Breeding Seasonality and Reproductive Success of White-Fronted Bee-Eaters in Kenya

Abstract

AssmAcT.-White-fronted Bee-eaters (Merops bullockoides) at Nakuru, Kenya, reproduce in an environment where food supply for provisioning nestlings is unpredictable and generally scarce. With data from 8 yr of study on three populations of bee-eaters, we examined how these environmental conditions have influenced the timing of breeding and patterns of nesting success. Breeding coincided with the general pattern of rainfall in the Rift Valley. Kenya experiences two rainy seasons per year, and individual bee-eaters bred with either the long or the short rains, but not both. Members of a given population bred colonially and quite synchronously, but members of adjacent populations were often out of phase with one another. As a result, the distribution of longand short-rains breeding populations formed a spatial mosaic. Reproductive success was highly variable but typically low. Mean (?SD) fledging success was 0.57 ? 0.83 young, and only one nest in four produced an independent offspring (6 months of age). Most prefledging mortality (88%) was due to egg losses and the starvation of young. Very little (7%) was attributable to predation. Social factors, primarily intraspecific nest parasitism, were responsible for nearly half of all egg losses, and represent an important cost of group living. Starvation, the single most important source of prefledging mortality, claimed the lives of 48% of all hatchlings. Starvation was greatest at times of low food availability, in nests with large broods, and in nests tended by pairs without helpers. Hatching was asynchronous. This enhanced the ability of older nestlings to monopolize limited food supplies, and resulted in the selective death of the smallest nestling first. Such brood reduction, coupled with the ability of nestlings to slow their development rate in response to food stress, is considered an adaptation for coping with the unpredictable variation in food supply commonly faced by these birds. Received 17 August 1990, accepted 18 January 1991.