The influence of phenology on double-brooding and polygyny incidence in the Sedge Warbler Acrocephalus schoenobaenus.

There is growing evidence that the fitness of birds in temperate zones depends strongly on the match between the timing of breeding and local phenology. We have analysed data collected from a 15-year study on a Sedge Warbler ( Acrocephalus schoenobaenus) population in its natural habitat. The highly skewed distribution of arrival dates of males indicated that they competed intensely to be the earliest arrivals. The distribution of mating dates, unlike arrival dates, was not skewed, rather it formed a bell-shaped distribution. Because females arrived much later, they could not base their mating choices on the arrival dates of males. Females, however, could use male song repertoire, a trait we found to be correlated with male arrival date. Double-brooded females mated early in the season, exactly within the peak of mating date distribution. Polygynous females, on the other hand, mated later in the season, choosing high-quality older males which had arrived significantly earlier. Male territory quality did not differ between double-brooded and polygynous females, nor did the quality of the territories occupied by an individual female change during her lifetime. Although longer lifespan and double-brooding influenced a female's fitness over the course of her lifetime, polygyny had no significant effect. We conclude that Sedge Warbler females do not compete for territorial resources, but they do prefer to mate with high-quality early-arriving males. Polygyny is an effective strategy used by females to compensate for their late arrival and breeding onset, since they pair with high-quality males. [ABSTRACT FROM AUTHOR]