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 manipula ted broods of the pallid swift Apus pallidus. Daily measurements of ch ick 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 lengt h and body mass were slightly but significantly smaller in larger broo ds, and the nestlings of enlarged broods needed longer to fledge. We d iscuss how these differences could influence survival after fledging. Hatching asynchrony caused a significant difference in growth among si blings, and the difference between the oldest and youngest chick was g reater in larger broods. Chick growth was independent of daily food av ailability. We suggest that this was due to an increased effort of the parents at their expense, when Food availability was poor. The abilit y of this species to raise an additional chick is in line with most fi ndings 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.