Decision between small and large prey: reduced energy acquisition by pre-migratory purple sandpipers, Calidris maritima, on Svalbard

On Svalbard, purple sandpipers forage on the shores when preparing for autu mn migration to the European continent after a period of food shortage on t he tundra in summer. Their food choice was studied on a beach when they wer e faced with the choice between two abundant prey species: a small collembo lan (springtail) and a much larger amphipod. In spite of several times high er gain of energy when foraging on amphipops, the birds did not show a dist inct preference for either of the two prey species in this situation. Alter native explanations for this behaviour are discussed. We suggest that this could be a consequence of adaptations to their summer diet on Svalbard, whe n they rely on Collembola as food to a considerable extent. Higher variance in energy gain rate when foraging on amphipods might also be important. Co llembolan densities high enough to trigger predation from birds occurred on ly on some days, with cloudy calm weather. This, together with the possible existence of a dietary bottleneck, may reduce the cost associated with the switch to Collembola.