Habitat associations of ten bird species in Washington, DC

Urban Ecology, 5 (1980/1981) 125-136 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

125

HABITAT ASSOCIATIONS OF TEN BIRD SPECIES IN WASHINGTON, DC

ROBERT D. WILLIAMSON* and RICHARD M . DeGRAAF** *U.S . Department of Agriculture, Forest Service, Southeastern Forest Experiment Station, Clemson, SC 29631 (U .S.A .) **U.S . Department of Agriculture, Forest Service, Northeastern Forest Experiment Station, Amherst, MA 01002 (U.S .A .) (Accepted 22 July 1981)

ABSTRACT Williamson, R .D. and DeGraaf, R .M ., 1981 . Habitat associations of ten bird species in Washington, DC. Urban Ecol., 5 : 125-136 . To determine the influence of habitat on songbirds in suburban, urban, and park areas, bird distribution in two neighbourhoods of Washington, DC, and in the corridor of natural vegetation which separates them, was quantified and compared by using an analysis of variance, and a correlation analysis was used to relate ten individual species with ten habitat components, Eight of the ten species showed no strong affinity for any one particular neighborhood, one species was most abundant in the suburban area, and one species was more abundant in the neighborhood where pavement, building structures, and people were most prevalent .

INTRODUCTION

For many urban and suburban residents, birds constitute the most conspicuous and abundant wildlife . The economic importance of nongame birds in the United States has been estimated at $500 million in 1974 (DeGraaf and Payne, 1975). A wildlife survey of homeowners in Waterloo, Ontario, Canada, indicated that more than 95% of the residents said they liked to see birds, particularly cardinals and robins, near their homes (Dagg, 1970) . Although there are a number of studies which show the influence of vegetation structures on bird populations (MacArthur and MacArthur, 1961 ; Balda, 1969 ; Thomas et al ., 1975), few studies have described the influence of habitat on songbirds in urban areas (Williamson, 1974 ; DeGraaf,1975) . The forester, wildlife manager, urban planner, and landscape architect need additional knowledge of habitat components necessary to urban bird populations in order to create and maintain suitable habitats and also enhance the urban landscape by adding the color, movement, and sound of birds (Thomas et al ., 1977) . Generally, expansive urbanization projects destroy natural areas to accomodate more people and buildings . We suspect that a high degree of urbanization, 0304-4009/81/0000-0000/$02 .50 © 1981 Elsevier Scientific Publishing Company

1 26

which often removes vegetation, also causes the impoverishment of birds in urban areas . Previous bird studies in suburbs have shown both positive and negative changes in bird populations following the transformation of natural environments (Woolfenden and Rohwer, 1969 ; Emlen, 1974 ; Geis, 1974) . Where areas are altered to a minor extent or are improved through careful design, an increase in bird species is a response to increased attractiveness of habitats (DeGraaf et al ., 1975 ; Thomas et al ., 1977) . However, some bird species readily exploit environments strongly modified by human activity (Nuorteva, 1971), even where the bulk of the original flora and fauna is removed . The purpose of our study was to identify habitat components that support bird life in environments influenced by humans to different degrees and to quantify and compare the exploitation of urban areas by the different bird species. STUDY AREAS

Park habitat

(Fig. 1)

Rock Creek Park (RCP) is a natural vegetation corridor between the suburban residential area and the crowded urban area of Washington, DC . This

Fig . 1 . Rock Creek Park is a natural vegetation corridor between the suburban and urban habitats where more than 70 species of trees occur .

1 27 687-ha park extends from the banks of the Potomac River into Maryland . Its rocj-strewn stream has formed a flood plain where oak (Quercus spp .), hickory (Carya spp.), tulip poplar (Populus spp .), and beech (Fagus gradifolia) predominate in a forest of more than 70 other species . Parts of this mixed hardwood forest have a dense understory . Suburban habitat (Fig. 2) Georgetown and other affluent neighborhoods are west of RCP in a sylvan setting. Characteristic of this community are privately owned older houses on quiet, shaded streets with well-landscaped lots of 0 .25-0 .50 ha. The houses and buildings are surrounded by sprawling green lawns, shrubs, and a variety of trees.

Fig . 2 . Characteristic of the suburban habitat are privately owned houses on quiet,, shaded streets with well-landscaped lots .

Urban habitat (Fig . 3) East of the park are low-income urban neighborhoods . Most housing is renter-occupied ; nearly 90% of the buildings contain more than one dwelling unit per structure with a density of 40 units/ha . These buildings are mostly two- and three-story row houses with an occasional large apartment complex . Scattered vegetation is generally limited to oak and maple trees along streets and hedgerows surrounding very small lawns .

1 28

Fig. 3 . East of Rock Creek Park are mostly two- and three-story row houses with an occasional towering apartment building . Scattered vegetation is generally in the form of maple trees along streets and hedgerows surrounding very small lawns .

METHODS

We recorded bird occurrence in 36 plots along four transects extending through the park into each neighborhood (Fig . 4) . Nine 0 .20-ha plots were located on each transect 0 .48 km apart between plot centers ; transects were 0 .64 km apart . We made six 5-min counts of birds on each plot between 25 May and 30 June 1972, starting at 05 .30 and ending at 09 .00 h on clear, calm days . Indices of species use of plots were produced from the total number of birds seen per plot (Table I) and birds seen per plot per visit (Table II) . Results for the six counts were averaged . Within our study area, ten bird species were selected because of their common occurrence in urban areas and because they were easily identified by sight and song . No data were collected on other birds present . To determine how songbirds utilize urban areas and to identify habitat variables correlated with species distribution, we attempted to account for the variation in the occurrence of species with their use of habitat features . The ten habitat variables were the number, diameter, and basal area of deciduous and coniferous trees, volume of deciduous and coniferous shrubs, lawn area, and pavement area . Tree diameter at breast height (1 .3 m, d .b .h .) was measured with a tape to the nearest centimeter, and basal area was determined



129 4

SUBURBAN 3 2

RCP

URBAN 2 3

4

Fig . 4 . Location of the study plots in Rock Creek Park and its adjacent neighborhoods, Washington, DC .

from conversion tables (Bruce and Schumacher, 1950) . Shrub volume was calculated by viewing each shrub as a rectangular prism and measuring the length, width, and height with a range pole . Lawn and pavement areas were recorded as percentage of plot covered . An analysis of variance (ANOV) and Duncan's Multiple Range Test were used to determine differences among the various habitat features and bird occurrence for plot groups at increasing intervals from RCP and for all plots east, west, and within RCP . A simple correlation analysis was used to relate habitat components to species occurrence . RESULTS AND DISCUSSION

Bird distribution

Two levels of comparison are used to show bird distribution within the study area . Urban, suburban, and RCP plots are compared to show the affinity of each bird species for a specific habitat (Table I) . Urban and suburban plot groups by distance from RCP are compared to show the impact of RCP on the distribution of each bird species (Table II) .

1 30 TABLE I Mean number of birds seen per plot in Rock Creek Park, Washington DC, and adjacent developed neighborhoods Bird species

RCP'

East' (urban)

I .13(1 .59)a2

4 .00(4 .69)a

52 .68(122 .00)a

2 .25(1 .73)a

1 .00(1 .16)ab

0 .32(0 .60)b

Mockingbird (Mimus polyglottos)

5 .06(4 .04)a

5 .00(5 .35)ab

1 .13(1 .96)b

Gray catbird (Dunetalla carolinensis)

2 .75(3 .02)a

4 .25(4 .57)a

1 .56(4 .98)a

American robin (Turd us m igrato ri us)

5 .63(3 .05)a

5 .00(2 .94)a

1 .38(2 .13)b

Wood thrush (Hylocichla mustelina)

0.19(0 .40)a

0 .50(1 .00)a

11 .31(10 .59)a

4 .50(5 .80)a

13 .38(10 .02)a

20 .38(12 .59)h

9 .50(8 .23)6

50 .75(27 .50)a

4 .56(2 .85)a

4 .26(5 .32)a

0 .63(1 .41)b

1 .44(1 .75)a

0 .25(0 .50)6

0 .38(1 .03)b

Rock dove

West' (suburban)

(Co/urn ha livia) Blue jay

(Cyanocitta cristala)

Starling

(Sturnus aulgaris) House sparrow

(Passer domeslicus) Cardinal

(Cardinalis cardinalis) Song sparrow (Helospiza melodia)

' 16 plots west and east, 4 plots in RCP ; each plot, 0 .20 ha . 'Means in a row followed by a different letter are significantly different (P < 0 .05) . Numbers in parentheses are standard deviations around the mean .

Neighborhood influence . From Table I it does not appear that most species showed strong affinities for any one particular neighborhood . The song sparrow was most abundant in the suburban neighborhood and the house sparrow was most abundant in the urban neighborhood . For all remaining species there were no statistically significant differences between the numbers in two or three neighborhoods . The American robin, the wood thrush, and the cardinal were more abundant in the suburbs and park than in the urban neighborhood, suggesting an affinity for less developed areas . We suspect a similar affinity for the blue jay and mockingbird, but the high variability of our samples makes statistical comparisons more difficult . For the remaining three species, the rock dove, the catbird, and the starling, we could detect no statistically significant difference among the numbers seen in all three neighborhoods . Two of these species, the rock dove and the starling might be considered cosmopolitan .



13 1 TABLE II Mean number of birds seen per visit to 020-ha plots in Rock Creek Park, Washington, DC, and adjacent neighborhoods, May-June 1972 . Columns represent groups of four plots at various distances from RCP Bird species

Suburban plot groups west of Rock Creek Park 4 1 .92 km

3 1 .44 km

Rock dove

0 .21(0 .32)a'

0 .13(0 .16)a

0 .17(0 .33)a

0 .25(0 .32)a

Blue jay Mockingbird Gray catbird American robin Wood thrush Starling House sparrow Cardinal Song sparrow

0 . 17(024)abed 0 .79(0 .32)ab 0 .13(0 .25)a 1 .08(0 .48)a 0 .04(0 .08)a 3 .08(3 .08)a 3 .50(1 .82)bc 0 .83(0 .45)a 0 .33(0 .36)a

0 .50(0 .41)a 0 .67(0 .45)ab 0 .42(017)a 0 .71(0 .48)abc 0 .04(0 .08)a 1 .71(0 .46)a 2 .38(1 .07)c 0 .63(0 .29)ab 0 .17(0 .33)a

0 .46(0 .25)ab 0 .79(0 .96)ab 0 .83(0.88)a 1 .08(0 .59)a 0 .04(0.08)a 1 .25(1 .47)a 2 .29(1 .07)c 0 .85(0.70)a 0 .33(0 .27)a

0 .38(0.21)abc 1 .13(0 .95)a 0 .46(0.25)a 0 .88(0 .60)ab 0 .00(0 .00)a 1 .50(1 .01)a 5 .42(2 .11)bc 0 .71(0.55)ab 0 .13(0 .25)a

Plots in Rock Creek Park

2 0.96 km

Urban plot groups east of Rock Creek Park U 1 2 3 0 .48 km 1 .44 km 096 km 21 .33(40 .67)a 0 .13(016)bcd 0 .21(0 .16)ab 0 .96(1 .60)a 0 .54(0 .37)abc 000(0 .00)a 2 .21(1 .40)a 7 .21(3 .26)abc 0 .29(0 .37)ab 0 .13(0 .25)a

3 .67(1 .89)a 0 .04(0 .08)cd 0 .29(0 .58)ab 0 .04(0 .08)a 0 .00(0 .00)c 0 .00(0 .00)a 1 .96(1 .27)a 8 .33(3 .85)ab 0 .00(0 .00)b 0 .13(0 .25)a

2 .29(1 .84)a 0 .04(0 .08)cd 0 .25(0 .32)ab 0 .04(0 .08)a 0 .29(0 .48)bc 0 .00(0 .00)a 2 .46(2 .32)a 7 .25(2 .95)abc 0 .13(0 .25)ab 0 .00(0 .00)a

1 0 .48 km

4 1 .92 km 7 .83(10 .17)a 0 .00(0 .00)d 0 .00(0 .00)b 0 .00(0 .00)a 0 .08(0 .17)c 000(0 .00)a 2 .29(2 .19)a 11 .04(7 .61)a 0 .00(0 .00)b 0 .00(0 .00)a

'Means in a row followed by a different letter are significantly different (P < 0.05) . Numbers in parenthesis are standard deviations around the mean .

Park influence. Examining the distribution of species abundance at increasing

RCP (Table II), it appears that RCP had little or no influence on bird populations in the more developed areas east or west of the park . For no species were we able to detect a significantly higher number seen in strata near or adjacent to the park relative to more distant strata . distances from

Habitat relationships Possible insight into why some of the birds were more abundant in one or more of the neighborhoods may be gained by examining each species' habitat

13 2

relationships (Table III) and the distribution of those habitat features among neighborhoods (Table IV). We were unable to correlate rock dove occurrence with any of the selected features . Like the rock dove, starling and catbird distribution did not change significantly among neighborhoods (Table I) . The only variable that correlated with starling distribution was the percent of unpaved area (Table III) . Therefore, very little can be inferred from our results on starling occurrence in a particular neighborhood . All but one of the habitat features correlated with the gray catbird - deciduous shrub volume, which appeared to be more closely associated with the habitat of RCP or the suburbs . Deciduous shrubs may be the key feature in control of catbird abundance and distribution because the volume of deciduous shrubs did not change significantly between neighborhoods (Table IV) . Mockingbird distribution among neighborhoods was similar to that of the blue jay, and we suspected that both species had a strong affinity for RCP and the suburbs (Table I) . Whereas the number of the two species was positive ly correlated with the percent of unpaved area, the number of mockingbirds did not correlate well with any other variable . The occurrence of blue jays, however, was positively correlated with three variables, and the highest correlation was with the basal area of deciduous trees (Table III) . From Table IV it can be seen that the quantity of unpaved area and deciduous basal area important habitat features for both the mockingbird and the blue jay - was greater in RCP and the suburbs . Three species, the American robin, wood thrush, and cardinal, did show a stronger affinity for RCP and the suburbs (Table I), Occurrence of each of these species significantly correlated with several habitat features ; among the highest were the percent of unpaved area for robins, the number of deciduous trees for cardinals, and building coverage for wood thrushes (Table III) . Occurrence of all three of these species was negatively correlated with building coverage which was significantly higher in the urban neighborhood . Because the percentage of unpaved area and the amount of vegetation were greater within RCP and the suburbs, we would not expect to find large populations of robins, cardinals, or wood thrushes east of RCP . Although the song sparrow showed a strong affinity for the suburbs (Table I) . none of the habitat components that correlated with number of song sparrows appeared to be in greater quantity in the suburbs than in one of the other neighborhoods . Two of four significant correlations (Table III) were best associated with RCP (Table IV) . Therefore, very little can be inferred about song sparrow distribution or abundance from our habitat relationships . The house sparrow showed a strong affinity for the urban neighborhood (Table I) . Among the variables that best correlated with house sparrow occurrence were percent building coverage, percent of unpaved area, and deciduous basal area (Table III) . Rather than being attracted to neighborhoods with high quantities of natural vegetation, the house sparrow appeared to be more attracted by the urban neighborhood where vegetation was less and the influence of humans was high (Table IV) .

0 .05 ; * * P

< o .oi .

0 .52* -0 .52**

*P <

0.51**

0 .50**

0 .43**

0 .48**

Gray catbird

Unpaved area (91) Building coverage 0 .36*

Mockingbird

Shrub vol. (m 3 ) Conifers Deciduous

0 .38* -0 .33*

0 .33*

No . of trees Conifers Broad-leavedhardwoods

Mean d .b .h . (cm) Conifers Broad-leaved hardwoods

0 .46**

Basal area (m 2 ) Conifers Broad-leaved hardwoods

Blue lay

0 .68** -0 .60**

0.34*

0 .55**

0 .44**

American robin

0 .46** -0 .63**

0 .48**

Wood thrush

-0 .39*

Starling

-0 .59** 0 .62**

-0 .34*

-0 . 49**

-0 34* -0.51"*

House sparrow

Correlation coefficient ( ) between the occurrence of nine b ird species and habitat variables in Washington, DC

TABLE III

0 .65** -0 .64**

0 .42**

0 .36* 0.72--

0 .52**

Cardinal

0 .45*" -0.37*

0 .34*

0 .47**

Son sparrow

134 TABLE IV Mean value of habitat variables recorded per 0 .20-ha plot in Rock Creek Park, Washington, DC, and adjacent developed areas Habitat variables

Basal area (m2) Conifers Broad-leaved hardwoods No . of trees Conifers Broad-leaved hardwoods

West' (suburban)

RCP'

East' (urban)

0 .08(0 .12)b'

0 .63(0 .36)a

0 .02(0 .06)b

2 .34(1 .43)a

2 .89(1 .92)a

1 .07(1 .02)6

5 .94(7,29)6

13 .50(3 .32)a

1 .38(2 .19)c

25 .13(15 .54)b

48 .50(37 .22)a

9 .88(8 .23)c

Mean d .b .h . (cm) Conifers Broad-leaved hardwoods

7 .65(6 .83)b

20 .65(8 .69)a

4 .85(7 .06)b

29 .19(13 .11)a

21 .72(5 .26)a

32 .26(14 .20)a

Shrub volume (m') Conifers Deciduous

86 .49(84 .48)a 51 .12(31 .33)a

10 .56(12 .65)a 95 .35(143 .76)a

44 .42(57 .15)a 63 .96(105 .40)a

Unpaved area(%)

51 .56(28 .51)a

78 .00(28 .33)a

27 .75(21 .42)b

Building coverage (%)

27 .00(16 .49)b

15 .00(17 .32)b

53 .44(15 .84)a

' 16 plots west and east, 4 plots in RCP . Means in a row followed by a different letter are significantly different (P < 0 .05) . Numbers in parentheses are standard deviations around the mean . 2

Some of these results are in general agreement with habitat associations described in the literature . The tolerance of some species for human habitation has been described by Geis (1974) . Robins are probably best adapated to suburbs (Howell, 1942), where they nest in hardwoods . Our data also show that occurrence of robins is best correlated with lawn cover around openly spaced buildings . Song sparrows and gray catbirds nest in shrubs; the catbird is also associated with a deciduous tree overstory of 15-40 feet (4 .6-12 .2 m) (Nickell, 1965 ; DeGraaf et al ., 1975) . The correlation between the variables in this study and the occurrence of mockingbirds and strlings gives little indication of the preferred habitat of these species . The occurrence of cardinals was best correlated with deciduous trees around openly spaced private homes with large areas of lawn . House sparrows are more associated with structures than with vegetation (Weber, 1975) . The blue jay seems to prefer habitats with mixed hardwoods and conifers, as does the wood thrush ; however, the wood thrush is more restricted to woodlots in suburbs . Both species show a negative relationship to building density (Lucid, 1974 ; DeGraaf, 1975)

135

Management implications

Fortunately, many of our cities have large parks or scattered tracts of forest as well as trees around houses that provide habitat for wildlife - especially birds . Unfortunately, many city areas have only small urban parks and playgrounds where one finds few of the habitat features necessary to many songbirds . The problem of ever-decreasing wildlife will become greater with increasing urban expansion that lacks planned areas of habitat . However, even urban areas with drawbacks (like the east side of RCP) have positive elements such as some tree-lined streets, vacant lots, and schoolyards that can be improved through careful design . Such areas, properly managed, would increase suitable habitat for wildlife and, at the same time, provide enjoyment for urban residents . REFERENCES Balda, R .P ., 1969 . Foliage use by birds of the oak-juniper woodland and ponderosa pine forest in southeastern Arizona . Condor, 71 : 399-412 . Bruce, D. and Schumacher, F.X ., 1950 . Forest Mensuration . McGraw Hill, New York, NY, 483 pp . Dagg, A .I ., 1970 . Wildlife in an urban area- Can . Nat ., 97 : 201-212 . DeGraaf, R .M ., 1975 . Suburban habitat associations of birds . Ph .D . Dissertation, Univ . Massachusetts, Amherst, MA, 295 pp . DeGraaf, R .M . and Payne, BR ., 1975 . Economic values of non-game birds and some urban wildlife research needs . Trans . 40th North Am . Wild] . and Nat . Resour . Conf., pp . 281287 . DeGraaf, R.M ., Thomas, J .W . and Pywell, H .R ., 1975 . Relationships between songbird nest height, vegetation and housing density in New England suburbs . Trans . 32nd Northeast . Fish and Wildl . Conf., pp . 130-150 . Emlen, J.T ., 1974 . An urban bird community in Tucson, Arizona : derivation, structure, regulation . Condor, 76 : 184-197 . Geis, A .D ., 1974 . Effects of urbanization and types of urban development on bird populations . In : J .H . Noyes and DR . Progulske (Editors), Wildlife in an Urbanizing Environment . Plan Resour . Dev . Ser . 28, Holdsworth Nat . Resour. Cent ., Univ . Massachusetts, Amherst, MA, pp . 97-105 . Howell, J .C ., 1942 . Notes on the nesting habits of the American robin . Am . Midl . Nat ., 28 : 529-603 . Lucid, V.J,, 1974 . Bird utilization of habitat in residential areas . Ph .D . Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA, 164 pp . MacArthur, R .H . and McArthur, J .W ., 1961 . On bird species diversity . Ecology, 42 : 594598 . Nickell, W .P,, 1965 . Habitats, territory, and nesting os the catbird . Am . Midl . Nat ., 73 : 433-478 . Nuorteva, P ., 1971 . The synanthropy of birds as an expression of the ecological cycle disorder caused by urbanization . Ann . Zoo] . Fenn ., 8 : 547-553, Thomas, J .W ., Crouch, G ., Bumstead, R . and Bryant, L ., 1975 . Silvicultural options and habitat value in coniferous forests . In : Proc . Symposium on Management of Forest and Range Habitats for Nongame Birds, May 5-9 1975, Tucson, AZ, pp . 272-287 .

1 36 Thomas, J .W ., DeGraaf, R .M . and Mawson, J .C ., 1977 . Determination of habitat requirements for birds in suburban areas . U .S . Dep . Agric . For . Serv . Res . Pap . NE-357, 15 pp . Weber, W .C ., 1975 . Nest-sites of birds in residential areas of Vancouver, British Columbia . Can . Field Nat ., 89 : 457-460 . Williamson, R -D-, 1974 . Birds in Washington, DC In : J .H . Noyes and D .R . Progulske (Editors), Wildlife in an Urbanizing Environment . Plan . Resour . Dev . Ser . 28, Holdsworth Nat . Resour . Cent ., Univ . Massachusetts, Amherst, MA, pp . 131-135 . Woolfenden, G .E . and Rohwer, S .A ., 1969 . Breeding birds in a Florida suburb . Fla . State Mus . Bull ., 13(1) : 1-83 .