Meeting Abstract

63.4  Jan. 7  Eastern Bluebird Sialia sialis Nestling Condition and Facultative Adjustment of Brood Sex Ratios KORDONOWY, LL*; MAUCK, RA; MAYS JR., HL; HEITHAUS, ER; Simon Fraser Univ.; Kenyon College; Georgia Southern University; Kenyon College lkordono@sfu.ca

One hypothesis to explain facultative adjustment among avian populations is that it is an evolutionary response to variable food resources in a promiscuous mating system. Unlike previous studies, our study focuses on chick condition to evaluate factors potentially associated with sex ratio manipulation in an Eastern Bluebird (Sialia sialis) population. We explored extra pair paternity (EPP) and food provisioning as possible contributors to facultative manipulation favoring a male biased brood sex ratio. Sex ratio data were collected for four years in a Knox County, Ohio nest box population. We observed a cohort level male biased sex ratio among nestlings in the summer of 2003, and even ratios from 2004-2006. The ratios of mass to wing length (Mass Wing Index) were correlated with male production in nests not parasitized by blowflies (Protocalliphora sialis), indicating a maternal facultative shift of sex ratios favoring males when parents provide more food for their clutch. The Trivers-Willard Hypothesis predicts facultative manipulation toward male chicks in a population with high rates of EPP. Previous studies of populations of Eastern Bluebirds have shown moderate rates of EPP; however, this population did not display EPP in 2005. We attribute male biased sex ratios in nestlings within heavy broods to maternal facultative manipulation to produce offspring with optimal reproductive success. We believe this manipulation has adaptively evolved in Eastern Bluebirds spanning wide geographic areas, but EPP is a relatively plastic behavior that need not occur at high rates yearly for populations to manipulate sex ratios in response to resource availability.