vegetation. This dependence is most evident when important vegetation components are rare. For example, the Ajo site (30 ha) had only one large chain cholla. The only pair of Cactus Wr:ens that bred on this site, which in other respects appears to be good habitat for Cactus Wrens, nested in this chain cholla. The only chain cholla on the Sonoyta site attracted a pair ofVerdins that probably would not have nested without the cholla. Tomoff(1974) emphasized that 'local variation in the densities of nest site plants, i.e. cacti and spinescents, significantly modify the occurrence and densities of breeding birds'. Small changes in vegetation stratification appear to have a significant effect on the bird community. Between the Sonoyta and Kuakatch sites (Fig. 3), foliage insectivores increased 600 per cent while foliage volume increased only 17'1 per cent. The most obvious difference between these two sites, and the apparent cause of the striking difference in foliage insectivores, is that the Kuakatch site has a sparse subtree stratum (4'5 subtrees/ha) which the Sonoyta site lacks. These subtrees were important nest sites for the foliage insectivores (six of seven nests were in subtrees) but were infrequently used as foraging substrates (17'8 per cent of foraging observations). For the community as a whole, most species (63'7 per cent) which were added to the bird community along the vegetation gradient became established with the establishment of the subtree layer (i.e, between the Sonoyta and Ajo sites). Thus, as observed by Karr & Roth (1971) and Willson (1974), the presence of a subtree layer, even though very sparse, has an important influence on bird community structure.
70
S. B. VANDERWALL & J. A. MACMAHON
Species that could modify their nest site selection or foraging behavior had wide distributional ranges along the bajada, For example, Black-throated sparrows seldom nested in creosote bush on the upper bajada (one of 14 nests) but used it frequently on the lower bajada (four of six nests) where few suitable alternatives existed. Nest site shifts by Ashthroated Flycatchers and Mourning Doves were discussed above. On the lower baiada, Black-tailed Gnatcatchers foraged at lower heights and foraged more frequently on shrubs to conform to changes in vegetation composition and foliage height profile. These species exhibit a wider distributional range than species which had narrower foraging (e.g. Verdin) or nesting (e.g. Gila Woodpecker) requirements. Replacement of congeners or ecologically similar species along an environment gradient has frequently been invoked as evidence for competitive exclusion (Terborgh, 1971; Terborgh & Weske, 1975; Able & Noon, 1976; Noon, 1981). Along this vegetation gradient, there were no species replacements and thus competitive interactions of this sort are not evident. Several characteristics of the vegetation may account for the lack of species replacement. For the most part, composition ofthe perennial vegetation along the gradient follows the same pattern as the bird community, i.e. vegetation composition of each site is largely a subset of the vegetation on the next higher site (Vander Wall, 1980). This pattern contrasts with many seres and elevational gradients where marked turnover in vegetation composition is characteristic (op. cit.). In addition, the subtree canopy on the upper bajada never becomes closed, and hence does not alter the configuration of the lower vegetation stratum, an important factor influencing the reassortment of bird species in forest situations (Willson, 1974). Other observations, however, suggest that some species distribution patterns may be affected by competitive interactions. For example, Cactus Wrens and Curve-billed Thrashers both use cholla extensively for nesting. When chollas are few, as frequently occurs on the lower portion of the bajada, competition for suitable nest sites may determine which species has the broadest distribution. Aggressive interactions between Ash-throated Flycatchers and Phainopeplas may serve to reinforce the Phainopepla distribution pattern. Ash-throated Flycatchers, which conduct most of their foraging in interwash areas, aggressively attack other insect-hawking species (Austin & Russell, 1972)including Phainopeplas (pers. observ.). These aggressive interactions may indicate interference competition. Competition may also limit the distribution of foliage insectivores. Densities of the three most important members of this guild, Ash-throated Flycatchers, Black-tailed Gnatcatchers and Verdins, were significantly correlated with foliage volume (Fig. 3). On the Kuakatch site, where the foliage volume is greatly reduced and subtrees occur infrequently, only the flycatcher and gnatcatcher were present. The Verdin, which forages predominantly in subtrees (Vander Wall, 1980), may have been excluded from this site through preemption of food resources by the gnatcatcher and flycatcher. Until experiments examine ecological release of species following removal of potential competitors, the importance of competition as a causative factor in shaping distribution patterns remains uncertain. In addition, we cannot exclude the possibility that competition with distantly related taxa (e.g. ants, spiders, lizards, rodents) has affected avian distribution patterns (cf. Pulliam & Brand, 1975; Brown & Davidson, 1977; Wright, 1981). The relative importance of various factors as determinants of avian distributional limits is similar for this and Able & Noon's (1976) study, despite striking differences in the nature of the vegetation gradient, but differs from that found by Terborgh & Weske (1975) (Table 3). Abiotic factors in all three studies were thought to be oflittle importance despite the fact that physical conditions along two of the gradients have broad ranges. The low importance of abiotic factors may be due to the broad physiological tolerances of birds and the concomitant accommodation to local conditions. Competitive exclusion as a factor determining distributional limits appears to be only low to moderately important along temperate gradients (Table 3) in comparison with tropical gradients where Terborgh & Weske (1975) estimated that two-thirds of the species
71
AVIAN DISTRIBUTION PATTERNS
Table 3. Gradient characteristics along a Sonoran Desert bajada (this study), a temperate elevational gradient (Able & Noon, 1976) and a tropical elevational gradient (Terborgh & Weske, 1975) Factor Climate Type of gradient Habitat Gradient characteristics Bird species richness
This study Arid temperate Edaphic (soil moisture) Desert scrub Habitat gradation no ecotones Low
Able & Noon Moist temperate Elevational
Terborgh & Weske Moist tropical Elevational
Deciduous-coniferous Evergreen forest forest Habitat zonation Habitat zonation ecotones ecotones Very high Medium
Relative number of avian distributional limits determined by: Abiotic factors Low Low Competitive exclusion Low to moderate Low Biologicalfactors (excluding High High competitive exclusion)
Low High Low
were limited by competitive interactions. Noon (1981) suggested that 'much greater species diversity of tropical elevational gradients, relative to temperate ones, accounts for the pattern of intrahabitat spatial exclusion observed for these communities'. In addition, in arid and semi-arid regions climatic variability (e.g. annual precipitation) is great (Logan, 1968; MacMahon, 1980). Such climatic variability is thought to cause widely varying food resource levels resulting in an inequable environment for consumers (Wiens, 1974). Bird species abundance may seldom rise to the point where food resources become limiting (Rotenberry, 1980). Populations maintained in a state of dynamic non-equilibrium may conform to the predictions of competition theory only infrequently (e.g. during 'ecological crunches', Wiens, 1977). A similar explanation may hold for the northeastern United States (the area of Able & Noon's 1976study). In that region 'strong seasonality fosters increased summer diversity, owing to strong resource oscillations combined with winter limitation of consumers. Temporarily superabundant resources in summer relax limits to ecological overlap and enhance diversity' (Rabenold, 1979). In contrast, less variable tropical environments may be in a state of equilibrium and thus competitive interactions may be a more potent force in structuring communities. However, other possible scenarios can be sketched. Biological factors (excluding competition) are important in determining species distributionallimits along temperate vegetation gradients but relatively unimportant along tropical vegetation gradients (Table 3). Able & Noon (1976) found that over 50 per cent of species' distributional limits were at ecotones whereas birds along our Sonoran Desert bajada, where ecotones were poorly defined appear to respond more to specific physiognomic characteristics of the vegetation. This difference may be due primarily to scale; the relatively steep elevational gradients of Able & Noon (1976) show distinct habitat zonation with fairly narrow ecotones, a common characteristic of mountains (Beals, 1969), while the edaphic structure and gradual slopes of baiadas result in habitat gradation with no distinct ecotones. We thank Douglas Andersen, Robert Bayn, Peter Landres, Barry Noon, Kimberly Smith, Rick Vetter & Eric Zurcher for their help. This research was aided by a Grant-in-Aid of Research from Sigma Xi and NSF Grant DEB 78-05328.
72
s. B. VANDER WALL & J. A. MACMAHON References
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