ORIGINAL PAPER H.P. Leinaas á W.G. Ambrose Jr. Decision between small and large prey: reduced energy acquisition by pre-migratory purple sandpipers, Calidris maritima, on Svalbard Accepted: 25 April 1999 Abstract 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 discus- sed. 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 den- sities 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. Introduction The concept of optimal foraging refers to how animals should behave in order to maximise ®tness through foraging. The underlying idea is that ®tness is closely correlated with energy (and essential dietary require- ments) available for allocation to survival and repro- duction, and that optimal foraging is a trade o between maximising gain and minimising risks associated with foraging (Charnov 1976a; Pyke et al. 1977; Stephens and Krebs 1986). Energetically costly deviations from opti- mal foraging behaviour, therefore, are expected to be rare. The optimal solution itself, however, may diverge from a simple maximisation of net energy gain due to a number of factors, such as exposure to predation, di- etary constraints, risk of starvation, and unpredictable environment (e.g. Stephens and Krebs 1986). An important limitation of an optimal foraging the- ory approach in empirical studies is that ®tness refers to lifetime reproductive success, while most foraging stud- ies only examine the animal's behaviour during a short period of their lives or in speci®c situations. Since the ability to make pro®table decisions in one situation may be constrained by adaptations and other choices, every single decision in life cannot be optimal when analysed separately. Evaluated on a short-term basis, therefore, even a well-adapted species would be expected occa- sionally to show suboptimal feeding activities. This may be dicult to distinguish from animals simply failing to make the most pro®table choices. Both these phenom- ena are most likely to occur when animals are facing uncommon situations. Here we report on the foraging behaviour of purple sandpipers (Calidris maritima BruÈnnich) on a Svalbard (Spitsbergen) beach in late summer, in a situation when faced with the choice between two prey species of very dierent size and abundance: the amphipod Gammarus setosus Dementieva and the collembolan (springtail) Archisotoma megalops (Bagnall). The two species were preyed upon on dierent levels of a beach: amphipods along the water line and Collembola 2±5 m further up. When the birds switched prey, they quickly moved be- tween the two levels of the beach. In order to maximise energy gain, a bird should not forage in subareas of the beach where the expected intake rate is less than average for the whole area (Charnov 1976b): i.e. they should concentrate on the level of the beach (prey species) that Polar Biol (1999) 22: 264±270 Ó Springer-Verlag 1999 H.P. Leinaas (&) University of Oslo, Department of Biology, Division of Zoology, P.O. Box 1050 Blindern, N-0316 Oslo, Norway e-mail: h.p.leinaas@bio.uio.no Fax: +47-22856016 W.G. Ambrose Jr. Department of Biology, Bates College, Lewiston, ME-04240, USA