Abstract
In seasonally breeding birds, the vernal growth of the song system is thought to result primarily from increased daylength and the associated increase in circulating testosterone. Other environmental factors such as social cues between mates influence the timing of reproduction, but less is known about how social cues might affect the song system and song behavior. We used white-crowned sparrows (Zonotrichia leucophrys gambelii) to test the hypothesis that the presence of a female in breeding condition influences song nuclei and song behavior of adult males. There were four treatment groups: (1) eight males housed individually in the same room on long days and paired with estradiol-implanted females; (2) eight males housed similarly on long days but without females; (3) four males isolated on long days; and (4) four males isolated on short days. The volumes of two song nuclei, HVc and RA, were significantly larger in males housed with females than in any other treatment group. Males isolated on short days had smaller HVc, RA, and area X volumes than all other groups. The volumes of Rt (a thalamic nucleus not involved in song) and the telencephalon did not differ among groups. Plasma androgen levels did not differ among the three long-day, social treatment groups at the times sampled, but were lower in the short-day isolates. Males paired with females sang at a higher maximum rate than males housed together, who sang at a higher rate than long-day isolates. These results suggest that seasonal plasticity in the adult song system is influenced by social cues.