Directionality of singing and non-singing behaviour of mated and unmated northern mockingbirds, Mimus polyglottos

Anim. Behav., 1987, 35, 331 339

Directionality ol singing and non-singing behaviour of mated and unmated northern mockingbirds, Mimus polyglottos R A N D A L L B R E I T W I S C H * & G E O R G E H. W H I T E S I D E S

Department of Biology, Unh~ersity of Miami, Coral Gables, Florida 33124, U.S.A.

Abstract. Early in the breeding season, unmated male northern mockingbirds displayed greater variability in singing direction within bouts of song than did mated males. Unmated males also sang more and used a greater proportion of singing perches in the interior of their territories than mated males. Although unmated males used more singing perches than mated males, they used fewer in proportion to the amount of song. Both unmated and mated males frequently sang into or across their territories, although mated males sang into their territories to a greater degree. Neither class of males directed song exclusively out of their territories when singing near boundaries. Unmated males also performed more visual displays associated with singing. Mated males chased both conspecifics and heterospecifics more than did unmated males. These findings support the hypothesis that mockingbird song at this time of the breeding season functions largely in male-female interactions.

Bird song has probably evolved by sexual selection. Little is known, however, about the relative contribution of Darwin's (1871) two processes of sexual selection to the evolution of bird song (Payne 1983). Does intrasexual (male competition for mates) or intersexual (mate choice by females) selection always predominate, or does their relative importance shift widely among species? If intersexual selection is of greater importance than intrasexual selection, then the singing behaviour of mated and unmated males within a population of passerines in the breeding season should be particularly revealing. Such comparisons have demonstrated clearly in some species that unmated males sing more than mated males (e.g. Catchpole 1973; Krebs et al. 1981; Greig-Smith 1982; Logan 1983; Cuthill & Hindmarsh 1985; Merritt 1985; Temrin 1986), in agreement with a prediction of the intersexual hypothesis. The northern mockingbird is well-known for its virtuosity and mimicry in song and its extensive repertoires (Merritt 1985 and references therein). Recent work by Logan (1983) and Merritt (1985) supports the intersexual hypothesis for song function in mockingbirds, and Merritt also favours this hypothesis for the evolution of song repertoires (contra Howard 1974). We studied the singing behaviour of territorial mated and unmated male * Present address: Department of Biology, Indiana University of Pennsylvania, Indiana, PA 15705, U.S.A.

mockingbirds early in the breeding season, documented the directionality of singing within bouts, and measured several additional aspects of behaviour. We asked four questions: do mated and unmated males (1) display similar variability in directionality of singing within bouts; (2) direct song out of their territories when singing near boundaries; (3) sing from perches similarly distributed relative to the centre and boundaries of their territories; and (4) sing from similar numbers of perches? If song in mockingbirds at this time of year functions primarily as a keep-out signal, we expect no differences in the distributions of these variables for mated and unmated males. If mockingbird song functions primarily in intersexual communication, then we expect differences in aspects of singing behaviour.

METHODS

We observed the singing behaviour of 24 individually colour-banded and two unbanded male mockingbirds on the main campus of the University of Miami, Coral Gables, Dade County, Florida. The habitat is sparsely wooded suburban grassland. All data were collected between 1 April and 1 May 1985 (i.e. early breeding season). We measured behaviour in 80 30-min sampling periods (40 for each class of male), all beginning between 0700 and 1000 hours. In 38 periods, we sampled 18

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mated males, and in 38 periods we sampled seven unmated males. In four periods, we sampled one male that was first unmated ( N = 2 ) , then mated ( N = 2 ) . We noted the focal bird's singing behaviour at 10-s intervals during the 30-rain sampling period. A 10-s interval was the shortest interval logistically possible, but was short enough to reflect the tendency of birds to change singing direction frequently within bouts. For each entry, we mapped the singing perch and noted the direction of singing. The singing direction was recorded as the octant (Fig. 1) in which the bird's bill pointed (frequently different from the long axis of the torso). This is the best estimate of the direction of maximum amplitude of song. Mockingbirds in this population are habituated to humans. Frequently, we were nearly directly beneath the perch of a singing bird, and this proximity gave us confidence in our accuracy in recording the direction of its bill. Singing perches were mapped within territories with reference to a 1 : 1600 scale aerial photograph of the study area. We also noted non-singing behaviour of focal males in consecutive 10-s intervals. Each aspect of behaviour recorded was poten-

3

,

5

6 7

Figure 1. Placement of the bill of a singing male mockingbird in an imaginary circle dividing 360 into octants, one of which is indicated by dashed lines. Long vector indicates direction of singing. Arrows to sides of the head indicate lateral rotation of the head within a singing bout.

tially short in duration, and thus more than one event could occur within a 10-s interval. We simply recorded presence or absence of each behaviour pattern in each interval. These behaviour patterns included chasing of conspecifics and heterospecitics, a stereotyped flight display associated with singing bouts (Merritt 1985), giving one or more harsh chat or chatburst calls (Logan et al. 1983), giving the rasping hew call (Logan & Fulk 1984), preening and foraging on the ground or in fruiting trees. We analysed singing directionality via circular statistical methods (Zar 1984). A song vector was constructed for each singing bout denoting mean direction and strength of directionality (or dispersion around the mean direction). Song vectors were between 0'0 and 1 "0, inclusive, with lengths directly proportional to the strength of directionality. For instance, a vector of unit length denoted constant direction of singing, and a vector of zero length denoted no overall directionality within a singing bout. We calculated the geometric centroid of each territory from territorial maps. We then calculated the angle (between 0 ~ and 180 ~ inclusive) between the song vector and the line connecting the singing perch with the centroid (Fig. 2). A male that sang directly toward the centroid of its territory had a 0 ~ angle, A male that sang directly away from the centroid had a 180 '~ angle (Fig. 2). We compared the distribution of angles for mated and unmated males. We defined a singing bout as continuous song from a single perch. Pauses in song within a bout were allowed if the duration of a pause did not extend beyond a single 10-s interval. Thus, a single song bout several minutes long might be punctuated by several pauses, but with no one pause extended beyond a single interval. We analysed all singing bouts greater than or equal to 1 rain long. This minimum length included six entries for singing direction; we decided a priori that fewer than six entries yielded a song vector with little meaning. If a bird flew from a singing perch and returned to sing from that perch later in the same sampling period, we scored the return as a new singing bout. We calculated relative distance between a singing perch and the territory's centroid by first calculating the radius of a circle with the same area as that of the irregularly shaped territory. We then compared the distance between the perch and the centroid to the length of the radius, yielding a ratio

Breitwisch & Whitesides: Singing directionality

333

/ \

r

/

j

\ \ CENTROID

J

/ ,

b

\ \ \

/

"

d \ \ \

~0o\\\

p /

Figure 2. Representative territory (male 334) showing boundary, centroid, singing perches (Pn), direction and length of song vectors at perches (vn), angles of song vectors relative to the centroid (0,), distances between singing perches and centroid (dn), and radius (r) of a circle containing the same area as the territory. Each song vector represents a single continuous singing bout. Distance ratio used in calculation of perch location is Rn = dn/r. Four singing perches are shown, one of which has two song vectors calculated from two singing bouts.

between 0.0 (a perch at the centroid) and 1"4 (the most distant perches from the centroid of some territories; Fig. 2). For a perch close to the centroid, the angle formed by centroid, perch and song vector could change markedly if the perch moved even slightly with respect to the centroid. By employing the ratio of distances, we could eliminate perches so close to the centroid that the angles formed were without analytical value. In comparing both song vector lengths and bout lengths between mated and unmated males, we treated each sampling period (N = 40 for each class of males) and each singing bout as independent from the others, even though we sampled most birds more than once. We justified this procedure with the results of Kruskal-Wallis one-way A N O VAs for vector lengths and bout lengths within each class of males. There was no significant variation in song vector lengths or bout lengths between males in either class. We used the same procedure to test for differences between males within classes in the amount of song and number of singing perches used. In no case was there a

difference. Similarly, we tested the frequency of each aspect of non-singing behaviour for differences between males within each class. There were no differences except for flight display by unmated males (H=16.39, d r = 7 , P<0.05). Therefore, except for flight displays, we used the number of sampling periods (Nm = Nu = 40) in comparing the frequency of each aspect of behaviour between mated and unmated males. For flight displays, we compared frequency using number of males (Nm = 19, Nu= 8), not number of sampling periods. We used M a n n - W h i t n e y U-tests to test for differences in distributions of each variable between mated and unmated males (i.e. whether the populations of values for the two classes of males were stochastically equal). For these analyses, we report the z-statistic (results of M a n n Whitney U-test corrected for ties). For analyses of correlation, we used the Kendall rank correlation coefficient (tau). For the analyses of non-singing behaviour for the two classes of males, we compared (via Mann-Whitney U-tests) the distributions of numbers of intervals with the occurrence of

Animal Behaviour, 35, 2

334

the particular behaviour pattern and report the zstatistic.

RESULTS

Directionality and Amount of Singing When we compared song vector lengths, unmated males displayed a greater variability in direction of singing within bouts than did mated males (z= 1.93, P = 0 . 0 5 , Nm =44, N~= 141 bouts). Mean vector length for unmated males was 0.559 (SD=0.211, No=141), for mated males 0.623 (SD=0.219, Nm=44). This difference was robust with respect to definition of minimal song bout length. Mean vector length for the two classes of males was of similar magnitude whether minimal length of song bout was, for example, as short as 1 rain or as long as 3 rain (Table I). Song vector length was inversely related to the length of a song bout for unmated males ( t a u = - 0 . 1 1 8 , P<0.05, N~ = 141) but not for mated males (tau = - 0 . 0 9 9 , P > 0 . 0 5 , Nm=44). This negative correlation for unmated males was largely a function of very long song bouts. Approximately 10% of all song bouts were greater than or equal to 10 min, and we

Table I. Song vector lengths for mated and unmated male mockingbirds

Vector length )? (sD)

Comparison of vector lengths

Song bout length

Mated males

Unmated males

z*

> 1.0min (N= 185) _> 1.5min (N = 147) _> 2.0min (N= 119) > 2.5min (N= 106) > 3.0min (N = 86) < 10.0min (N= 167)

0'623 (0"2190) (N=44) 0.617(0.1973) (N = 34) 0.626(0.2062) (X= 23) 0-612 (0.2088) (N= 20) 0.623 (0.1997) (N = 17) 0.637 (0-2142) (N=40)

0.559 (0.2109) (U= 141) 0.552(0.2074) (N = 113) 0.544(0.2109) (N=96) 0.538 (0.1996) (N= 86) 0-528 (0.2016) (N= 69) 0.568 (0-2128) (N= 127)

1.93 0"05

P

1.70 0.09 1.83 0.07 1-70 0.09 1.92 0.06 1.90 0-06

*z is the test statistic for the Mann Whitney U-test (corrected for ties).

arbitrarily termed these long. When long bouts ( N = 14; 9.9% of 141) were eliminated, the strength of the negative correlation was reduced ( t a u = - 0 . 0 8 4 , P = 0 . 0 8 , Nu= 127). The elimination of long bouts from the analysis did not alter the degree of difference between classes of males ( z = 1.90, P = 0 . 0 6 , Nm=40, No= 127; Table I). Unmated males sang more than mated males (z=5.26, P<0"05, N m = N u = 4 0 ) . There was no difference in the length of singing bouts between mated and unmated males ( z = l . 1 2 , P>0-05, Nm=44, N , = 1 4 1 ) ; unmated males simply sang more bouts than mated males ( z = 6.27, P<0.05, Nm = Nu : 40).

Broadcasting of Song and Number and Location of Singing Perches Neither class of males broadcast song exclusively out of their territories. Both mated and unmated males, even when perched near their territorial boundaries, frequently sang back into their territories. Mated males directed song closer toward the centroid of their territories than did unmated males (z=2.74, P<0.05, N ~ = 4 2 , Nu=141), but mean angles for both classes of males were less than 90 ~ i.e. into the territory. The mean angle for unmated males was 81 ~ (SD=45.3 ~ Nu=141); the mean angle for mated males was 60 ~ (SD=45.0 ~ N~=42). This occurred regardless of the location of singing perches with their territories. Successive elimination from the analysis of perches moving from the centroid to the territorial boundaries changed neither the mean angles for the two classes of males nor the difference between mean angles of mated versus unmated males (Table II). For males that sang in a sampling period, unmated males used a greater number of singing perches than mated males (z=3.27, P < 0 . 0 5 , Nm=34, Nu=40). However, mated males had higher ratios of perches used to amount of song as measured by the number of singing perches/ number of 10-s intervals with song in a sample (z =4.36, P < 0.05, Nm = 34, Nu =40). Similarly, an analysis of the ratios of the number of singing perches used to the number of singing bouts within a sample showed that mated males used more perches per bout (z=2.72, P < 0 ' 0 5 , Nm=26, Nu = 39). Unmated males used a greater proportion of singing perches in the interior of their territories than did mated males (Fig. 3). This was true in an

Breitwisch & Whitesides." Singing directionality Table II. Mean angles of singing relative to territory

335 MATED~UNMATED MALES MALES

0.35

centroid for mated and unmated male mockingbirds CO hi

Location* Alldata R>0'20 R>0"40 R > 0.60 R>0.80 R > 1.00

Mated

Unmated

zt

P

:z:

o

60. F'(45.0):~ 81-0-(45-3) 2-74 <0-05 (N--42) (N= 14l)

a,

59.6 (46-4:)

~

(N-38) 58"4 (46-3 :) ( N - 35) 56.9 (45.5) (N-31) 60.1 :(47.8) (N-22) 70.1:' (55.4-) (N=13)

81.3 (45.2)

(N= 126) 83'5 (44-6) (N = 89) 85.6 (45-6) (N-57) 91,1 (45-3:) ( N - 30) 99.7 (36.0:) ( N - 10)

2.69

2'88

<0"05 <0'05

0.30

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0.25

s Z Z 03

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O.15

7"

2.81

< 0.05

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2.41

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0.05

1~56

NS

* R equals the ratio of (the distance to the song perch from the centroid) to (the radius of a circle with the same area as the territory). t Mann-Whitney U-tests; z statistic corrected for ties. ++sD in parentheses.

2

0.20

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0.00

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Figure 3. Distribution of singing perches relative to

unweighted comparison in which each singing perch was counted only once (z=4.21, P < 0 . 0 5 , Nm = 39, Nu = 69) and in a weighted comparison in which a perch was counted each time it was used (z=4.18, P<0.05, Nm=42, Nu=141). For unmated males, 52% of the singing perches were located at a distance less than half of the calculated territorial radius from the centroid (versus 21% for mated males). These differences were not related to territory size; the mean size of those territories measured was not different for mated and unmated males (z=0.51, P>0-05, Nm= 12, No=7).

Non-singing Behaviour of Males There were differences in the frequencies of several aspects of non-singing behaviour (Table III). Unmated males performed many more flight displays than mated males. Mated males chased both conspeciflcs and heterospecifics more than did unmated males and also preened more. There was no difference between classes of males in the amount of foraging. Although unmated males sang more, mated males gave more hew calls. Mated and unmated males gave similar numbers of chat and chatburst calls.

terrilory centroid for unmated and mated male mockingbirds. X indicates that unmated males used no singing perches in the 1.2-1.4 range of distance ratios.

Change in Mating Status One male was unmated during the first part of the study, then became mated. The mean song vector length for this male when unmated was 0.628 ( N = 4 bouts), and when mated was 0.721 ( N = 3 ) . This difference, although in the same direction as the larger sample, was not significant (Mann Whitney U-test for small samples, U = 3 , P=0-20). This male decreased its amount of singing upon becoming mated, from 235 of 360 10-s intervals (65.3%) to 73 of 360 intervals (20.3%) (Gtest, G= 155.1, df= 1, P<0.05).

DISCUSSION

Variability in Singing Direction and Female Attraction The greater variability in the direction of singing by unmated male mockingbirds is in agreement with the intersexual hypothesis for the primary function of song. If song were primarily a form of male-male communication (i.e. a keep-out signal),

Animal Behaviour, 35, 2

336

Table I!!. Frequencies of several behaviour patterns for mated and unmated male mockingbirds* Frequencies of behaviour patterns )? (SD) Behaviour Conspecific chase Heterospecific chase Forage Flight display:~ Hew call Chat and chatburst Preen

Mated

Unmated

zt

P

1-42 (1'947) 0-68 (1.071) 3.55 (5.602) 1.03 (2.232) 1.88 (4"831) 0-52(2.253) 9.05 (8.494)

0.30(0.823) 0.30(0.687) 2.70(4.014) 10.24(15.112) 0-18(0-712) 0.22(0-660) 4.42(6.042)

4.00 1.94 0.97 3-41 3-51 0.66 2.88

<0"05 =0.05 NS <0.05 <0.05 NS <0"05

* Frequencies of occurrence per 30-min sampling period; Nm= Nu= 40. t Mann- Whitney U-tests;z statistic corrected for ties. :~There was significantheterogeneity among unmated birds, and this comparison is therefore between the two classes of males, using mean values for each male (Nm = 19, Nu= 8); the standard deviation is of means (= sz).

there would be no reason to expect unmated birds to display a greater variability in the direction of singing within bouts than mated birds. In contrast, if unmated birds attract females via song, then we would predict that they should display a greater variability in direction as they send the signal to any females within vocal range, but at unknown locations. This is what they appeared to do. However, the singing behaviour of unmated males is certainly more complex than simply broadcasting song with equal probability pandirectionally within bouts. We occasionally found unmated males singing persistently in particular directions within bouts. This observation suggests that events on adjacent or nearby territories may stimulate unmated males to sing in the direction of those territories. For example, both Logan (1983) and Merritt (1985) demonstrated seasonal cyclicity in mockingbird singing. Mockingbirds in this southern Florida population, as in many parts of the species' range, nest at least twice in a season (Breitwisch et al. 1984), and may build as many as six nests per season (Breitwisch, unpublished data). Within the breeding season, mated male mockingbirds sing most during nest-building, decrease song production as incubation proceeds, and sing little during the nestling period. They then increase song production as they build the succeeding nest of the season (Logan 1983; Merritt 1985). Thus, unmated males could monitor the nesting stage for

neighbouring pairs and direct song at neighbouring females at the most opportune times for luring these females away from their mates. Occasionally, mated females in this population change mates within the breeding season; such changes frequently follow nesting failure (Breitwisch, unpublished data; P. G. Merritt, unpublished data). However, the proximate details of mate choice and mate switching in mockingbirds are unknown. Undoubtedly, there are other possible reasons for unmated males singing in particular directions. For example, if a neighbouring mated male sang little (e.g. at the nestling stage), then an unmated male could broadcast song over the mated male's territory to points beyond with little acoustical interference from that mated male. Nonetheless, any such directed song by unmated males is subsumed within a general pattern in which unmated males display a greater variability in singing direction than mated males. The finer resolution of this pattern remains to be investigated.

Singing Perches The extensive use by mockingbirds of singing perches deep within territories was previously documented by P. G. Merritt (unpublished data). Use of these perches may be related to perch height. Some perches were quite high (greater than 10 m),

Breitwisch & Whitesides: Singing directionality and the detection distance of song from these perches must be great. Unmated males may use elevated perches more than mated males. As part of a different study, Merritt estimated perch heights of singing males from 6 May to 17 June 1981. The mean height of the perches of singing mated males was 6-9 m and of unmated males 10.1 m (MannWhitney U-test, z = 1.81, P = 0-07, Nm= 15, Nu= 8). Unmated males also frequently sang quite loudly from elevated perches, although we lack quantitative measures of this. Broadcasting song in highly variable directions from elevated perches near the centre of the territory should be an efficient means of extending the vocal range of unmated males maximally in all directions. Our observation that many singing perches were located in territorial interiors further argues against the use of song at this time of the season as a keep-out signal to other male mockingbirds. If song had this function, we would predict singing perches to be located at the edges of territories in order to associate the signal with the location of territorial boundaries. Our results cannot be explained by a scarcity of territorial behaviour at the time of sampling. In this population, the seasonal distinction between territory establishment versus maintenance is not as clear as in the temperate zone. Territorial boundaries shift during the breeding season (Merritt 1985), and chases and fights between neighbouring males occur throughout the breeding season, not just in the early spring. P. G. Merritt (personal communication) has also noted neighbouring male mockingbirds singing from perches within their respective territories, then flying toward one another and engaging in defensive territorial boundary dances (Hailman 1960) at locations between the singing perches. Male mockingbirds may benefit from associating the song of a neighbour with occupation of a territory, but song does not appear to function at this time of year in sharply defining the boundaries of the area occupied. The practice of singing back into territories from perches near boundaries by both classes of males was not predicted. This observation raises the question of whether mated and unmated males sing for the same reason. Results here and in Merritt (1985) suggest that unmated males sing to attract females. However, mated males that sing back into their territories may be singing to their mates rather than to other females. Unfortunately, we were not able to determine the locations of the mates of

337

males while we sampled singing by these birds. The functions of broadcasting song to a mate may include bringing the female into reproductive condition anew or synchronizing the reproductive cycle of mates (Brockway 1969; Kroodsma 1976; Logan 1983). Our impression was that song by mated males was frequently rather low in amplitude and did not project far beyond territorial boundaries. This was in marked contrast to our impression of the amplitude of song of unmated males. We note a different interpretation of our results offered by S. M. Green (personal communication). Green hypothesizes that birds perched near one territorial boundary and singing toward the centroid were sending a signal across the territory. Such behaviour may maximize the advertisement by a male to females by visually placing the male on one side of its territory yet providing for auditory detection from the opposite side of the territory. The efficacy of this behaviour would depend on the rate of degradation of both kinds of signals with distance in the particular habitat. Mated males may sing for the same reason as unmated males: to attract (additional) mates. Bigamy occurs in this population (Breitwiseh et al. 1986b; P. G. Merritt, unpublished data) and elsewhere (Logan & Rulli 1981), and bigamous males usually acquire their second mates within the breeding season. However, both the limited total amount of song and the low amplitude of much of the song of mated males appear to contradict the hypothesis of attraction of additional mates by mated males. The portion of mated male song that is low amplitude may be sexual song to mates (Payne 1983), and thus may display other differences from high amplitude song.

Singing and Attacks The higher attack rate by mated males than unmated on both conspecifics and heterospecifics agrees with previous findings for this population (Merritt 1985; Breitwisch et al. 1986a) and likewise supports the functional separation of song from territorial behaviour at this time of the breeding season. Although unmated males sang far more than mated males, they did not engage in as much chasing of territorial intruders. Further, if song is used as a keep-out signal, we expect mockingbirds to sing while chasing intruders, as do indigo buntings (Passerina cyanea; Payne 1983). Mock-

338

Animal Behaviour, 35, 2

ingbirds, however, infrequently sing while chasing intruders (Logan et al. 1983; Merritt 1985).

Directionality of Vocal Signalling To our knowledge, the techniques employed here have not been used before to quantify broadcasting of vocal signals (see Witkin 1977). Although directionality in signalling is well-known for other modalities of communication (Hailman 1977; Shorey 1977), our techniques still appear to be novel, regardless of modality. It is known that vocal signallers use properties of the atmospheric medium to their advantage (Morton 1975; Wiley & Richards 1982) and position themselves for the most effective signal transmission. We expect similar use of directionality in broadcasting vocal signals to particular target individuals or places where these individuals are likely to be located. Further observations on such directionality in a variety of animals can provide information pertinent to questions concerning the evolution of this form of communication. In particular, the relative importance of the two components of sexual selection to the evolution of vocal signalling may be further elucidated by the techniques described here. Our findings support the hypothesis that mockingbird song during the early breeding season functions intersexually (see Logan 1983). Although it is possible that song functions in male competition during territory establishment, many mockingbirds in this population remain on the same territories year-round, and territory establishment or re-establishment in early spring is not as dramatic as in the temperate zone. It is not clear to what extent our findings apply to other monogamous passerines. Payne (1983) concluded that song in monogamous indigo buntings is shaped by malemale interactions with no aspect obviously related to female choice. However, Payne (1982, 1983) sampled bunting song both at territory establishment and later in the season, but analysed these data as a single set. The time of sampling is potentially critical, and song may function differently at different times of the year (D. E. Kroodsma, personal communication). Finally, Merritt (1985) cautioned against the assumption that because males respond to song by other males, male male interaction has necessarily shaped song in mockingbirds. We should expect that vocal signals destined for females will be intercepted by

males and responded to in a manner that benefits these interceptive males.

ACKNOWLEDGMENTS We thank Steve Green, Don Kroodsma, Peter Merritt, Gene Morton, Charles Snowdon and Keith Waddington for comments on a previous version of this paper and Green and Colin Pennycuick for advice on statistical analyses. We have acted on many of their suggestions. Steve Green's expertise in the evolutionary theory of communication largely provided the intellectual atmosphere conducive to formulating this study. Members of the graduate ethology class in the Department of Biology, University of Miami participated in this project, and we thank M. Diaz, W. Evoy, N. Gottlieb, N. Kline, R. Lee, M. Nelson, J. Partak, B. Rose, W. Sheldon and J. Zaias for their assistance. Peter Merritt kindly made available to us his unpublished observations on perch heights of singing males. Jill Pearson and Natasha Gottlieb helped with data handling. Natasha Kline drew the mockingbird in Fig. 1. This research was partially funded by a grant to R.B. from the Tropical Audubon Society, Miami. Contribution No. 212 from the Program in Behaviour, Ecology and Evolution, Department of Biology, University of Miami.

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(Received 6 December 1985; revised 27 February 1986; MS. number: A4675)