Age and sex composition, biometrics, site fidelity and origin of Brambling Fringilla montifringilla wintering in Norfolk, England.

Abstract

We analysed data collected from approximately 2,600 Brambling Fringilla montifringilla caught while attracted to artificial food at a site in Norfolk, England over seven winters. The age and sex composition of trapped birds varied between winters in relation to the total number of birds caught, but no systematic pattern was apparent. During invasion years, when a higher number of birds were present on the study site, the proportion of males was higher. In any one winter, 38-70% of the Brambling were male and 63- 86% were juvenile. Very few adult females visited the study site. Males had longer wings than females, by an average of 5-6 mm and the wings of adults averaged 1-2 mm longer than juveniles. There was sexual dimorphism in body weight, with males being on average 2 g heavier than females. Adults were on average 1 g heavier than juveniles. Within winters, mean body weights were higher in warmer months. Between winters, there was significant variation in wing length and body weight, which may have been associated with variation in the geographical origin of the birds or food availability. Approximately 70% of juveniles had undergone a partial moult of the greater coverts, with an average of between 2.7 and 3.4 old greater coverts being retained. Annual variation in the progression of moult was probably due to food availability and weather conditions on the breeding grounds or where the juveniles moulted. The mean proportion of Brambling retrapped within-winters was 5.7% (range 0-15%) and there was no evidence to suggest that site fidelity was different in invasion years. Recoveries of ringed birds suggest that Brambling over-wintering in Norfolk originate from Scandinavia and pass through the Low Countries on migration.