Comparing the Effects of a Social Cue on Reproductive Development in Seasonally Breeding Migrant and Resident Female Songbirds (Zonotrichia leucophrys)

Preparation for seasonal events such as breeding begins months in advance in response to diverse cues. The type of cue(s) that organisms respond to may depend upon their ability to predict future conditions. Theory predicts that reproductive preparation in resident bird species responds more to variation in local climate and social environment than that of migrants, which begin gonadal maturation far from breeding grounds. Consequently, residents may shift reproductive phenology with a changing climate more rapidly than migrants. While photoperiodic cue use across avian taxa has been studied heavily, less is known about non-photic (e.g. temperature or social) cue use. This experiment compared the response of migratory and resident white-crowned sparrows (Zonotrichia leucophrys) to a male song cue. Wild caught migratory (Z.l. gambelii) and resident (Z. l. nuttalli) juvenile females were held on a naturally changing wintering ground photoperiod and experimental birds were exposed to subspecies-appropriate male song recordings. Reproductive preparation was tracked using monthly laparotomies, bi-weekly blood samples, and morphology (brood patch) and molt scoring. After 83 days, terminal measurements (ovary mass, oviduct mass, follicular size) were made. Reproductive development was highly variable and resident birds had more advanced ovaries and oviducts than migrants. Effects of song treatment were small, but suggested that residents respond more to song than migrants as predicted. Effects on reproductive hormones and pre-nuptual molt are also reported. Results suggest that further study of mechanisms that permit or constrain flexibility in reproductive phenology in migrants and residents is promising.