Breeding Biology of White-Fronted Bee-Eaters at Nakuru: The Influence of Helpers on Breeder Fitness

Abstract

SUMMARY (1) Among white-fronted bee-eaters (Merops bullockoides) near Nakuru, Kenya, helpers dramatically increased the fitness of breeders. Using 5 years of data, we examine the relative importance of food availability, breeder attributes and group size on reproductive success. (2) Both food availability and size of the group attending the nest had significant effects on productivity, while the characteristics of breeders (age, experience) did not. Helpers had their effect almost entirely through increased productivity per nesting attempt, each helper on average increasing the number of fledglings by 0 50 (+0.06 S.E.). The activities of helpers did not affect either breeder survival or the number of nesting attempts per year. (3) The presence of helpers did not affect clutch size, had no influence on the probability of successful hatching, but dramatically increased the rate of provisioning per nestling. As a consequence of their feeding contributions, helpers significantly decreased both the probability of nestling starvation and the degree of nestling developmental retardation due to food stress. More young fledged, and fledged in better condition, from helped nests. (4) The mechanisms by which helpers have their effects are largely ecologically determined. The ability of bee-eater helpers to increase fledging success through their feeding contribution is related to the environmentally unpredictable, and often harsh, conditions at Nakuru. Nestling starvation rates were high (48% of all hatchlings), whereas predation losses were low (4% of eggs laid). In other, cooperative breeding species where helper feeding contributions have increased nestling growth and survival, sparse or unreliable food resources have also been the rule. In contrast, predation has been implicated as the major source of mortality in many other cooperative breeding species, and it is in these species that the anti-predator activities of helpers have been reported to have an important impact on nesting success. (5) Helper effects of the magnitude recorded in the Nakuru population of beeeaters are unusual. The presence of one helper effectively doubled the fledging success of an unaided pair, and the effect of increasing numbers of helpers was linear across all commonly observed group sizes. This large helper contribution has important implications for the behavioural options of both breeders and helpers. Not only should the presence/absence of helpers influence the decision to breed, but breeders are expected to compete for potential helpers. From the perspective of a potential