Sex Ratios of Nestling and Fledgling Red-Cockaded Woodpeckers (Picoides borealis) Favor Males

Abstract

Red-cockaded woodpeckers breeding in costal South Carolina [USA] produce nestling and fledgling sex ratios dominated by males. Differences in the costs of daughters versus sons in terms of the growth of nestlings, an earlier death of sons, or local resource competition between parents and daughters are not evident. However, local resource enhancement is a possible source of the theoretically reduced costs of sons. Because red-cockaded woodpeckers breed in clans consisting of pairs alone and pairs assisted by adult male helpers, we hypothesized that progeny sex depends on the presence or absence of helpers. We found no strong support for this notion; nestling sex, however, depends on the tenure class of breeding females, suggesting that sex-ratio variation may be facultative (in the sense of Williams 1979) and related to kinship asymmetries among the clan types in this species.