Reactions of Some Panamanian Birds to Human Intrusion at the Nest

Abstract

Current thinking about variation in life-history patterns among species of animals stresses variation in the effort expended by parents to rear their young (Steams 1976). Following Williams (1966), many authors have suggested that optimum reproductive effort is a compromise between augmenting any one year's productivity and increasing the probability of survival to the following breeding season. Reproductive effort presumably increases the risk of the parent's dying. If we assumed that the number of young reared is determined by both the level of parental effort and the availability of resources, tests of hypotheses concerning the optimization of effort would require the measurement of effort independently of resources and production. Adequate tests of such hypotheses have not been made, in part because reproductive effort is difficult to define and measure (Hirschfield and Tinkle 1975). Effort has many components, one of which may be the risk that parents assume to defend their offspring against potential predators. In this study, I report on the defensive behavior of adult birds near their nests and its relationship to other aspects of breeding behavior. Certainly behavior is not an adequate measure of risk, but if we assumed that the intensity of defense is a crude index to reproductive effort, we could test, at least tentatively, the hypothesis that effort and breeding productivity are directly related. Although Alexander Skutch and others have reported the breeding biology of many species of tropical birds, certain aspects of breeding behavior, including the reactions of adults and nestlings to human intrusion at the nest, have received little attention. The results reported in this paper are based on observations made during a study on development rates in tropical birds (Ricklefs 1976).