1982 年 14 巻 2-3 号 p. 281-292
A small population of Dunnocks Prunella modularis in a semi-natural habitat in southern England was studied for 51/2 years. The social organization during the breeding season was complex. Males outnumbered females in the proportion 1.25:1. Some territories were shared by two males, one of which was dominant. All such territories were in high-quality habitat. Dominant males did most of the mating; in a few cases subordinate males mated with females before the laying of a second or later clutch. In addition, one quarter of the males were bigamous. Bigamous males fed the young of both females. There was no significant difference in the breeding success of females associated with one or two males. In five years with mild winters females survived slightly better than males; in one very severe winter males survived normally but all colour-ringed females disappeared. The data suggest that males are more at risk than females during the breeding season, but that females, because they are subordinate to males at feeding places, are more at risk during periods of food shortage. Cutaneous disease was rather common in the population, and affected males more than females. Different patterns of male and female mortality may result in temporally and spatially fluctuating sex ratios in the breeding population, and this may affect pairing relationships; but it is suggested that the main underlying factor involved in the Dunnock's unusual social organization is its feeding ecology, linked with the independence of male and female territories. Foraging on small, well-dispersed food items on the ground in thick cover, Dunocks probably cannot effectively exclude intruders from their territories. Males may thus tolerate an intruder which accepts subordinate status. It may be a better strategy for a male without a territory to accept subordinate status in a territory of high quality (i. e. in habitat where females are densest), with the possibility of eventually taking over the territory, rather than to take up a territory of poorer quality (where females are sparse or may be absent).