Predicting polygynous settlement while incorporating varying female competitive strength

The rationale behind the polygyny threshold model is that the breeding situation quality (BSQ) males have to offer females varies, and that differences in BSQ offset females' costs of sharing with other females, thus favoring polygynous settling. It predicts that the first chosen territories become polygynous first, and that breeding success of secondary and contemporary monogamous females is similar. This is not generally found. Testing of the polygyny threshold model (PTM) assumes that females are equal competitors and distribute ideally free around available breeding resources, a condition probably not often met. If sharing a male is costly, and competitors differ in quality, weaker individuals should experience degrees of competitive exclusion. Setting female competitive abilities proportional to arrival order, we use an individual-based interference-competition model to examine settlement patterns. Shifts in the ratio of variance in interfemale competitiveness to interterritorial differences in BSQ result in various settlement patterns, with different predictions concerning settlement order and fitness returns. We find support for the novel predictions from data on northern lapwing (Vanellus vanellus), starling (Sturnus vulgaris), pied flycatcher (Ficedula hypoleuca), and blue tit (Parus caeruleus). We suggest that before testing polygyny predictions, an evaluation of the settlement sequence should be made, which may help to generate more accurate predictions. We argue that violation of the "equal female" assumption may explain much of the discrepancy between predictions and empirical findings in previous tests of the PTM, and that secondary females in general have lower success than do monogamous breeders because they are of lower quality. Copyright 2003.