Sexually dimorphic eggs, nestling growth and sibling competition in American Kestrels Falco sparverius

1. American Kestrel (Falco sparverius) nestlings are sexually dimorphic, with daughters larger than sons. The larger daughters have an advantage during sibling competition for food in excess of their higher per capita food requirements, and we predicted that parents would reduce this competitive disparity by differentially enhancing the growth of sons, specifically by laying them in larger eggs. 2. In a captive breeding population, eggs producing sons were significantly larger than eggs producing daughters; laying order effects were controlled. 3. The influence of sibling egg size ratios on post-natal size relationships persisted through the nesting period, providing parents with a tool to manipulate size-related phenomena in their offspring.