Predictors of natal dispersal in great reed warblers: results from small and large census areas

For most species, it is currently not known whether individual characteristics and population parameters affecting dispersal distances within the local population are also affecting dispersal of individuals emigrating longer distances. We evaluated this in the great reed warbler Acrocephalus arundinaceus by analysing data of natal dispersal distances within (1) the local hatching area in southern Central Sweden and (2) the species' whole breeding range in Sweden. The different scales gave conflicting results. Site of birth was the main predictor of natal dispersal distance in the local census area, whereas in the larger census area the dispersal distance differed significantly between hatching years. Previous studies have shown that the recruitment rate of great reed warblers within the local study area differs between hatching years and also that it declines during the course of the breeding season. It was not known, however, whether these relationships reflected variation in survival or dispersal. The results from the present study suggest that the differences in local recruitment rate between hatching years is partly caused by annual variation in emigration rate, whereas the decline in local recruitment rate with season mainly reflects mortality of late-hatched nestlings. Our study shows a twofold advantage of a large census area when studying dispersal. First, predictors of longer-distance dispersal will be revealed and, second, the variation in local recruitment rate can be separated into the two components, survival and dispersal.