IMMIGRATION PATTERN AND SUCCESS IN RED SQUIRRELS

We studied the characteristics of immigrants and the effects of immigr ation on reproductive activity and spacing behaviour in red squirrels living in high-quality woodlands. Male immigration peaked in spring, f emale immigration in autumn. There was no sex bias in dispersal distan ce of local recruits or in the proportion of male/female immigrants, b ut more subadults than adults immigrated on the study plots. Hence, hy potheses explaining sex-biased dispersal were irrelevant in explaining immigration patterns in our study populations. Immigrant females were not in breeding condition, nor had they produced a litter prior to im migration. Hence breeding dispersal did not occur. Red squirrels are p romiscuous, and females defend intrasexual territories while males hav e overlapping home ranges with a dominance hierarchy (Wauters et al. 1 990; Wauters and Dhondt 1992). Site fidelity is very important to repr oductive success and most parents still have a high residual reproduct ive value after having produced a litter. Under such circumstances, th e resident fitness hypothesis (RFH; Anderson 1989) predicts that paren ts can benefit by forcing emigration of offspring if the latter are li kely to find nearby vacancies. The settlement pattern of successful im migrants, which had a higher probability of becoming established when they had high body mass and when they were settling in plots with redu ced intrasexual competition, agreed with the RFH and with the proximat e dispersal mechanism suggested by Gliwicz (1992), that dispersal tend ency in both sexes depends on the degree of intrasexual competition un der local conditions. The fact that close inbreeding was never observe d could indicate that random immigration of both sexes, within the soc ial environment of a partly territorial, relatively long-lived species , has evolved not only to reduce competition for resources between par ents and offspring but also as an inbreeding avoidance mechanism.