Co-Operative Breeding in Riflemen (Acanthissitta Chloris) Benefits To Parents, Offspring and Helpers

Abstract

Abstract Two types of helpers were observed feeding young of the New Zealand rifleman (Acanthisitta chloris): "regulars" and "casuals". The former contributed significantly to feeding young from only one nest while the latter contributed a trivial amount to young from a number of nests. Regulars were usually unpaired adult males some of which acquired a mate from the brood they helped. While some casuals were unpaired adult males, most were current season offspring helping with their parents' second clutch. The relative contributions of parents and helpers in raising broods were measured to establish the benefits of co-operative breeding to offspring and feeders. Whether or not helpers were present while feeding young, the breeding female's contribution was the same. By contrast, the work load of the breeding male was significantly reduced when helpers were present, but even then he usually contributed more than this mate. That male parents benefited most from helpers feeding young was probably related to their being responsible for most of the feeding in the absence of helpers. A helper's presence did not improve productivity or the male parent's chance of survival. However, female parents of nests with regular helpers survived better than those without. Offspring fed by helpers were not heavier upon fledging nor were their chances of survival improved. The interval between fledging first clutch broods and laying second clutches was the same whether helpers were present or not.