Reduction of cost of polygyny by nest predation in the Black-browed Reed Warbler

Abstract

When female birds choose already-mated males as their mates, they suffer some costs. One major cost is a reduction in male parental care. However, since nest predation disturbs the nesting cycles of polygynously mated females, it might change the allocation of male parental care between the females. I investigated the effect of nest predation on the relationship between female mating order and their brood status during the nestling-rearing period in the Black-browed Reed Warbler Acrocephalus bistrigiceps. Polygynous males did not feed later-hatched broods. The nest predation rate was high (56% of nests), which gives subsequently mated females a chance to receive male assistance. Four subsequently mated females acquired monogamous status by the time their eggs hatched, because the previously mated females had failed in their breeding attempts and disappeared from the territories. In addition, a subsequently mated female's nest was preyed upon and her renesting delayed her own nesting cycle, which resulted in the disappearance of the previously mated female with her fledglings and gave monogamous status at egg-hatching to the subsequently mated female. Furthermore, a case of inversion in the hatching order in polygynously mated females occurred by the prolonged pre-laying period of the previously mated female, which resulted in the subsequently mated female obtaining primary status at egg-hatching. As a result, 43% of the females that paired with already-mated males acquired monogamous or primary status at egg-hatching, whereas 69% of the females that paired with unpaired males did so. This suggests that nest predation reduces the cost of polygynous mating in this reed warbler population.