Divorce in birds: a review of the hypotheses
Despite the considerable number of studies of avian mating systems, the 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 continuously faced with the decision of whether to stay with the old mate or divorce in favour of a different partner. The key to understanding when divorce will occur lies in determining the costs and benefits of both pair fidelity and divorce. These are likely to be species-specific and related to the varying life histories. Individuals are expected to divorce when the benefits outweigh the costs. Although a number of hypotheses have been proposed to explain why and when individuals may divorce, these are scattered throughout the literature and have not been critically compared and contrasted either theoretically or empirically. Confusion still exists as to the underlying assumptions and predictions of the individual models, mainly because a sound theoretical framework is lacking. Empirical studies to date are few and have been mainly post-hoc interpretations. Further advances in the field require sound mathematical modelling of the hypotheses and more experimental studies 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 enlarging clutch sizes to alter perceived fitness, and manipulating the availability or quality of territories or mates to determine to what extent this affects divorce rates.