AN EXTREME EXAMPLE OF DELAYED VOCAL DEVELOPMENT - SONG LEARNING IN A POPULATION OF WILD BROWN-HEADED COWBIRDS

To determine whether vocal ontogeny in migratory brown-headed cowbirds, Molothrus ater artemisiae, of the Sierra Nevada was open at the age of 1 year, two kinds of songs were recorded from recently captured birds who had been given testosterone implants. As these birds were held in captivity for only 3-4 days before recordings were begun, their vocalizations reflected the results of learning in the wild. Only two (12.5%) of 16 yearlings ever gave a complete version of the local dialect of the 'flight whistle' song while nine (81-8%) of 11 adults did so. Yearlings had smaller repertoires of the 'perched' song and unlike adults, their repertoires contained song types that were not shared with any other bird. Yearling cowbirds apparently complete their vocal development during their second winter when they learn to match memorized copies of the songs of adult males from the areas where they were trapped as yearlings. These findings are consistent with an explanation for the maintenance of dialects based on social benefits for birds matching the songs of resident adults. Data on five birds recorded as yearlings and as adults are presented to support this hypothesis. Delayed learning in yearlings was predicted because juveniles have little opportunity to hear conspecifics vocalize during their hatching year. The present results offer further evidence for plasticity in the timing of learning and suggest that there may be more variation than was previously thought in the vocal ontogeny of some oscines as a result of delayed opportunity to learn. © 1993 The Association for the Study of Animal Behaviour. All rights reserved.