Do males hatch first and dominate sex ratios in White Stork Ciconia ciconia chicks?

Abstract

Sex allocation has been a fertile topic in the development of evolutionary theory. The dominant models for vertebrates have provided predictions of sex ratios based on asymmetry in breeding success between sexes and the relative effect of local competition. In birds, empirical work has provided some support for these models, but has also generated apparently contradictory observations. Recent models have provided some predictions for both individual and population sex ratios, showing that the availability of high quality habitat patches and dispersal rates can critically affect both population- and individual-level expectations. We explore patterns in offspring sex ratio among chicks of the White Stork Ciconia ciconia, a large migratory bird species with bi-parental care, which does not show strong sexual dimorphism in size and survival patterns. We checked the hatching order and body weight of 342 nestlings from 124 broods, and their sexes were recorded over 4 years (2005–2008) in a dense healthy stork population in western Poland. Sex ratio in the study population was skewed to males (57.5%) which was consistent between years. We have assumed that chick age was directly related to their size. The heavier (and assumed older) chicks were significantly dominated by males.