Sibling competition in asynchronously hatched broods of the Pallid swift (Apus pallidus)

Parent-offspring interactions during the rearing period were studied in the Pallid Swift, to examine competition for food among nestlings of different ages. We videotaped 1572 feeding events, in 26 nests over a period of 2 years. Both parents and all chicks, in broods of two or three, were individually marked. Both male and female adults allocated food (insect boluses) with no apparent patterns of preference for any one nestling. Siblings hatched asynchronously and the last born chick obtained less food than the first one. Success in obtaining food was related positively to a nestling’s begging activity. The area in the cavity where nestlings waited for parent arrival was not random, but tended to be in sectors close to the nest, where the majority of feeding events occurred ("activity centre"). The observed behaviours have been compared with those of other nidico-lous birds with asynchronous hatching and a general pattern emerges: parents are apparently unselective in feeding chicks, and the competitive capacities of siblings in begging and positioning themselves in a proper central area have the largest effect on their ability to obtain food.