SOCIAL STABILITY AND TERRITORY ACQUISITION IN BIRDS

This study tested the hypothesis that one strategy birds use to acquire territories is to seek and exploit areas where conspecific social stability is weak. Predictions were that nonterritorial individuals, 'floaters', should be attracted to areas of social instability and that they should obtain territories in unstable areas. An experiment with red-winged blackbirds (Agelaius phoeniceus), designed to test whether floaters are attracted to boundary disputes between territorial neighbors, showed that floaters approached multiple male models in simulated boundary disputes at higher rates than they approached single models. Both drug implants that interfered with testosterone physiology in territory owners and manipulations of breeding habitat rendered certain territory aggregations socially unstable, and floaters obtained territories in those areas. Therefore, evidence generally supported the instability hypothesis of territory acquisition. No evidence was found to support an associated prediction, that a high-density breeding area should attract more floaters and thus suffer a higher territory owner turnover rate than a low-density site. Experiments with models of two species of blackbirds showed that one method floaters may use to establish territories is to settle on boundaries between adjacent owners. The potential generality of the instability hypothesis, and its implications for the expression of avian territorial behavior, are discussed. If floaters are attracted to signs of social instability, then it would be advantageous for territory owners to behave in ways that reduce the appearance of instability.