Risks and Rewards of Nest Defence by Parent Birds

Nest and offspring defence by birds can be treated as an optimization problem wherein fitness benefits are determined by the survival of the current brood and fitness costs depend upon the probability that the parent will survive to breed again. At the optimal intensity of defence, net fitness benefits are maximized. Unlike many other aspects of animal behavior, the reproductive consequences of nest defence can often be measured directly. Within this optimality framework, we review the current adaptive hypotheses to explain both interspecific and intraspecific variation in nest defence behavior, and we present some new ideas of our own. Most research to date has focused on seasonal patterns of nest defence to test the prediction that the intensity of nest defence should increase through the nesting cycle either because renesting potential declines or because the probability of offspring survival increases rapidly relative to that of the parents. Studies testing the renesting potential hypothesis have both supported and rejected it, but few studies to date have been controlled well enough to allow us to distinguish between the two hypotheses. The intensity of nest defence is also predicted to increase with parental experience and confidence of parenthood; offspring number, quality and vulnerability; and nest accessibility and conspicuousness. The response of parents is also expected to vary with the relative armament and mobility of parent and predator and the relative roles of the parents in caring for their offspring. Although there is some evidence supporting many of these predictions, most have not been explicitly tested while holding other factors constant. The tendency for researchers to ignore variables that might influence the intensity of nest defence makes the conclusions of most studies that find support for any single adaptive hypothesis relatively unconvincing. Like Optimal Foraging Theory, however, a comprehensive functional theory of nest defence based on life-history theory can help us to elucidate many of the patterns observed in this important aspect of the parental care behavior of a wide variety of animals.