Costs of parasites in common eiders: effects of antiparasite treatment

Parasites are exploiting their host for the resources required for the parasite's reproduction and survival. The effects of antiparasitic treatment during incubation on body mass and return rate in female common ciders Somateria mollissima were examined in a two year study (1998 and 1999). There was no effect of treatment on nest success, but unsuccessful females had lower body mass. Treatment had no effect on incubation time, mass-loss during incubation or return rate among successful birds. However, treatment lead to higher return rate among unsuccessful birds (treated = 69%, n = 13; controls = 18%, n = 11). This pattern remained significant also after including the birds that returned two years after the experiment. We suggest that the unsuccessful females are of lower quality and had higher costs from parasite infections than birds which completed incubation, and that the higher return rate for treated unsuccessful females may mean that costs of parasites are substantial, and that the potential fitness gain from investment in parasite defense and avoidance is large.