Helping a Sibling or Helping a Stranger: Does Bird Nestling Relatedness Influence Fledging Asynchrony?

Science Center A255

In multi-offspring broods, some bird nestlings may be ready to leave the nest, fledge, before their nest mates. Kin selection theory predicts that more developed individuals should delay fledging until their nest mates are also ready to fledge: although staying in the nest can increase predation risk, leaving early can reduce survival of less developed siblings. Thus degree of relatedness within nest mates should affect the length of period in which the nestlings leave the nest, called fledging asynchrony. To our knowledge, this topic has been investigated within species but not across species. We tested the prediction by conducting a cross-species study of the relationship between fledging asynchrony and nestling relatedness in song birds and ducks, two large orders in which extra-pair paternity and brood parasitism are common and well documented. Data on relatedness and life history variables were gathered from published sources. We used generalized linear mixed models controlling for phylogeny and compared models with AIC. We found no support for the prediction that nestling relatedness influences fledging asynchrony, suggesting that something other than kin selection accounts for the variation in fledging asynchrony across species.