Variation in parental effort of Pallid Swifts (Apus pallidus) was investigated for 3 years in a colony in northwestern Italy. The masses of adults and of bolus loads brought to chicks were monitored by electronic balances inserted under nests, and feeding rates were monitored by video cameras. Fluctuations in daily food availability were measured with an insect-suction trap. Manipulation experiments on broods originally consisting of three chicks were performed to increase (four chicks) or reduce (two chicks) adult effort, with the aim of determining if parents tend to allocate food primarily to themselves or to their offspring, and if mass loss in adults results from reproductive stress or from adaptive programmed anorexia. With the enlargement of brood size, mean bolus mass remained constant, but the visitation rate increased significantly. Daily food abundance did not influence the amount of food allocated to chicks (neither time spent foraging nor the bolus mass changed), but positively influenced the mass of adults, which showed large daily variations. These results indicate that parents tend to invest constantly in offspring, at their own expense when food is scarce. Our data lend support to the cost of reproduction hypothesis instead of adaptive anorexia, since adults lose mass mainly in the brooding period, when demand is highest, and always regain mass when prey availability is greater.
Brood size, sibling competition, and the cost of begging in great tits (Parus major)