The social system and reproduction in a New Zealand magpie population : and a test of the cooperative breeding hypothesis : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University

Abstract

Magpie social behaviour was studied at Linton in the Manawatu, from June 1978 to November 1982. Social behaviour patterns and reproduction were investigated, and the hypothesis that magpies were cooperative breeders was tested. There were two population phases. Nomadic flocks formed in spring and foraged on open, treeless pasture. The flock sex ratio was determined from morphometric measurements, and was between 33:66 and 50:50 males to females. Flock density stabilised at 1 magpie per hectare, and the estimated daily survival rate was 0.9. Territorial magpies defended all-purpose areas averaging 5 hectares in size, and there was no correlation between territory size and the number of occupants. The mean adult survival rate was 0.85, and annual productivity was 0.96 juveniles per breeding female. Flock magpies associated randomly, and flock membership changed frequently. The primary activity of flock birds was foraging, which intensified during the afternoons, and in the month of March. The proportion of flock magpies foraging was positively correlated with the number of birds present. Territories were defended by pairs and groups. Non-kin groups may have formed in the flock, and kin groups formed when juvenile dispersal was delayed. Non-breeders did not help at the nest, and male parents made as many visits to nestlings as females did. Time of day strongly influenced the distribution of activities, so that foraging occupied more time in the afternoons. Group-territory magpies spent less time perching and more time defending the territory than pair-territory birds. The average national clutch size was 3.5 eggs. There was no difference in the breeding output of pair-territory and group-territory hens, but per capita production was lower in the groups. Average individual fitness estimates were prepared for each of four magpie lifestyles, and were highest for pair-territory birds. Supplementary food did not inhibit juvenile dispersal, and lead to polygyny in the formerly pair-defended territories. Hens receiving extra food fledged more juveniles than they had in the previous spring. It was argued that magpies were not cooperative breeders, but selfish opportunists which exploited various social lifestyles in order to obtain reproductive status.