Female preference for the songs of older males and the maintenance of dialects in brown-headed cowbirds (Molothrus ater)

Abstract

Female preferences for local song types have been implicated in the maintenance of local song dialects in songbirds. Such preferences can be assessed by copulation solicitation displays (CSDs) of females in response to playback of male songs. We used CSDs to determine if female brown-headed cowbirds (Molothrus ater) in the Sierra Nevada of California discriminate between yearling and adult male vocalizations based on two vocal characteristics that often distinguish songs of these age classes, incomplete renditions of the local dialect flight whistle and 'unique', unshared perched song types. We predicted stronger responses to adult songs because yearlings are rarely involved in copulations observed in these dialects although they are sexually mature. Eleven estradiol-treated females trapped in the Convict flight whistle dialect were more sexually stimulated by complete Convict flight whistles recorded from either adults or yearlings than by incomplete flight whistles typical of yearlings. Females also found the shared perched song types of local adults more stimulating than the 'unique' types of yearlings. These sexual preferences for complete and appropriate local songs are consistent with the hypothesis that females use vocal differences between the age classes as cues in their choice of adults as mates. Furthermore these preferences would create selection pressure on young males to conform to the local song culture. Mainly because of a learning constraint in their hatching year, yearlings do not complete their development of local repertoires until the start of their second season when they are two years old. The vocal deficits of yearling males serve as uncheatable indicators of age, and age in turn is a likely correlate of male genetic quality, which may explain female preference for songs that reliably identify older males. This system shows that signal honesty can be maintained by developmental constraints rather than by high signal cost alone.