Dynamics of Foraging Behavior Associated With Variation in Habitat and Forage Availability in Captive Black Brant (Branta Bernicla Nigricans) Goslings in Alaska

Abstract

Abstract We conducted an experiment to study the effect of gosling density on food abundance and feeding behavior of Black Brant (Branta bernicla) goslings in two habitat types important to Black Brant on the Yukon-Kuskokwim Delta, Alaska: (1) Carex subspathacea grazing lawns and (2) slough levees that contain Triglochin palustris. Within each habitat, we manipulated grazing pressure by allowing goslings to graze specific plots every 6, 9, or 12 days. We randomly assigned six goslings to one of six treatment groups (three grazing frequencies × two habitats). Biomass of C. subspathacea was higher in lightly grazed plots (grazed every 12 days) than in the heavily grazed plots (grazed every 6 days). Offtake in C. subspathacea was also greater in the lightly grazed plots. Within C. subspathacea stands, proportion of time spent feeding varied among grazing intensities. Goslings within heavily grazed plots spent more time feeding than goslings in lightly grazed plots. Within slough levee habitat, there were no differences between heavily and lightly grazed plots in either biomass or offtake of T. palustris. Whereas overall percentage of time feeding did not vary between the two habitats, percentage of time feeding declined as gosling mass increased in all treatments and habitats. Although the trend in peck rate over time varied among treatments and between habitats, during trials very early in the season we observed a reduced peck rate in the heavily grazed treatment, but only within the C. subspathacea grazing lawns. There was no variation in peck rate among treatments within slough levee habitat; however, peck rates for goslings fed on C. subspathacea was double the rate of goslings in slough levee. Thus, the varied growth responses of plants between habitats to variation in grazing pressure that we observed during this experiment suggest the importance of density-dependent effects on brood-rearing habitat and the resulting variation in gosling growth.