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 c olony in north-west Italy. The observed variability was related to lay ing order and laying date, but was not influenced by the wing length o r mass of the parents. Food abundance in the days preceding egg laying did affect egg mass, but the variation was small with respect to tile average egg mass. As previously reported for the Common Swift, the la st (third) Egg lain had the smallest mass. The heaviest Eggs were laid early in the season. As there is no seasonal decrease in aerial insec t abundance, this trend may be because high quality parents lay earlie r in the season. The mass of the egg positively influenced nestling we ight at age 20 days. We have analysed to what extent egg mass per se a ffected chick survival.