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 autumn migration to the European continent after a period of food shortage on the tundra in summer. Their food choice was studied on a beach when they were faced with the choice between two abundant prey species: a small collembolan (springtail) and a much larger amphipod. In spite of several times higher gain of energy when foraging on amphipops, the birds did not show a distinct preference for either of the two prey species in this situation. Alternative explanations for this behaviour are discussed. We suggest that this could be a consequence of adaptations to their summer diet on Svalbard, when they rely on Collembola as food to a considerable extent. Higher variance in energy gain rate when foraging on amphipods might also be important. Collembolan densities high enough to trigger predation from birds occurred only 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. Accepted: 25 April 1999