Increase of parental effort in experimentally enlarged broods of pallid swifts

Apus pallidus were studied in a colony in NW Italy. Manipulation experiments on broods originally consisting of three chicks were performed to increase (four chicks) or reduce *two chicks) adult effort. With enlargement of brood size, mean bolus mass remained constant, but 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. Parents thus tend to invest constantly in offspring, at their own expense when food is scarce. 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. -from Authors