Seasonal patterns of age and sex ratios, morphology and body mass of Bramblings Fringilla montifringilla at a large winter roost in southern Europe

During the winter, Bramblings Fringilla montifringilla are known for concentrating in huge communal roosts, normally situated along a NE-SW axis from west Russia to France in southern Europe. Irruptions of Bramblings in Iberia are rare and normally only in an attenuated form. Exceptionally, in November of 2010 a large roost of c 900,000 Bramblings formed in the Basque mountains of northern Iberia. We caught and ringed Bramblings from this roost from November 2010 to February 2011, and compared age and sex ratios, wing length and body mass to data reported in the literature. Overall, first-year birds comprised more than half the roost and the sex ratio in first-year birds was biased to females. This is in agreement with the prediction that first-year birds and females migrate to regions further south than males and adult birds. Wing length was constant across the season, but not body mass. Body mass was also negatively correlated with temperature and positively correlated with snowfall.