Cooperative breeding in birds                                                                    (Kenya, 1975-1985). Explaining the evolution of cooperation and apparent altruism (e.g. in social insects) posed a problem for Darwin because such behaviour to the benefit of others seemed to defy his theory that natural selection produces organisms that maximise their own fitness. Today, we know various evolutionary routes that can lead to the evolution of cooperative behaviour: direct fitness gains through mutualism or reciprocity and indirect fitness gains through helping genetically related individuals (kin selection). For African Pied Kinfishers (Ceryle rudis) I could show that both routes to higher fitness can occur within the same species. In this bird species, a strongly biased sex ratio results in several unmated males which help breeding pairs in feeding young. Through the helping activity, primary helpers achieve an indirect fitness gain from increasing the survival of younger siblings, while secondary helpers raise their direct fitness by improving their own future reproductive success. Breeders tolerate primary helpers - who are their offspring from previous years - under all conditions. Unrelated secondary helpers, however, which com- pete with the male breeders over the rare females, are accepted only under poor food conditions, i.e. when help is needed. Under benign conditions they are rejected. Hence, the relative fitness benefits from the two helping strategies vary with ecolo- gical conditions. Lake Naivasha, Kenya