Bruce E. Byers

Female mate choice and songbird song repertoires

Males of many songbird species use song repertoires that contain multiple song types. A commonly encountered evolutionary explanation for this trait and its recurrent appearance among songbird species is that repertoires have arisen in response to widespread female preference for mates with larger song or syllable repertoires. To assess whether the available evidence supports this hypothesis, we reviewed the literature on the relationship between mate choice and song repertoire size. Our review revealed an array of results that was ultimately inconclusive with regard to the generality of the mate choice hypothesis. Given the ambiguity of these results, we also examined patterns of song use and development and found that some patterns (e.g. overproduction during song ontogeny, repertoires of non-interchangeable songs, hidden repertoires and the prevalence of small repertoires) are at odds with outcomes expected if female preference for larger repertoires were common. We conclude that these cross-species patterns of song use suggest that female preference for larger repertoires is not widespread. We propose that song repertoires have generally arisen not through selection for repertoire size per se, but rather as a by-product of social conditions that favoured the evolution of complex signalling systems.

An evaluation of powerline rights-of-way as habitat for early-successional shrubland birds

Recent population declines among bird species that breed in early-successional shrubland habitats in the eastern United States have been associated with declines in habitat availability. Forest succession has eliminated shrublands in many locations, but powerline rights-of-way constitute a potential reservoir of shrubland habitat for birds. We studied 2 populations of an early-successional shrubland bird, the chestnut-sided warbler (Dendroica pensylvanica), in powerline rights-of-way in western Massachusetts over five breeding seasons to evaluate the potential conservation value of these habitats. Our goals were to l) measure reproductive success and adult survival of birds nesting in powerline rights-of-way, 2) test for edge-related increases in nest predation that might compromise the health of bird populations in powerline rights-of-way, and 3) evaluate whether reproductive success and adult survival rates of birds nesting in powerline rights-of-way were sufficient to maintain these populations. Our results indicated that nesting and fledging success in these populations were high, but nest success was marginally lower (P=0.09) near edges in 1 of 2 years for which distances from nests to edge were measured. Also, reproductive success and adult survival were sufficient at both sites to balance losses from mortality, suggesting that powerline rights-of-way can support populations of early-successional shrubland birds.

Geographic variation of song form within and among Chestnut-sided Warbler populations

Chestnut-sided Warbler (Dendroica pensylvanica) song repertoires include both accented-ending (AE) and unaccented-ending (UE) songs, and the two song categories are used in different behavioral contexts. To determine if the song categories also differ in patterns of spatial distribution, I analyzed local geographic variation in song form within a Massa- chusetts population and also compared songs from this local population with songs from other geographic regions. I found that: (1) UE repertoires of individuals were more similar to those of nearby individuals in the local population than to those of more distant birds; (2) most UE song types and song elements tended to cluster on groups of adjacent territories; and (3) UE songs from geographically distant sites tended not to match types from the main study site. In contrast, my findings indicate that: (1) AE-repertoire similarities were not significantly related to local interindividual distances; (2) AE song types were dispersed at random over the local area; and (3) nearly all AE song types from distant sites were identical to the AE types found at the main study site. The contrast between variable, geographically clustered UE songs, and stereotyped, geographically dispersed AE songs suggests that the two song categories serve different communication functions. Received 21 December 1994, accepted 28 June 1995.

Independent cultural evolution of two song traditions in the chestnut-sided warbler.

In oscine songbirds, song phenotypes arise via gene‐culture coevolution, in which genetically transmitted learning predispositions and culturally transmitted song forms influence one anothers evolution. To assess the outcome of this process in a population of chestnut‐sided warblers (Dendroica pensylvanica), we recorded songs at intervals over a 19‐year period. These recordings revealed the pattern of cultural evolution of songs in our study area, from which we inferred likely learning predispositions and mechanisms of cultural transmission. We found that the species’ two song categories form two distinct cultural traditions, each with its own pattern of change over time. Unaccented‐ending songs have undergone continual, rapid turnover of song and element types, consistent with a model of neutral cultural evolution. Accented‐ending songs, in contrast, persisted virtually unchanged for the entire study period, with extraordinarily constant song form and only one appearance of a new song type. Our results indicate that in songbirds, multiple independent cultural traditions and probably multiple independent learning predispositions can evolve concurrently, especially when different signal classes have become specialized for different communicative functions.

Development of two song categories by chestnut-sided warblers

Abstract Unlike most songbirds, individuals of several wood-warbler species have song repertoires that contain two song classes, each with distinct patterns of use and geographical variation. These patterns suggest that the modes of song development of the two categories could differ in important ways, such as in the nature of auditory and social experience required during song ontogeny. That possibility was tested in an experiment with a representative paruline species, the chestnut-sided warbler Dendroica pensylvanica . Of three experimental groups, one heard no tutor songs. A second group heard tutor tapes of both the unaccented-ending (UE) and accented-ending (AE) song categories, and a third group heard the tutor tapes but also saw and heard live adult males. The untutored birds developed no species-typical songs. Some of the tutored individuals learned songs from tapes alone, but learning and performance quality were greatly enhanced by the presence of a live tutor. Birds exposed only to tape tutoring imitated AE songs but failed to learn UE songs; live-tutored birds imitated songs from both categories. These results, interpreted in the context of field data, support the hypothesis that chestnut-sided warbler song development is somewhat compartmentalized, with AE song development more restricted by a neural ‘template’ and UE song more dependent on, and subject to influence by, social interaction.

Estimating the complexity of bird song by using capture-recapture approaches from community ecology

Repertoire size, the number of unique song or syllable types in the repertoire, is a widely used measure of song complexity in birds, but it is difficult to calculate this exactly in species with large repertoires. A new method of repertoire size estimation applies species richness estimation procedures from community ecology, but such capture-recapture approaches have not been much tested. Here, we establish standardized sampling schemes and estimation procedures using capture-recapture models for syllable repertoires from 18 bird species, and suggest how these may be used to tackle problems of repertoire estimation. Different models, with different assumptions regarding the heterogeneity of the use of syllable types, performed best for different species with different song organizations. For most species, models assuming heterogeneous probability of occurrence of syllables (so-called detection probability) were selected due to the presence of both rare and frequent syllables. Capture-recapture estimates of syllable repertoire size from our small sample did not differ significantly from previous estimates using larger samples of count data. However, the enumeration of syllables in 15 songs yielded significantly lower estimates than previous reports. Hence, heterogeneity in detection probability of syllables should be addressed when estimating repertoire size. This is neglected using simple enumeration procedures, but is taken into account when repertoire size is estimated by appropriate capture-recapture models adjusted for species-specific song organization characteristics. We suggest that such approaches, in combination with standardized sampling, should be applied in species with potentially large repertoire size. On the other hand, in species with small repertoire size and homogenous syllable usage, enumerations may be satisfactory. Although researchers often use repertoire size as a measure of song complexity, listeners to songs are unlikely to count entire repertoires and they may rely on other cues, such as syllable detection probability.

EXTRAPAIR PATERNITY INCREASES VARIABILITY IN MALE REPRODUCTIVE SUCCESS IN THE CHESTNUT-SIDED WARBLER (DENDROICA PENSYLVANICA), A SOCIALLY MONOGAMOUS SONGBIRD

Abstract A monogamous mating system that includes extrapair fertilization can potentially generate higher variability in male reproductive success than monogamy without extrapair fertilization. That increased variability could provide a correspondingly higher opportunity for sexual selection and, thus, for the origin and persistence of sexual dimorphism in monogamous species. To determine whether extrapair fertilization enhanced the opportunity for sexual selection in a sexually dimorphic, monogamous bird species, we used microsatellite DNA typing to assess the prevalence of extrapair fertilization and its effect on variation in male reproductive success in a population of Chestnut-sided Warblers (Dendroica pensylvanica). We found that the level of extrapair fertilization in our study population was at the upper end of the range reported for bird populations (47% of nestlings had extrapair fathers; 61% of broods contained extrapair offspring). We also discovered that almost all extrapair offspring were sired by paired males resident on nearby territories. In addition, we found that variation in male reproductive success was substantially higher than variation in female reproductive success, and that extrapair fertilizations made a significant contribution to variation in male reproductive success. Together, those findings suggest that extrapair fertilization creates an opportunity for sexual selection on male traits in this population.

Birdsong, migration and sexual selection: a skeptical view

In the early 1980s, Catchpole (1982) speculated that intersexual selection might be more intense on migratory than on nonmigratory songbirds, thereby fostering larger song repertoires and/or more elaborate songs inmigratory species. Other investigators have proposed a variety of mechanisms by which migratory behaviour could intensify sexual selection (e.g. Møller 1992; Fitzpatrick 1994; Reinhold 1996; Morbey & Ydenberg 2001). However, Catchpole’s additional prediction, that a migration-related increase in sexual selection will yield increased song elaboration, can be valid only if elaborate songs do in fact arise as a result of intersexual selection. An assumption that song elaboration is sexually selected is central to a recent paper by Collins et al. (2009) that describes an attempt to test the hypothesis that migratory behaviour affects sexual selection. In the paper, Collins et al. claim to have shown ‘for the first time, in contrast to what was generally believed, that sexual selection exerted through female choice is more intense for migratory than sedentary populations, but that sedentary populations may be under stronger selection than migratory populations from male competition’ (page 589). The authors reach this conclusion on the basis of a descriptive analysis of a small sample of Sylvia atricapilla (blackcap) songs from four locations in Spain. The logic connecting the authors’ data to their conclusions relies on a series of four assertions. (1) In songbirds, female mate choice selects for more complex songs. (Collins et al. use the modifier ‘complex’ to denote singing that incorporates a large number of syllables and a large variety of song or syllable types.) (2) Competition among male songbirds selects for short songs. (3) In blackcaps, one portion of a song functions in male-to-female signalling and a different portion functions in male-to-male signalling. (4) Among Spanish blackcaps, both the female-directed the maledirected portions of the song are more elaborate in migratory than in sedentary populations. Each of the four assertions is of dubious validity. Below, I comment on each assertion in turn.

Singing by Female Chestnut-sided Warblers

Abstract We recorded examples of complex, highly variable, song-like vocalizations uttered by female Chestnut-sided Warblers (Dendroica pensylvanica) in Massachusetts and New Hampshire. These songs occurred only during a brief period just after females arrived on the breeding grounds and were almost invariably associated with intense social interactions. Although the timing and context of the songs might suggest a territorial defense or other communicative function, the songs bore no resemblance to male songs, were uttered infrequently, and were used by only a small proportion of females. Therefore, we believe it is unlikely that the songs could be reliably recognized and interpreted by conspecifics. Perhaps the vocalizations are byproducts of seasonally high levels of circulating sex hormones, as suggested by the prolonged singing that we observed in a captive-reared female that had been implanted with testosterone.

Migration and song elaboration in wood-warblers (Geothlypis)

ABSTRACT Although some comparative studies of oscine songbirds have found that long-distance migration is positively correlated with elaborate songs, an analysis of singing by species in the genus Geothlypis (Parulidae) found no evidence of such a correlation. The migratory species in the genus sing relatively simple songs, whereas the singing of nonmigratory species varies; some species have simple songs, and others have more elaborate songs. Elaborate songs are found in the nonmigratory species G. semiflava, G. aequinoctialis, and G. poliocephala. For example, in Costa Rican populations of these species, songs are longer, contain more notes, have greater note-type diversity, and (in G. semiflava and G. aequinoctialis) have more phrase types than the songs of the migratory Geothlypis species. However, in other nonmigratory species (G. nelsoni, G. flavovelata, G. speciosa, and G. rostrata), the duration, note count, and note-type diversity of songs are similar to those of the migratory species. Thus, there seems to be no consistent relationship between migration and song elaboration in Geothlypis. In accordance with this inconsistency, ancestral-character-state reconstruction showed that evolutionary loss of migratory behavior was associated with increased song elaboration in some clades within the genus, but not in others. Overall, song variation in wood-warblers (Geothlypis) provides no support for the hypothesis that long-distance migration favors the evolution of elaborate songs.