Oil-spills have killed thousands of birds during the last 100 years, but non-lethal effects of oil-spills on birds remain poorly studied. We measured phenotype characters in 279 eiders Somateria mollissima of which 13.6% were oiled. We tested the hypotheses that (1) the morphology of eiders does not change due to oil contamination; (2) the anatomy of organs reflects the physiological reaction to contamination e.g. increase in metabolic demand, increase in food intake and counteracting toxic effects of oil; (3) large locomotion apparatus that facilitates locomotion increase the risk of getting oiled; and (4) individual eiders with a higher production of secretions from the uropygial grand were more likely to have oil on their plumage. We tested whether 19 characters differed between oiled and non-oiled individuals, showing a consistent pattern. The final model retained seven predictor variables showing relationships between eiders contaminated with oil and food consumption, flight and diving abilities. We tested whether these effects were due to differences in body condition, liver mass, empty gizzard mass or other characters that could have been affected by impaired flight and diving ability. There was no evidence of such negative impact of oiling on eiders. We found that significant exposure to oil was associated with increased diversity of antibacterial defence. Oiled eiders did not constitute a random sample, and superior diving ability as reflected by large foot area were at a selective disadvantage during oil spills. Thus, specific characteristics predispose eiders to oiling, with an adaptation to swimming, diving and flying being traded against the costs of oiling. In contrast, individuals with a high degree of physiological plasticity may experience an advantage because their uropygial secretions counteract the effects of oil contamination. --

We sampled 819 eiders Somateria mollissima during 2014-2019 in Danish waters of which 279 were whole specimens and 540 were wings sent in by hunters to the Danish Wing Survey. We measured 19 phenotype characters. In total 817 eiders were inspected for oil and of these 111 specimens (13.6%) had oil on their plumage. The uropygial grands were removed from 119 eiders and uropygial secretions were extracted to analyze anti-bacterial activity.