Flock composition, agonistic behaviour and body condition of wintering Bullfinches Pyrrhula pyrrhula

Abstract

During October-April of 1989-2003 I made observations on size and sexual composition of Bullfinch Pyrrhula pyrrhula flocks (ringed and unringed birds), recorded interactions between members and estimated the body condition of birds observed in a mixed forest in central Norway. The flock size did not change with ambient temperature and ranged from two to seven with a mean of 4.1 in October-January and 2.3 in February-April. During March-April, 28 of 29 flocks consisted of one male and one female. Seventeen flocks fol- lowed over 6-17 days in November-December did not change in size or sexual composi- tion. The adult sex ratio was equal. In flocks consisting of two, four or six birds, 88-97% consisted of equal numbers of males and females. Few birds stayed in the area throughout the winter. One colour-ringed mated pair breeding in the area was observed during two winters. Two ringed females, each observed in the company of an unringed male, were re- peatedly observed during two subsequent winters. Females were socially dominant over males. Adult and juvenile females had a better body condition (body mass to wing length ratio) than juvenile males, but not better than that of adult males. The body condition of the birds increased with decreasing ambient temperature.