The Grey Warbler's Care of Nestlings: A comparison between unparasitised broods and those comprising a Shining Bronze-Cuckoo

Abstract

Grey Warblers Gerygone igata at Kaikoura, New Zealand, raised either a brood of up to four of their own young or one nestling Shining Bronze-Cuckoo Chrysococcyx lucidus. The Cuckoo never weighed as much as four nestling Warblers of the same age, was visited with food less often than three or four Warblers, and had fewer faecal sacs removed. Therefore raising a Cuckoo to fledging required less effort in terms of feeding-visits and hygiene than raising a brood of several Warblers. Single Cuckoos were brooded more on average than three or four Warblers of the same age, for a greater part of the nestling period and in longer spells. Thus the pattern of brooding by adult Warblers was not determined entirely endogenously but was modified by external stimuli.