Disentangling genetic and prenatal sources of familial resemblance across ontogeny in a wild passerine.

Cross-fostering experiments are widely used by quantitative geneticists to study genetics and by behavioral ecologists to study the effects of prenatal investment. Generally, the effects of genes and prenatal investment are confounded and the interpretation given to such experiments is largely dependent on the interests of the researcher. Using a large-scale well-controlled experiment on a wild population of blue tits (Cyanistes caeruleus), we are able to partition variation in body mass across ontogeny into the effects of genes and the effects of between-clutch variation in egg characteristics.

Intraclutch differences in egg characteristics mitigate the consequences of age-related hierarchies in a wild passerine.

The relative age of an individual's siblings is a major cause of fitness variation in many species. In Blue tits (Cyanistes caeruleus), we show that age hierarchies are predominantly caused by incubation preclutch completion, such that last laid eggs hatch later than early laid eggs. However, after statistically controlling for incubation behavior late laid eggs are shown to hatch more quickly than early laid eggs reducing the amount of asynchrony.