Biometrics were taken from 242 Red-breasted Geese Branta ruficollis caught in summer on the Taimyr, Yamal and Gydan Peninsulas, arctic Russia in 1996 and 2007-2014, and from 94 birds during four catches on the wintering grounds in Bulgaria in 2011-2014. These biometrics represent the first published data of body measurements, flat wing lengths and mass for Red-breasted Geese using sample sizes of more than 14 birds. Males were larger than females amongst adults and first-winter birds. Adult

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... body mass was lower in winter than during moult, whereas females showed no significant difference. In common with some other arctic-moulting goose species, the mass of most adult geese remained constant throughout the flightless moult period; however, the mass of non-breeding adult females declined. An index of adult winter flat wing length x body mass was a 100% accurate predictor of sex determined by cloacal eversion (n = 22), but was less successful in determining the sex of first-winter birds (92-93%, n = 27).