The evolutionary significance of co-operative breeding in birds

Abstract

Although the phenomenon had barely been hinted at before Skutch’s review of 1961, co-operation by three or more adult birds in some aspect of reproductive effort is remarkably widespread as a usual feature of breeding biology, and is now known to involve at least 1.5 per cent of the world’s avifauna. I hold co-operative breeders to include the many birds with ‘helpers at the nest’, and the several species which breed in pairs but are group-territorial and provision the young communally, as well as the few in which several females lay in a common nest. For reasons given below we should exclude the half-dozen species that cooperate in building a supernest in which each pair rears its young unaided — those are best designated communal nesters. I include, however, birds in which later broods in the same season are regularly fed by their siblings of earlier broods, since this seems similar to their retaining family bonds and, when adult, helping parents at the nest in subsequent years. Details of social organisation and the nature of the co-operative effort vary considerably with species; they typically involve pre-breeding adult individuals helping a breeding pair with nest building, incubation, territorial defence, feeding nestlings and attending fledglings.