Structure of a Brambling Fringilla montifringilla roost according to sex, age and body-mass

In a winter roost of several hundreds of thousands of Bramblings Fringilla montifringilla , the composition of birds roosting at the periphery and of birds flying into the centre of the roost was studied regarding sex, age, size and body-mass. At the periphery of the roost, more females were present than in the centre, and their proportion increased with decreasing density at the periphery. No significant difference in age classes was found between centre and periphery, but adults flew into the roost later than first year birds. Within sex-age classes, birds in the centre were heavier but not larger than birds at the periphery. The difference in body-mass between centre and periphery was more pronounced in small than in large birds within each sex-age class.
Because strong competition for particular positions within the roost was observed, it is inferred that the quality of roosting positions varied and that females and/or light birds were displaced toward the periphery. Predation and microclimate are discussed as possible reasons for the variation in the quality of roosting positions and for the lower energy reserves of birds roosting at the periphery.