POLYGYNY IN THE BLUE TIT - UNBALANCED SEX-RATIO AND FEMALE AGGRESSION RESTRICT MATE CHOICE

The origin of polygyny and its effects on reproductive success was studied in a population of the blue tit, Parus caeruleus. Each year about 20% of males and about 35% of females engaged in a polygynous mating. Polygynous males fledged more young and survived better than monogamous ones. On average primary females did not fledge fewer young than monogamous ones, but they paid a cost because they received less male help. Secondary females had lower reproductive success than either primary or monogamous females. Polygyny could arise when a male annexed a neighbouring territory after the owner disappeared (replacement polygyny), or when a female settled on an already occupied territory. Females either settled early during winter (year-round polygyny) or during the breeding season (successive polygyny). Replacement and successive polygyny seem to result from a female-biased sex ratio during the breeding season. This biased sex ratio was caused by a female-biased immigration from January to March. Predation of males could also be significant. The biased sex ratio caused severe competition for breeding opportunities between females. Settlement of floater females was limited as a result of strong female-female aggression, especially early in the breeding season. In this study, the polygyny-threshold model could not be used to explain polygyny, since one of its basic assumptions, that females are free to settle where they choose, was violated. It is argued that the model can work on a larger scale. © 1994 The Association for the Study of Animal Behaviour.