Family values: Costs and benefits of communal nesting in the moorhen

Moorhens, Gallinula chloropus, are predominantly monogamous, but, annually, 21-35% (N=74-78) of the territories in a high density breeding population of moorhens at Peakirk, Cambridgeshire, were occupied by cooperative breeding groups in which three or more adults collaborated in caring for young from a single nest. In most cases, these consisted of a pair and their previous offspring. Often a mother and daughter laid their clutches synchronously, together in a communal nest. Eggs experimentally introduced into the nest before either female began to lay were rejected, suggesting that communal nesting could not begin until both females had begun to lay. DNA fingerprinting revealed that a polygynous male gained full paternity of all offspring in the communal nest, even when one of the females was his daughter. The deleterious effects of inbreeding were reflected by a low survival rate of offspring that were the products of father-daughter incest. Communal nesting may be the only breeding option for some first-year females that hatch late in the season and overwinter on their natal territories. Mothers do not incur a cost from allowing their daughters to nest communally. However, daughters would do better by leaving and breeding in a monogamous pair if they could. Limited evidence suggested that daughters favoured this option. Consistent with the predictions of current models, the degree of reproductive skew was greater when communally breeding females were relatives than when they were unrelated. The ways in which a predisposition for communal nesting could have facilitated the evolution of intraspecific brood parasitism in this population are discussed. (C) 1996 The Association for the Study of Animal Behaviour