Covariation between egg size and rearing condition determines offspring quality: an experiment with the alpine swift

A positive correlation between egg size, early growth and nestling survival has been frequently reported in the ornithological literature. Albeit of interest, most of these studies did not determine whether the relationship between egg size, early growth and nestling survival was confounded by the quality of rearing conditions. However, this is of importance in order to assess the extent to which a life-history trait like egg size causally affects fitness. In a colony of the alpine swift Apus melba, we cross-fostered complete clutches between nests to determine the relative contribution of egg size and rearing condition on nestling growth and survival. In foster nests, nestlings that hatched out of larger eggs were significantly heavier at birth and at the age of 10 days; at 25 days, however, the relationship was no longer significant. The likelihood of a chick surviving from birth to 25 days of age was not correlated with its original egg size, but with the size of the eggs laid by its foster parents. This experiment therefore lends support to the hypothesis that in the alpine swift the relationship between egg size and nestling growth and survival is mainly due to a covariation between egg size and parental care rather than to a direct contribution of egg size.