Understanding Sex Differences in Parental Effort in a Migratory Songbird: Examining a Sex-Specific Trade-off between Reproduction and Molt

Abstract

Migratory birds are expected to experience a trade-off between reproductive effort, the timing and pace of molt, and initiation of fall migration. The purpose of this study was to investigate if males and females of a songbird that migrates to the neotropics, the Wood Thrush (Hylocichla mustelina), experience a trade-off between parental care and the timing of molt. We determined the relative contribution of males and females in feeding young, assessed how reproductive effort influenced timing of molt, and tested the prediction that the sex provisioning nestlings at the higher rate should molt later. Males fed significantly more than females throughout the breeding period, and males with higher feeding rates had greater nesting success. Males compensated for females' reduced provisioning rate at late nests, and females initiated molt earlier than males. We found a significant negative relationship between the timing of molt and the number of young fledged per season. Males that began molt earlier than females and late-molting males also increased their pace of molt. For males, the cost of reduced parental care is likely higher than the benefit of earlier molt, but delayed onset of molt is partially mitigated via a faster pace of feather growth.