Predation and antipredator tactics of nesting black brant and lesser snow geese

Abstract

Coloniality and nest defence were examined in black brant Branta bernicla nigricans and lesser snow geese Chen caerulescens caerulescens. Increased nest density had no effect on nest survival, egg survival, or likelihood of partial clutch predation in snow geese. In brant, nest survival declined as nest density increased in 1992 and with increased distance from shore in 1993. Brant with conspecific nearest neighbours were less likely to suffer partial clutch predation in 1993, but not in 1992. Egg survival in brant increased with nest density in 1993, but decreased as density increased in 1992, however, the decrease occurred only in nests with three or four eggs. Nesting at high densities, in central positions, or far from shorelines commonly travelled by glaucous gulls Larurs hyperboreus and parasitic jaegers Stercorarius parasiticus, the primary egg predators in this study, did not provide geese with a nest or egg survival advantage because effects were lacking in snow geese and were inconsistent and contradictory in brant. Female snow geese had very high nest attendance and both sexes had high territory attendance so snow goose nests were rarely unattended during incubation. Brant had lower nest attendance than snow geese, and due to a lack food near their nests, brant left their territories to feed resulting in lower territory attendance as well. Although male brant were capable of defending the nest from avian predators and usually remained on their territories when females were absent, males were less effective defenders than incubating females. Increased vigilance and decreased resting by female brant as incubation progressed provided support for the prediction, from parental investment theory, that nest guarding effort would increase with offspring age, but there were no changes in male brant or in snow geese. However, declining nest and territory attendance by female brant and males of both species contradicted predictions from parental investment theory but were consistent with an increased need to forage as nutrient reserves declined through incubation as expected due to energetic constraints.