ENVIRONMENTAL DEGRADATION, FOOD LIMITATION AND REPRODUCTIVE OUTPUT - JUVENILE SURVIVAL IN LESSER SNOW GEESE

1. Long-term changes in pre-fledging gosling survival, in relation to food availability, were investigated in a population of lesser snow geese (Anser caerulescens caerulescens) at La Perouse Bay, Manitoba. Both colony size and the density of birds using brood-rearing areas increased over the duration of the study (the former from 2000 to 8-9000 pairs). 2. Annual mean size of broods using traditional brood-rearing areas (as a proportion of initial brood size) declined significantly between 1979 and 1991, from 0.9-1.0 to 0.6-0.8. 3. Mean proportion of marked goslings in a brood that survived from hatch to ringing (5-6 weeks of age) declined from approximately 65% prior to 1980 to 35-40% in recent years. Annual mean daily survival rate of goslings declined from 70-100% to 40-55% over the same period. 4. Within-seasons there was a small but significant decline in gosling survival rate over time for weeks 1-4 post-hatch, while the decline in week 5 was highly significant and 12-14 times greater than for weeks 1-4 combined. 5. Mean size of broods on the feeding area early in the season which were subsequently resighted or not resighted did not differ significantly between 1979-86, but broods not resighted were on average 0.33 goslings (13%) larger than resighted broods between 1989 and 1991. 6. Over the same period (1979-91) mean above-ground biomass of graminoid salt-marsh sward declined significantly from approximately 50 g m-2 to 25 g m-2 . This rate of decline was greatest during late brood-rearing (>21 days post-hatch). 7. Annual and seasonal variation in gosling survival is consistent with a density-dependent effect of habitat degradation and food limitation. Birds are responding to this in two-ways: (i) those remaining on the traditional areas are suffering decreased reproductive output (feeding site fidelity may now be maladaptive); and (ii) those dispersing to alternate Tearing areas show higher rates of growth and survival.