(1) We studied breeding dispersal in male and female great reed warblers (Acrocephalus arundinaceus L.) at lake Kvismaren in South Central Sweden from 1984 to 1990. In the two study plots, Marsh A and Marsh B, the numbers of breeding birds increased over the years. (2) About 55% of the breeding birds returned to the study site between years and 80% returned to the same breeding marsh. Birds that had bred successfully were more likely to return to their breeding marsh than those that had failed. (3) Between years, 18% of females and 6% of males returned to breed in their former territories and males tended to move further than females. Less than 15% of the females with their previous year's mate alive, Te-established the pair bond. (4) The order of territory settlement was similar between years. Both males and females settled earlier in year(t + 1) in territories that were occupied early in year(t), than in territories occupied late or in territories that had been unoccupied. (5) Males and females that arrived relatively earlier than they did the year before occupied a territory of higher rank. Such males increased their reproductive success significantly whereas females did not. (6) The attractiveness of territories was fairly constant over the study period. By dividing the territories into two groups, attractive and less attractive, based on the average order they became occupied by males, we show that males became mated with more females and had higher reproductive success in the attractive territories. Reproductive success of females did not differ between attractive and less attractive territories. (7) The high marsh fidelity for both males and females cannot be explained by benefits from optimal inbreeding, re-mating with former mates or breeding in the same territory. Instead, we suggest that the adaptive significance of returning to their breeding marsh (neighbourhood fidelity) is that the birds can use knowledge about the quality of several territories, gathered in year(t), and settle in the best territory available at the time of their arrival in year(t + 1). (8) In most bird species, females are less faithful to their breeding sites than are males. In the polygynous great reed warbler, the variance in reproductive success is much larger among males than among females. Although there might be a higher cost for males to disperse this might be overcome by a greater benefit if they succeed in occupying a high quality territory. This potential fitness gain should entice males to disperse.
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