Breeding of the grey warbler with special reference to brood-parasitism by the shining cuckoo

Abstract

Between 1976 and 1979 I sought to document and understand territoriality and breeding in the grey warbler (Gerygone igata), and brood-parasitism of the warbler by the shining cuckoo (Chrysoccyx lucidus), at a kanuka-forest near Kaikoura (South Island, New Zealand). Grey warblers held territories of about 0.7 ha. The average annual mortality of breeding-birds was only 20%, and the predicted average life-expectancy of adults five years. The warbler's breeding strategy was unusual: the breeding-cycle was protracted; eggs of a clutch were laid 48 hours apart; each egg was 23% of the mean adult weight; nestlings apparently gained weight according to a truncated normal curve, and they exceeded adult weight by up to 40%. This strategy may be an evolutionary response to a relative scarcity of food during breeding. Each parasitised nest received one cuckoo-egg, apparently swapped for a host's egg. Shining cuckoos lacked egg-mimicry. The cuckoo's egg was similar in size to the warbler's, but was heavier relative to adult weight than in other cuckoos. Cuckoos laid during 10 weeks, the modal week of laying following the apparent peak of arrival of cuckoos in New Zealand by seven weeks. Cuckoos often laid after warblers began incubating. Nestling cuckoos evicted all other nest-contents at 3-7 days old. Parasitism reduced by only 17% the production of fledgling warblers from late nests. Cuckoos had little effect on the warbler's overall reproductive success because no first nests and only 55% of late nests were parasitised. The mean weight of nestling cuckoos never reached that of a brood of four warblers of comparable age. On average a cuckoo was brooded more than broods of warblers, but was visited with food less often than the host's own young in broods of three or four.