Can you really change your stripes? Stable wing ornamentation suggests limited condition-dependent signalling in female common eiders

Abstract

Variation in the expression of white ornaments, such as flank spots or wing stripes, has previously been related to indices of individual quality in birds. We measured the white wing stripes of colour-banded female common eiders Somateria mollissima in three islands in Iceland from 2014 to 2023, to determine whether the expression of these ornaments is (1) an individually fixed trait that shows consistency among years, or (2) a flexible parameter that shows annual variation within known individuals. While clutch size varied among the three islands, and body condition and nest initiation date varied among years, linear models supported wing stripe variation among females being a fixed, repeatable trait independent of clutch size, nest initiation date or body condition. A categorical analysis indicated that wing stripe dimensions were similar among birds that were categorized as caught often, re-sighted rather than caught, or transients. Wing stripe dimensions were not related to apparent survival or capture probabilities in a mark–recapture analysis. Over 90% of females had five or more of 10 possible feathers with wing stripes, suggesting that poorly visible wing stripes are not optimal for this study population. Low variation in the expression of wing stripes may be explained by the good health of this study population relative to other eider populations. Local adaptations may explain the discrepancy between studies of wing stripes in eiders.