Biometrics and pre-migratory fattening in the snow bunting plectrophenax nivalis

Abstract

Biometric data for Snow Buntings overwintering at three sites in Highland Region, Scotland are reported. A significant sexual dimorphism (males larger) was found for eight of nine linear measures of body size recorded (namely wing length, wing span, tail length, tarsus length, bill length to feathers, bill length to nostril, bill width and bill depth); only hind claw length showed no significant sexual dimorphism, mainly because of the variability of this character. Wing lengths of museum skins collected in various parts of the breeding range (Arctic Canada, S.W. Greenland, N.E. Greenland, Iceland and Scandinavia) showed significant variation in males, with those from N.E. Greenland being the largest, but not in females. Skins of males collected in winter in the British Isles were most similar in size to breeding birds of Arctic Canada, S.W. Greenland, Iceland and Scandinavia, and least like those of N.E. Greenland. Weights remained roughly constant for a given site during the winter months, but varied appreciably between sites, between seasons and between the sexes. From about mid-February weights increased dramatically as the birds developed extensive fat reserves prior to their spring migration. Various parameters associated with this pre-migratory fattening are estimated (mean date of commencement of fattening, mean duration of fattening period, mean rate of weight gain, mean lean weight, mean pre-migratory fat weight etc). Estimated fat loadings in both males and females were about 22%., sufficient to take the birds to Iceland or southern Norway, but not Greenland, in a single non-stop flight in still air. Fat weights of males of the Icelandic race P.n. insulae, were appreciably higher than those of the nominate race, P. n. nivalis, suggesting that birds of the latter race were more likely to be heading for Scandinavia than Greenland. © 1989 Taylor & Francis Group, LLC.