Decline in Body Mass, Size, and Condition of Greater Snow Geese, 1975-94

Abstract

In some colonies of arctic-nesting geese, declines have been observed in body mass, size, and condition (body mass relative to size) of goslings at fledging, with a resulting decrease in juvenile survival during fall migration. These declines have been attributed to overgrazing by increasing goose populations on brood-rearing areas. We examined 25,761 greater snow geese (Anser caerulescens atlanticus) shot by hunters at the main fall staging area on the St. Lawrence River, Quebec, from 1975 to 1994, to determine whether there were long-term trends in body mass, size, and condition in this increasing population. Mean annual body mass and culmen length (an indicator of body size) of adult males, adult females, juvenile males and juvenile females showed annual fluctuations and declines over time. Body mass declined by 9.6% (a loss of 312 g, SE = 61 g) over 19 years (1975-94) in adult males, 11.2% (333 g, SE = 51 g) in adult females, 9.2% (245 g, SE = 30 g) in juvenile males, and 8.7% (211 g, SE = 34 g) in juvenile females. Culmen length declined by 2.2% (1.5 mm, SE = 0.4 mm) over 18 years (1976-94) in adult males, 0.9% (0.6 mm, SE = 0.3 mm) in adult females, 1.5% (1.0 mm, SE = 0.5 mm) in juvenile males, and 1.5% (0.9 mm, SE = 0.3 mm) in juvenile females. Body mass adjusted to a standard culmen length showed a decline in body condition. Predicted body condition declined by an average of 6.5% (loss of 209 g, SE = 79 g) over 18 years (1976-94) in adult males, 8.5% (248 g, SE = 56 g) in adult females, 6.0% (158 g, SE = 50 g) in juvenile males, and 5.9% (142 g, SE = 41 g) in juvenile females. We suggest that changing conditions on arctic brood-rearing areas are responsible for these declines. Increasing numbers of goslings may be encountering a reduced per capita availability of food and therefore experiencing lower growth rates and reduced final size. The long-term decline in the size and condition of these geese has not yet led to measurable reductions in reproductive output. With continued population growth, we predict further deterioration of body condition of the geese and an eventual increase in the mortality of juveniles during fall migration.