Determinants of clutch size in the tropics;with reference to the White-rumped Swiftlet


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A series of experiments involving clutch and brood-size manipulation, supplemental feeding and nest enlargernent were conducted on White-rumped Swiftlets (Aerodramus spodiopygius chillagoensis) nesting in savannah habitat in Queensland Australia, so that the birds' reproductive performance might be compared with that of A.s.assimilis (Tarburton 1987a) which nests in the tropical rainforests of Fiji. These experiments show that nest-size, predation, synchrony of moult and breeding, and 'competitive release' afe each inadequate to explain why the subspecies in the Queensland savannah has a smaller clutch than the subspecies in the rainforests of Fiji. While an inadequate food supply prevents Queensland birds from raising two nestlings at a time it is clear that current interpretations of food limiting theories afe inadequate to explain why birds of the Queensland savannah produce a smaller clutch than their conspecifics in rainforests.