Context-dependent effects of castration and testosterone treatment on song in male European starlings

Most seasonally breeding songbirds display dramatic seasonal fluctuations in plasma testosterone (T) levels and mate attraction behaviors, including song. However, males of some songbird species, such as the European starling (Sturnus vulgaris), continue to sing at high levels after the breeding season, when T levels are basal. In male starlings song during the breeding season functions mainly to attract mates, whereas song during the nonbreeding season appears unrelated to reproduction. This suggests that song produced in a context unrelated to female courtship, unlike song directed toward females, is not regulated by plasma T. In captive males housed in large outdoor aviaries we explored the relationship between plasma T and song produced during the breeding season within and outside a courtship context. This was achieved by determining the effects of castration and subsequent T treatment on song and mate attraction behaviors in both the presence and the absence of a female. Compared to intact males, castrated males did not show reduced song activity in the absence of a female for at least 6 months after the operation, strongly suggesting that the expression of noncourtship song is not regulated by plasma T. Likewise, we found that experimentally elevating T levels in castrated males did not affect noncourtship song rates. However, control castrated males receiving empty implants tended to show reduced noncourtship song rates after implantation. This may have been due to a suppressive effect caused by the presence of the T-implanted castrated males in the same aviary. In contrast, courtship singing was clearly controlled by plasma T: it was abolished by castration and restored by subsequent T replacement when males were housed both individually and in a group situation. High plasma levels of T also appeared necessary for the activation of three other behavioral traits critical for mate attraction, namely, nesthole occupancy, spending time (singing) in a nesthole, and carrying green nesting material into a nesthole.     

Juvenile sparrows preferentially eavesdrop on adult song interactions

Recent research has demonstrated that bird song learning is influenced by social factors, but so far has been unable to isolate the particular social variables central to the learning process. Here we test the hypothesis that eavesdropping on singing interactions of adults is a key social event in song learning by birds. In a field experiment, we compared the response of juvenile male song sparrows (Melospiza melodia) to simulated adult counter-singing versus simulated solo singing. We used radio telemetry to follow the movements of each focal bird and assess his response to each playback trial. Juveniles approached the playback speakers when exposed to simulated interactive singing of two song sparrows, but not when exposed to simulated solo singing of a single song sparrow, which in fact they treated similar to heterospecific singing. Although the young birds approached simulated counter-singing, neither did they approach closely, nor did they vocalize themselves, suggesting that the primary function of approach was to permit eavesdropping on these singing interactions. These results indicate that during the prime song-learning phase, juvenile song sparrows are attracted to singing interactions between adults but not to singing by a single bird and suggest that singing interactions may be particularly powerful song-tutoring events.     

Species-specific song responses emerge as a by-product of tuning to the local dialect

1. Download : Download high-res image (186KB)
2. Download : Download full-size image

     

Changes in Performance of Shared and Unshared Songs Within and Between Years in the White‐crowned Sparrow

Song of passerine birds is one of the few animal signals that is learned and that improves with practice. Vocal practice is crucial early in life to perfect a song imitation, but it also occurs throughout life and may continue to improve aspects of song performance. Differences in song performance among males that share song types, that is sing structurally similar songs may be particularly salient to receivers. We here test the hypothesis that aspects of song performance improve in a songbird species that deletes song types from its repertoire early in the first breeding season to share their final single song type with territorial neighbours. Over 3 yrs, we recorded songs in a population of Puget Sound white‐crowned sparrows Zonotrichia leucophrys pugetensis and measured percentage peak performance and consistency thereof in all of the song types in each male's repertoire. We found that within the first year on territory, percentage peak performance was higher in shared than unshared songs but did not change from first to second recording. Contrary to the hypothesis that song performance improves with age, song performance declined from the first to the second year. Our results support the hypothesis that high‐performance singers share songs. We did not find support for song performance improving within or between years, like it does in some other songbird species.