Opportunity costs influence food selection and giving‐up density of dabbling ducks

Abstract

The interaction of animals with their food can yield insights into habitat characteristics, such as perceived predation risk and relative quality. We deployed experimental foraging patches in wetlands used by migrating dabbling ducks Anas spp. in the central Illinois River Valley to estimate variation in seed removal and giving‐up density (GUD; i.e. density of food remaining in patches following abandonment) with respect to seed density, seed size, seed depth in the substrate, substrate firmness, perceived predation risk, and an energetic profitability threshold (i.e. critical food density). Seed depth and the density of naturally‐occurring seeds outside of experimental plots affected seed removal and GUD in experimental patches more than perceived predation risk, seed density, seed size or substrate firmness. The greatest seed removal and lowest GUDs in experimental patches occurred when food resources in alternative foraging locations outside of plots (i.e. opportunity costs) appeared to be near or below a critical food density (i.e. 119–181 kg ha–1). Giving‐up densities varied substantially from a critical food density across a range of food densities in alternative foraging locations suggesting that fixed GUDs should not be used as surrogates for critical food densities in energetic carrying capacity models. Foraging and resting rates in and near experimental foraging patches did not reflect patterns of seed removal and were poor predictors of GUD and foraging habitat quality. Our results demonstrated the usefulness of GUDs as indicators of habitat quality for subsurface, benthic foragers relative to other available foraging patches and suggested that food may be limited for dabbling ducks during spring migration in some years in the midwestern USA.