Causes of Natal Dispersal in the Lesser Kestrel: Inbreeding Avoidance or Resource Competition?

Abstract

1. Natal dispersal of seven cohorts (1988-94) of the colonial lesser kestrel (Falco naumanni) was studied in southern Spain. We ringed 1852 nestlings and resighted 321 (17.3%) which survived at least the first migration to Africa. Of the returning individuals, 57% settled in their natal colonies and the remaining 43% dispersed to other colonies. Resights fell off markedly with increasing distance from the natal colony; more than 90% of resights were within 30 km of the birthplace. 2. Contrary to most avian studies, dispersal from the natal site was not sex-biased in our population. The proportion of dispersing males (41%, n = 171) did not differ significantly from that of females (45%, n = 150). Excluding philopatric individuals, the median dispersal distance for males (19 km) did not differ from that of females (18.5 km). 3. Siblings (72 twos) dispersed or were philopatric independently of each other. This result provides no support for either a genetic basis of dispersal (which predicts the same strategy for both siblings) or the inbreeding avoidance hypothesis (which predicts greater dispersal in one sex than the other). This latter hypothesis also conflicts with our observation that the presence of the parent of the opposite sex in the natal colony has no effect on whether returning individuals disperse. 4. Generalized linear models (GLMs) showed that the probability of dispersal for an individual increased when the absolute growth of its natal colony decreased and also from beginning to end of the study, coincidental with a general population increase. These results point to a link between colony saturation and dispersal. GLMs also showed that dispersal distances tended to increase with size of the natal colony and higher breeding success. These results are also consistent with intraspecific competition for resources. 5. Some individuals were observed in two different colonies in their year of first return. These observations, and the lack of active inbreeding avoidance, suggest a scenario for the dispersal process: individuals would prefer to settle in their natal colonies. If they cannot secure a nest site or a mate, they will have to decide whether to stay as nonbreeders or to disperse in search of another breeding chance elsewhere. The actual decision to disperse could be triggered by the level of aggression the unmated individuals encounter in their natal colonies.