Studies on the adaptive significance of behavioral strategies are most promising when there is high variability, both in the behavioral trait and the ecological conditions under which it occurs. This allows comparison of the costs and benefits of pursuing different strategies under the same conditions and of pursuing the same strategies under different conditions. The Pied Kingfisher (Ceryle rudis) shows such variability in its helper system. I have studied this species over eight breeding seasons (1976–83) at Lake Naivasha and Lake Victoria in Kenya. The following account of the results consists of four sections: (1) a description of the life history and the helper structure; (2) a functional interpretation of the cooperative breeding from the helpers' and from the breeders' points of view; (3) an analysis of the causal mechanisms which allow the birds to choose the strategy which maximizes their fitness under the prevailing ecological conditions; and (4) some speculations as to the origin of this helper system.

Life history and helper structure

General biology

Pied Kingfishers range, in three subspecies, from eastern Asia through Asia Minor to South Africa. They occur along many rivers, but are particularly frequent in the marginal regions of big freshwater lakes. They feed almost exclusively on fish. To catch their prey, they either dive from papyrus stems, dead trees or other perches along the shore, or – more often – fly over the water searching, sometimes hovering above the surface, and plunging swiftly when they see a fish.