Factors affecting egg mass in the Pallid Swift Apus pallidus

In order to understand the sources of egg mass variation in the Pallid Swift, we weighed 142 eggs (0.1 mg accuracy) from 1990 to 1993 in a colony in north-west Italy. The observed variability was related to laying order and laying date, but was not influenced by the wing length or mass of the parents. Food abundance in the days preceding egg laying did affect egg mass, but the variation was small with respect to the average egg mass. As previously reported for the Common Swift, the last (third) egg laid had the smallest mass. The heaviest eggs were laid early in the season. As there is no seasonal decrease in aerial insect abundance, this trend may be because high quality parents lay earlier in the season. The mass of the egg positively influenced nestling weight at age 20 days. We have analysed to what extent egg mass per se affected chick survival.