CAUSES OF NATAL DISPERSAL IN THE LESSER KESTREL - INBREEDING AVOIDANCE OR RESOURCE COMPETITION

1, Natal dispersal of seven cohorts (1988-94) of the colonial lesser k estrel (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 oic 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 c olony: more than 90% of resights were within 30 kin of the birthplace. 2, Contrary to most avian studies, dispersal from the natal site was not ses-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 dist ance for males (19 km) did not differ from that of females (18.5 km). 3, Siblings (72 twos) dispersed or were philopatric independently of e ach other, This result provides no support for tither a genetic basis of dispersal (which predicts the same strategy for both siblings) or t he inbreeding avoidance hypothesis (which predicts greater dispersal i n one sex than the other), This latter hypothesis also conflicts with our observation that the presence of the parent of the opposite sex; i n the natal colony has no effect on whether returning individuals disp erse, 4, Generalized linear models (GLMs) showed that the probability of dispersal for an individual increased when the absolute growth of i ts natal colony decreased and also from beginning to end of the study. coincidental with a general population ina ease. These results point to a link between colony saturation and dispersal. GLMs also showed th at dispersal distances tended to increase with size of the natal colon y and higher breeding success. These results are also consistent with intraspecific competition for resources. 5, Some individuals were obse rved in two different colonies in their year of first return, These ob servations, and the lack of active inbreeding avoidance, suggest a sce nario for the dispersal process: individuals would prefer to settle in their natal colonies, If they cannot secure a nest site or a mate, th ey will have to decide whether to stay as nonbreeders or to disperse i n search of another breeding chance elsewhere. The actual decision to disperse could be triggered by the level of aggression the unmated ind ividuals encounter in their natal colonies.