SIBLING COMPETITION IN ASYNCHRONOUSLY HATCHED BROODS OF THE PALLID-SWIFT (APUS-PALLIDUS)

Parent-offspring interactions during the rearing period were studied i n 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 t hree, 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 r elated positively to a nestling's begging activity. The area in the ca vity where nestlings waited for parent arrival was not random, but ten ded to be in sectors close to the nest, where the majority of feeding events occurred (''activity centre''). The observed behaviours have be en compared with those of other nidicolous birds with asynchronous hat ching and a general pattern emerges: parents are apparently unselectiv e in feeding chicks, and the competitive capacities of siblings in beg ging and positioning themselves in a proper central area have the larg est effect on their ability to obtain food.