Colony size selection determines adult survival and dispersal preferences: allee effects in a colonial bird.

Avian coloniality traditionally has been investigated by examining how breeding success varies with colony size, but other crucial fitness components rarely have been examined. This may lead to wrong conclusions because unmeasured parameters may change the final fitness balance. We used multistate capture-recapture models to investigate adult survival and dispersal in relation to colony size within a long-term monitored population of lesser kestrels (Falco naumanni). Nest predation probability decreases with colony size, and adult survival is predicted to show the same trend because adults are exposed to the same suite of predators. As expected, survival probability was higher in large colonies (0.72+/-0.015; mean+/-SE) than in medium or small colonies (0.65+/-0.02). Additionally, dispersal probabilities were higher going from small to large colonies (0.20+/-0.01) than from large to small (0.08+/-0.01), as predicted by theory of habitat selection shaped by fitness maximization. These asymmetries are likely to generate size-specific colony population dynamics, so they should be taken into account in studies of colonial birds and other metapopulation-like systems. Allee effects, that is, positive density dependence, appear to be the cause of the evolution of dispersal behavior and may explain the maintenance of coloniality in this species.