Delayed Access to Local Songs Prolongs Vocal Development in Dialect Populations of Brown-Headed Cowbirds

Abstract

A recent study by O' Loghlen and Rothstein (1993) found that ontogeny of the Mammoth dialect flight whistle (FW) and repertoires of local perched songs (PS) was not complete by the first breeding season for male Brown-headed Cowbirds (Molothrus ater artemisiae) in the eastern Sierra Nevada of California. To determine how typical of the region these results were, I compared adult and yearling vocal behavior in the Convict dialect adjacent to Mammoth and the Round Valley dialect, 55 km south and 900 m lower elevation. In the Convict group, a majority of 20 yearlings produced either incomplete local (35%) or foreign (35%) IWs. In the Round Valley, 15 of 19 yearlings (79%) gave foreign whistles. In both dialects, yearling PS repertoires lacked local song types, and in Convict, repertoires were significantly smaller than those of local adults. As was found in the Mammoth dialect, incomplete FWs of yearlings were resistant to modification during the season, and modification of yearling PS repertoires was also arrested. I propose a scheme for vocal development in these Sierran populations and discuss possible reproductive consequences of delayed vocal learning