Demography and Lifetime Reproductive Success in the Polygynous Great Reed Warbler

I analysed demographic parameters and lifetime reproductive success in a Swedish population of Great Reed Warblers. The population was followed on a daily basis in 1985-1993 when all breeding Great Reed Warblers were individually marked and >95% of the young fledged in the study area were ringed. Between-year breeding site fidelity of adults was relatively high both for males (55%) and females (51%), and it seemed not to be affected by previous breeding results. Natal site fidelity was also relatively high as 14% of the fledglings hatched in the study area returned to breed in subsequent years. Each year, however, about 50% of the breeders were hatched outside the study area (i. e. immigrants). The production of lifetime recruits was biased so that 17% of the males and 13% of the females produced more than 50% of the recruits in the population. There was no difference in lifetime reproductive success between native birds and immigrants, suggesting that immigrants contributed substantially to the local gene pool. Despite this, effects of non-incestuous inbreeding, measured as lowered hatchability of eggs among broods of genetically more similar pair members, were found in the study population. This may be a result of the dispersal pattern between local Great Reed Warbler populations in Sweden and also a consequence of the short history of these small populations.