DIVORCE IN BIRDS - A REVIEW OF THE HYPOTHESES

Despite the considerable number of studies of avian mating systems, th e reasons for divorce in monogamous birds are still poorly understood. It is now generally believed that divorce is a reproductive strategy by an individual to maximize its own fitness, and that birds are conti nuously faced with the decision of whether to stay with the old mate o r divorce in favour of a different partner. The key to understanding w hen divorce will occur lies in determining the costs and benefits of b oth pair fidelity and divorce. These are likely to be species-specific and related to the varying life histories. Individuals are expected t o divorce when the benefits outweigh the costs. Although a number of h ypotheses have been proposed to explain why and when individuals may d ivorce, these are scattered throughout the literature and have not bee n critically compared and contrasted either theoretically or empirical ly. Confusion still exists as to the underlying assumptions and predic tions of the individual models, mainly because a sound theoretical fra mework is lacking. Empirical studies to date are few and have been mai nly post-hoc interpretations. Further advances in the field require so und mathematical modelling of the hypotheses and more experimental stu dies that rigorously test a priori predictions. These include removal of mates to create vacancies under controlled situations, manipulating sex ratios to increase intra-sexual competition, reducing or enlargin g clutch sizes to alter perceived fitness, and manipulating the availa bility or quality of territories or mates to determine to what extent this affects divorce rates. (C) 1995 The Association for the Study of Animal Behaviour