A density-dependent decline in the average clutch size of lesser snow geese (Anser caerulescens caerulescens) occurred from 1973 to 1989 at the breeding colony on the shores of La Pérouse Bay, Manitoba. An increase in average parasite load was hypothesized to be one of the two most likely causes of this decline. We shot 28 incubating adult female lesser snow geese at the La Pérouse Bay colony and examined the carcasses for parasites to determine if there was any proximate association between parasitic infections and the size of the clutch a female laid. We found no convincing evidence that parasitic infections were the proximate cause of any reduction in clutch size. In the absence of evidence of any direct effect of parasites, we conclude that an increase in the average parasite load is probably not the cause of the long-term decline in clutch size at La Pérouse Bay. By default, we suggest that increased intraspecific competition for food at the staging areas on the migratory flyway is the most likely cause of the decline in clutch size.
Polygenic Variation and Stabilizing Selection in a Wild Population of Lesser Snow Geese (Anser caerulescens caerulescens)