Sexual dimorphism and offspring growth: Smaller female Blue Tit nestlings develop relatively larger gapes

Sexual size dimorphism results in asymmetric sibling competition, and nestlings of the smaller sex are expected to prioritise the development of those morphological characters that maximise effective sibling competition. In this study, we test the prediction that female Blue Tit Cyanistes caeruleus nestlings, which are smaller than males, preferentially develop relative gape area in a trade-off against growth of mass and head-bill length. We found that whilst male nestlings were heavier, female and male nestlings had similar head-bill lengths, but females had relatively larger gape areas. Therefore, female nestlings were investing relatively more resources in gape area than males because of their overall smaller body size, presumably because gapes are an integral part of the process used by nestlings to solicit food from their parents.