LONG-TERM DECLINE IN BODY SIZE IN A SNOW GOOSE POPULATION - EVIDENCE OF ENVIRONMENTAL DEGRADATION

(1) The effect of environmental factors on annual body size variation was investigated in a breeding population of lesser snow geese (Anser caerulescens caerulescens L.) La Perouse Bay, Manitoba, using data from 1969 to the present. The population size has increased from approximately 2000 to 10 000 pairs during this period (Cooch et al. 1989). (2) Annual mean gosling body mass, tarsus and culmen length (measured at fledging) declined significantly by approximately 16% (240 g), 4% (3.4 mm), and 2% (0.93 mm) respectively in cohorts hatching between 1976 and 1988. The decline in gosling size has led to a significant decline in size of locally bred adults. Annual mean adult female body mass, tarsus and culmen lengths declined by approximately 15% (270 g), 6% (5.4 mm), and 4% (2.3 mm), respectively, in cohorts hatching between 1969 and 1986. (3) Gosling growth and subsequent adult size was also reduced in years with cold, wet weather after hatch. (4) The decline in gosling size was not dependent on the pattern of annual variation in egg or hatch mass, nor systematic changes in annual mean hatch date or post-hatch weather. Goslings reared by individual adult females showed a decline in size over time, suggesting that the general decline reflects a non-genetic change in gosling growth rates during the fledging period. The most probable non-genetic factor is a long-term reduction in food available to the geese at La Perouse Bay. This reduction in food availability may reflect over-exploitation of primary food plants by the geese.