Prey choice affects the trade-off balance between predation and starvation in an avian herbivore

Animals make economic decisions on where to feed based on balancing intake rates with the risk of starvation. When foraging, they must also manage time allocation to feeding, antipredator behaviour and resource guarding. These behaviours are usually believed to be mutually exclusive. The most intense foraging effort is predicted to occur in low-risk situations, while more antipredator behaviour is seen in high-risk situations. We studied the time allocation of individual light-bellied brent geese, Branta bernicla hrota, and the proportion of the flock engaged in foraging and vigilance behaviour in high- and low-risk habitats. Geese faced with a depleted resource in a low-risk intertidal situation responded by foraging in a higher-risk terrestrial habitat. Individual intake rates and levels of agonistic interactions were higher during terrestrial feeding, but group vigilance was maintained, with certain flock positions showing disproportionate levels of vigilance. In contrast to previous studies, agonistic interactions were more frequent when the geese were feeding on a nonpatchy resource, suggesting that different mechanisms drive agonistic encounters in different habitats. We discuss these results in the context of a growing population with limited resources. By remaining behaviourally plastic, the geese are able to adapt novel trade-off strategies to use alternative prey types.