Delayed sensory learning and development of dialect songs in brown-headed cowbirds, Molothrus ater

In captivity, male brown-headed cowbirds can develop adult song repertoires by the time they are sexually mature at approximately 1 year, as is the case for most songbirds studied to date. However, in the wild it generally takes 2 years for cowbirds to reach this stage. We have postulated that a crucial step in this protracted development involves a sensory learning phase during a male's first breeding season when he memorizes de novo many of his adult songs. To test this hypothesis, we exposed yearling cowbirds that had been housed in acoustic isolation since being taken from the wild as juveniles to different song culture environments. One cohort of yearlings was housed with adult males (tutors) from the Mammoth Lakes, California, U.S.A., dialect (Mam) where the juveniles had been trapped, while another cohort was housed with adults from an unfamiliar dialect, Santa Barbara, California, U.S.A. (SB). One year later when the tutee birds were recorded as 2-year-old adults, all 10 males in the Mam cohort had developed song repertoires typical of adults in the Mam dialect. In contrast, only 1 of 11 SB males had a single Mam song. Four of the SB cohort produced copies of their SB tutors' songs while the majority of SB tutees produced new song types that were shared exclusively by the SB tutees in 2007. The current results offer new experimental evidence to support our model for delayed vocal development in wild cowbirds based on environmental constraints that can also affect vocal ontogeny in other songbirds.