Alternative Models of Territorial Polygyny in Birds

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Abstract
Much of the empirical work on territorial polygyny has centered on attempts to prove or disprove a single hypothesis, the polygyny-threshold model. Here we advocate instead a research strategy of testing between alternative explanations for the occurrence of this mating system. To this end, we propose a set of models for the explanation of territorial polygyny. The models are classified hierarchically to facilitate testing. We outline general methods for testing the models and apply the tests in detail to four species. In all four species, polygyny seems to be based on female choice rather than on male coercion. For red-winged blackbirds, experimental evidence indicates that mating with already-mated males is not costly to females. Polygyny in this species is probably best explained by models in which polygyny either has a net benefit or has no cost or benefit to females and in which the degree of polygyny is increased by female choice based on territory quality. In pied flycatchers, polygynous mating does seem to be costly to females. We can reject models in which females are forced to pay the cost because of a skewed sex ratio and those in which females are compensated for the cost. Polygyny in the pied flycatcher, then, is explained by models in which females pay a cost of polygynous mating for which they receive no compensation; discrimination among the cost no-compensation models is difficult. Evidence regarding whether polygyny is costly to females is less conclusive in the other species examined, but what evidence there is supports a cost in marsh wrens and no cost in yellow-headed blackbirds. Although we cannot completely discriminate among the models for even the best-studied species, it is nonetheless clear that no one model explains territorial polygyny in all species with this system.