Nest invaders under scrutiny
AUCKLAND – This breeding season, when shining cuckoos carry out their furtive practice of laying their eggs in the nests of grey warblers, a Massey University researcher will be following developments closely. While grey warblers have always accepted the eggs of shining cuckoos, and raised the young, very little is known about the dynamics of the relationship between the shining cuckoo (pipiwharauroa) and the grey warbler (riroriro).
Michael Anderson is a PhD student at the Institute of Natural Resources’ Auckland-based Ecology and Conservation Group and his study of the grey warbler is thought to be the first significant study in the past 30 years, of this widespread, tiny native bird.
He is carrying out the research at Tawharanui Regional Park, an open sanctuary east of Warkworth, and will study the song of the grey warbler to contribute to the well-established song bird research led by Associate Professor Dianne Brunton at the centre.
As part of the wider evolutionary project, he will also study the relationship known as brood parasitism, between the two species, research in association with Dr Mark Hauber of the University of Auckland.
The grey warbler lays twice in the breeding season in a pear shaped nest with a small opening while migratory shining cuckoos return to New Zealand around September or early October and lay a single egg in the nests of grey warblers at the time of the second laying. The grey warbler is New Zealand’s smallest bird by weight, just 6.5g at maturity, while the shining cuckoo grows to 25g, about the same as a sparrow.
The cuckoo chick hatches first, grows more quickly and pushes the grey warbler nestlings and any unhatched eggs out of the nest but despite this aggression is then fed to maturity by the adult grey warblers.
In some examples of brood parasitism the eggs may be of similar size or appearance but that is not the case in this relationship. The shining cuckoo egg is slightly larger and a different colour, olive green.
The shining cuckoo and the long-tailed cuckoo are the only New Zealand land-based birds that migrate regularly, spending the winter from about April to August in the Solomon Islands and the Bismark Archipelago.
Mr Anderson says he will be recording the calls of the nestlings to see if the young cuckoos mimic the calls of the grey warbler chicks. He also intends to place nestlings of another species – probably house sparrow – into the nests to establish whether of not the adult grey warbler will in fact accept any other young to raise as their own.