Effect of food availability on nestling growth and fledging success in manipulated pallid swift broods

Growth rate and fledging success were assessed in natural and manipulated broods of the pallid swift Apus pallidus. Daily measurements of chick mass, wing length, and insect abundance allowed us to examine the short-term variation of chick growth in relation to food availability. The number of fledged nestlings increased with brood size. Wing length and body mass were slightly but significantly smaller in larger broods, and the nestlings of enlarged, broods needed longer to fledge. We discuss how these differences could influence survival after fledging. Hatching asynchrony caused a significant difference in growth among siblings, and the difference between the oldest and youngest chick was greater in larger broods. Chick growth was independent of daily food availability. We suggest that this was due to an increased effort of the parents at their expense, when food availability was poor. The ability of this species to raise an additional chick is in line with most findings on birds, but partially in contrast with results for the common swift in which, at least during poor seasons, the additional nestling caused an increased mortality and lowered the reproductive success.