Behavioral Ecology of Fledgling Brown-Headed Cowbirds and Their Hosts

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Abstract
Fledgling brown-headed cowbirds (Molothrus ater) (22) raised by 9 host species were monitored between leaving the nest and independence. The objectives were to record as much as possible about the fledgling period (especially those features that enable cowbirds to become independent) to determine if fledglings behaved differently with different hosts and to determine if fledgling cowbirds possess adaptations for brood parasitism. Cowbirds usually left the nest when 10 or 11 days old, first fed themselves when 20-22 days old and became independent when 25-39 days old. They went through 3 stages: inactive, active and superactive, differentiated by how often they flew and by other behavior. General development was probably unmodified by hosts. Perching heights and home ranges of fledglings were modified in a host-specific manner. Cowbirds perched mainly at the hosts'' foraging heights and their home ranges corresponded to their hosts'' territories; both are related to feeding efficiency. Cowbirds did not recognize hosts as individuals; they generally responded positively only to their host species. Hosts fed fledgling cowbirds more than they fed an equivalent weight of their own young. The loud persistent calling of fledgling cowbirds may cause them to be fed more and is probably their main adaptation for brood parasitism.