Ecological effects on song learning: delayed development is widespread in wild populations of brown-headed cowbirds

The timing and extent of early exposure to conspecific song can have critical influence on subsequent male vocal development in songbirds. Opportunities to memorize local song models may vary among populations depending on local ecological conditions that determine the length of the breeding season. In populations with comparatively short breeding seasons, such as northern or high-elevation populations, restricted access to local songs may delay development in a large proportion of juveniles. Previous studies have described extreme examples of delayed development in high-elevation populations of brown-headed cowbirds, Molothrus ater, in the Sierra Nevada of California, U.S.A. In the current study, we determined that delayed development also occurs in a low-elevation population located at a more northerly latitude than those in the Sierra. We recorded two kinds of songs from yearling and adult males who had been given testosterone (testosterone increased song output but did not change the nature of songs in males' repertoires) soon after being trapped at two adjacent sites in New York state, U.S.A. The average size of ‘perched’ song repertoires of 17 yearlings was significantly smaller than that of 20 local adults (2.8 versus 4.6 types, respectively) and yearlings generally lacked the shared songs typically found in adult repertoires. Only 40% of yearlings trapped at one site produced the correct local dialect ‘flight whistle’ compared with 91.7% of adults. These results strongly suggest that juvenile access to local songs is also restricted in this population and that delayed vocal development is widespread in cowbirds. In addition, these findings indicate that reliance on field recordings may underestimate adult-yearling differences in vocal competency in cowbirds because yearlings that sing readily in nature may not be representative of yearlings in general, a result that may also apply to other songbird species.