In some birds a direct competition between siblings occur. Several differences within clutches and broods influence this competence. The main asynchronies are present in egg components, egg size and age. These differences were studied and experimentally manipulated to investigate their influence on sibling rivalry. Kittiwake mothers lay mainly two eggs asynchronously provoking chicks to hatch with a difference of 12 to 24 hrs. This difference in hours is enough to favour first hatched siblings in terms of age and size. When food is not enough, first hatched chicks will aggressively expel...