Corticosterone and stable isotopes in feathers predict egg size in Atlantic Puffins Fratercula arctica

Examining factors that operate outside the breeding season may provide new insights into life-history traits such as egg size, in which individual variation has not been fully explained. We measured corticosterone (CORT) levels and 15N values (trophic level) in feathers grown several months before egg-laying to test the prediction that a female's physiological state and feeding behaviour prior to the breeding season can influence egg mass in Atlantic Puffins Fratercula arctica. As predicted, egg mass increased with both CORT and 15N values in feathers, suggesting that the ability of female Puffins to meet the nutritional costs of egg production is related to CORT promoting increased foraging effort during moult and to consumption of a higher trophic-level diet.