Evidence of a state-dependent trade-off between energy intake and parasite avoidance in Steller's eiders

Abstract

We examined the hypothesis that Steller's eiders, Polysticta stelleri (Pallas, 1769), in good body condition avoided nutritious and abundant prey that were intermediate hosts of acanthocephalans, while birds in poor condition accepted the long-term costs of parasitism to minimize the short-term risk of starvation. We predicted that the intensity of the acanthocephalan Polymorphus phippsi (Kostylev, 1922) should be positively related to the intake of intermediate hosts and that the intake of such prey should be negatively related to body condition. All Steller's eiders were infected (2–1142 parasites/bird). Only the intake of the amphipod Gammarus oceanicus (Segerstråle, 1947), a known intermediate host of P. phippsi, was significantly related to parasite intensity. Juvenile Steller's eiders were in poorer condition than adults and fed more on amphipods (44% vs. 9% of total biomass). On the contrary, adults preferred isopods (26% of total biomass vs. 12% for juveniles), which were less abundant than littoral amphipods but were not intermediate hosts of P. phippsi. Moreover, in juveniles there was a negative relationship between body condition and the proportion of amphipods in the diet. Hence, juveniles in poor body condition consumed potentially infected prey while adults and juveniles in good condition avoided such prey. The cost of avoiding littoral amphipods was probably a lower energy return per unit feeding effort.