Adrian L. O'Loghlen

Culturally correct song dialects are correlated with male age and female song preferences in wild populations of brown-headed cowbirds

Variation in vocal behavior among local populations of songbirds may be significant to females in mate choice. In a study of dialect populations of brown-headed cowbirds, estradiol-implanted females from two dialects held the pre-copulatory lordosis posture longer in response to playback of the local flight whistle song than to foreign whistle types from adjacent and more distant dialects. Females were held in captivity for a relatively brief period prior to testing and received no tutoring so discrimination was based solely on experience in the wild. This is the first study to show evidence of discrimination by female cowbirds based on flight whistle type. Evidence is presented from one of the study dialects that the majority of yearling male cowbirds are vocally distinct from resident adults in having either foreign or incomplete local whistles. Although these yearlings are fully mature sexually, they have little or no mating success. The correlation between female whistle preference and male mating success suggests that the ability to give the correct local whistle type may be a characteristic used by females to assess age and quality of a potential mate. A learned female preference for the predominant local song type may be a factor in the stability of these dialects by making it adaptive for males to conform to the local dialect.

VOCAL DEVELOPMENT IS CORRELATED WITH AN INDICATOR OF HATCHING DATE IN BROWN-HEADED COWBIRDS

Abstract The timing and ecological circumstances under which individual songbirds acquire memorized copies of their species-typical songs can vary significantly within a population. Males that hatch later in the breeding season are likely to hear less conspecific song as juveniles than earlier-hatched individuals. In addition, late-hatched males will experience shorter days and decreasing photoperiods during their song acquisition phase, factors known to affect vocal development. We tested the prediction that yearling Brown-headed Cowbirds (Molothrus ater) that hatched earlier the previous season are more advanced in their development of repertoires of local songs than those hatched later. We recorded perched songs from 17 yearling and 20 adult males trapped at two adjacent sites in New York state and found that yearling perched song repertoires were smaller and contained few of the perched song types common to the repertoires of local adults (adult-shared types). As found in previous field studies of cowbirds, yearlings did not alter the content or size of their repertoires during the season. We used underwing juvenal-feather retention as a measure of relative hatching date in a subset of 15 yearlings and found that perched song repertoires of earlier-hatched yearlings contained more local adult-shared types than repertoires of younger, later-hatched birds. We also investigated flight whistles of males from one site and found that only 4 of 10 yearlings produced the flight whistle type typically given by local adults. Evidence linking flight whistle development and the plumage character we used as an indicator of hatching date was inconclusive. Correlación Entre el Desarrollo Vocal y un Indicador de Tiempo de Eclosión en Molothrus ater Resumen. El tiempo y las circunstancias ecológicas bajo las cuales las aves canoras adquieren copias memorizadas de los cantos típicos de su especie pueden variar entre poblaciones. Los machos juveniles que eclosionan tarde en la temporada tienen (1) menor probabilidad de escuchar cantos coespecíficos que los individuos que eclosionan antes y (2) experimentarán días más cortos y una disminución del fotoperiodo durante la fase de adquisición del canto, factores que afectan el desarrollo vocal. Probamos que juveniles de Molothrus ater que eclosionaron tempranamente durante la temporada anterior presentan un mayor avance en el desarrollo de repertorio de cantos locales que aquellos que eclosionaron más tarde. Registramos los cantos de percheo de 17 machos juveniles y de 20 machos adultos capturados en dos sitios adyacentes en el estado de Nueva York. Los repertorios de canto de juveniles fueron menores y contenían sólo unos pocos tipos de canto de percheo típicos de adultos locales (tipos compartidos entre adultos). Los juveniles no cambiaron el contenido ni el tamaño del repertorio durante la temporada. Utilizamos la retención de plumas juveniles bajo el ala como una medida relativa del tiempo de eclosión en un subconjunto de 15 juveniles. Los repertorios de canto de percheo de los juveniles que eclosionaron tempranamente contenían más tipos adultos que los de aves más jóvenes. En machos de uno de los sitios encontramos que sólo 4 de 10 juveniles produjeron el silbido al vuelo típicamente emitido por los adultos locales. La evidencia que conecta el desarrollo del silbido al vuelo y el indicador del plumaje que utilizamos como indicador del tiempo de eclosión no fue conclusiva.

East and west coast female brown-headed cowbirds agree: both categories of male song are sexy

Abstract Some prior studies of brown-headed cowbirds, Molothrus ater , suggest geographical variation in female copulatory responsiveness to the two categories of male courtship songs, flight whistles (FWs) and perched songs (PSs). Females from the eastern M. ater ater subspecies and midwestern populations of the M. a. artemisiae and M. a. obscurus subspecies were reported to give copulation solicitation responses only to playback of PSs. But females from far western populations of M. a. artemisiae and M. a. obscurus responded to both PSs and FWs. An east–west difference in the social salience of FWs could have an ecological basis. FWs are used mostly for long-distance signalling and because western birds have larger home ranges they may use FWs more often. However, the disparate FW findings may result from different procedures used to assess female responsiveness to playbacks. Determining whether the disparate findings are due to geographical variation or to methodological differences is important because many potentially significant insights into song development and female preferences in oscines are based on cowbirds. Furthermore, either explanation would elucidate important links between female choice and male sexual displays. To resolve this issue, we tested M. a. ater females from New York using identical procedures to those we used in the far west and found that they were highly responsive to both FWs and PSs. This does not support the hypothesis that there is pronounced geographical variation in the courtship salience of FWs. Rather, it suggests that different procedures used by researchers in different areas of the cowbird’s range are responsible for the disparate findings. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .

Bacteria-killing ability is negatively linked to epaulet size, but positively linked to baseline corticosterone, in male Red-winged Blackbirds (Agelaius phoeniceus)

ABSTRACT Secondary sexual traits in males are thought to convey information about the quality of the male to rivals and potential mates. Semi-static signals such as sexually selected plumage are thought to convey information about how well a male fared through a critical period, such as development or molt, but it is unclear if they should relate to his current condition during the breeding season. We examined the relation between epaulet size and two measures of immune function, levels of baseline corticosterone (CORT), and body condition in adult male Red-winged Blackbirds (Agelaius phoeniceus). Epaulet size was significantly negatively related to the bacteria-killing ability (BKA) of the blood but was not related to the phytohemagglutinin swelling response. There was no significant relationship between CORT and epaulet size, but there was a significant positive relationship between CORT and BKA. In addition, BKA was significantly positively correlated with date, suggesting a possible tradeoff with breeding-related activity levels. However, body condition was not related to measures of immunity, CORT, or epaulet size. Our results indicate that male Red-winged Blackbirds with larger semi-static signals experience reduced BKA during the breeding season and that baseline levels of CORT can be positively associated with measures of immune function.

DIVERGENT SEXUAL RESPONSES TO DIFFERENT CATEGORIES OF FOREIGN COURTSHIP SONGS IN FEMALE BROWN-HEADED COWBIRDS (MOLOTHRUS ATER)

Abstract Female songbirds generally have an innate ability to distinguish between con-specific and heterospecific song, and may learn to discriminate among variants of conspecific song. By observing female copulation-solicitation displays elicited by playback of perched songs and flight whistles from distant (>2,000 km) widespread populations, we assessed the extent to which responses of female Brown-headed Cowbirds (Molothrus ater) to the two categories of male courtship song depend on experience. We hypothesized that female responsiveness to flight whistles is more dependent on experience than their responsiveness to perched song, because, though both perched songs and flight whistles vary spatially, perched songs always conform to strict species-specific structural and syntactic rules. Flight whistles, in contrast, are so variable that some types may not be recognizable as conspecific vocalizations to birds that have never experienced them. The species-wide structural similarities of perched songs make it possible for females to have innate responsiveness to these songs, as shown by isolate-reared females. In contrast, isolate-reared females do not respond to flight whistles. In the present study, females readily responded to foreign perched-song types, but showed as little response to foreign flight whistles as they did to heterospecific control songs. A previous study had shown that the same females were highly responsive to the local flight whistle. Thus, in accord with our hypothesis, females must have direct experience with a flight-whistle type to become responsive to it, but will respond to any unfamiliar perched-song type. Our findings for females are concordant with results on variation in the role experience plays in development of male production of songs from these two categories.

Changes in Immunocompetence and Other Physiological Measures during Molt in Brown-Headed Cowbirds (Molothrus ater)

ABSTRACT. We tested one of the foundational hypotheses of the field of ecological immunology: that it is difficult for animals to simultaneously carry out two or more especially demanding physiological processes at optimal levels because of energy needs or other factors that cause tradeoffs among competing components of life history. We investigated possible effects of molt (a costly life-history stage that all birds share) on three physiological parameters in 32 male and female Brown-headed Cowbirds (Molothrus ater). We measured hematocrit, corticosterone levels (CORT), and the bactericidal competence (BC) of blood plasma by drawing blood from the birds before, twice during, and twice after their molt from late July to early September 2009. In accordance with our predictions, BC dropped during molt for both males and females. Interestingly, BC recovered faster after molt ended in males, and female BC remained depressed for the rest of our sampling period. Hematocrit also dropped during the molt but returned to initial levels after molt in both males and females. CORT dropped during molt, but the change was significant only for males. Our results highlight possible physiological consequences of molt in Brown-headed Cowbirds even when the birds are maintained in optimal conditions (i.e., shelter, ad libitum food and water, relatively low-stress environment). Furthermore, we suggest that the slower recovery of female immune function following molt may be related to higher female mortality resulting in the ubiquitous phenomenon of male-biased sex ratios in Brown-headed Cowbird populations.

Cultural Evolution and Long-Term Song Stability in a Dialect Population of Brown-Headed Cowbirds

Abstract. Knowing the extent to which the acoustic structure of songs is temporally stable is essential to understanding how cultural evolution affects song dialects in oscines. The acoustic structure of the most prevalent variant of the flight-whistle song recorded from male Brown-headed Cowbirds (Molothrus ater) in the Mammoth dialect (Mammoth Lakes, California) from 2005 to 2009 differed significantly and consistently from whistles recorded in 1989 and 1978–1985. The most common whistle variant in the 2005–2009 sample had structural features absent from whistles described in the earlier studies and overall was produced at a consistently lower acoustic frequency. Besides the emergence of this new variant sometime between 1989 and 2005, the prevalence of other variants of the whistle also changed from 1978 to 2009. Changes reported in other studies of cultural evolution in oscines have been based on lower-level structural elements (notes and syllables), whereas we found that entire songs appear to have evolved as cultural units or memes. We discuss possible mechanisms as to how these changes may have occurred. Despite these changes, the Mammoth whistle still retained the same basic three-syllable structure it had 31 years ago. This stability is notable because of the potential for extreme variation in whistle structure exemplified by the distinct whistles of nearby dialects in the region.