Chick mortality and hatching asynchrony in the Pallid Swift    Apus pallidus


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In swifts (Apodiformes), hatching asynchrony has been frequently reported and it has been suggested (Lack, 1947) that it can enhance reproductive success because in favourable conditions an extra chick can be reared, while in poor conditions it can be eliminated (brood reduction hypothesis). In this paper hatching asynchrony is studied in relation to nestling mortaIity. A four year study of the Pallid Swift in two Piedmontese (NW Italy) colonies has been conducted. Clutch size and other measures of reproductive success were record ed in 313 nests, either of the first or of the second c1utch. Hatching success and f1edging success were similar to those observed in previous years in the same North-Italy area. Mortality increased, both in the first and second brood, in late-laid c1utches. Chicks died mainly during the first ten days after hatching. The most important factor affecting nestling surviva1 was hatching hierarchy: the last hatched, especially in three-egg c1utches, experienced higher mortality. Hatching asynchrony was nearly always observed in three-chick broods, and in 751110 of the cases in two-chicks broods. In the latter situation, asynchronous and synchronous broods did not differ significantly as to mortality and mass of the f1edging birds. It is discussed why in two-chick broods hatchìng asynchrony did not affect strongly the breeding success, while in three-chick broods the survival value of asynchrony was apparently fully expressed.