Wing Molt and Weights of Barnacle Geese in Spitsbergen

Abstract

This paper examines the wing molt and measurements of 1241 barnacle geese caught during the flightless period in Spitsbergen (Svalbard) in 1977. Birds were individually marked and observations on the wintering grounds established pair and breeding status of individuals caught in molt. Yearlings were about 9% lighter than adult non-breeders although other body measurements were similar. Breeding birds which molted later in the season were heavier than non-breeders, presumably because feeding conditions were better. Repeat captures showed that the round-ups caused large weight losses in some groups, although geese caught more than 4 days after their original capture showed only small daily weight losses. Catching operations could, in certain circumstances, cause increased mortality of molting geese. Primary feathers grew at the rate of 7.5 mm/day on average. This was similar in all age and sex classes and the rate was constant through the flightless period, which lasted about 25 days. Because the birds could fly before their primaries were fully grown, the period of wing molt is longer, 35-40 days. Unsuccessful nesters molted 9-10 days later than non-breeders, who became flightless as soon as coastal pools became ice-free. Parental adults began their molt 7-8 days later than unsuccessful breeders. The timing of molt of parental males was correlated with the size of their young, and began when young were a month old. Molt of parental females was independent of that of their mates. The timing of their molt may be determined by the need to attain a minimum body weight after the stresses of laying and incubation. Most classes of geese lose weight gradually during molt but females who nested unsuccessfully made significant gains. Birds beginning the molt with large fat reserves probably are at an advantage in depleting these because lighter birds have a shorter flightless period. Geese beginning the molt in poor condition (many of the yearlings in the sample) would suffer if feeding and weather conditions were unfavorable. Non-breeders probably molt early in order to maximize the time available for acquiring reserves for autumn migration. Post-molting geese are able to use better feeding areas and a sample recaptured in autumn had gained up to 500 g in weight following molt. Many goose species apparently undergo a long northward molt-migration because it enables them to molt at the beginning of the growing period in these areas, when the vegetation is more nutritious and more easily digested.