The frequency of polygyny in great reed warblers is one of the highest among European passerines. Males arrive to the breeding area about two weeks before the females. In an experiment with females tagged with radiotransmitters , we found that they visit the territories of three to twelve males before choosing a breeding partner . Females that choose to breed with already mated males enjoy similar fledging and recruitment success as females that choose to breed with unmated males, and this supports the view that the polygyny threshold model can explain this polygynous mating system. The fitness gain seems to be increased survival of young to adulthood as male song repertoire size correlates with the relative post-fledging survival of offspring.
Females engaging in extra-pair copulations obtain no direct benefits from the cuckolderer.
This suggests that good singers are attractive because they carry genes that promote fledgling survival. The great reed warbler initially started to breed at Kvismaren in 1978. Since 1987 we have collected blood samples from almost all individuals in the population at Kvismaren. Rather, we believe that recent population bottlenecks have increased the relative frequency of deleterious alleles and that inbreeding, through non-incestuous effects, in fact is much stronger than the pedigree data suggest. References Bensch, S. 1996.