Limited evidence that asymmetries of multiple Whiskered Auklet (Aethia pygmaea) feather ornaments could serve a display function

Abstract

Elaborate avian feather ornaments have proven to be enigmatic because their function is often unclear, even though they are used in courtship and social displays. Male and female Whiskered Auklets Aethia pygmaea display on their faces four elaborate feather ornaments that serve both courtship and mechanosensory functions: three bilateral pairs of white facial plume tracts (superorbital, suborbital and auricular) and a slender black forehead crest, each consisting of several filoplumes. We studied left–right symmetry in the three antenna‐like bilateral white head plumes of 721 wild‐caught marked individuals (162 of known sex, 94 of known age that were 1–16 years old) during 1992–2009. Auricular and suborbital plumes were slightly more asymmetric in subadults (1‐year‐olds) than in adults (≥ 2 years old) but the opposite was true for superorbital plumes. Ornament asymmetries were not sexually dimorphic, nor were they significantly related to individual body condition, body size or age, except that superorbital plume asymmetry decreased significantly with tarsus length. Relative asymmetry (scaled for ornament size) of all three ornaments was negatively correlated with plume size, as predicted by some sexual selection models, but variation in asymmetries was large and differences between left (L) and right (R) sides in most birds were probably too small to be detected visually. Marginal mean absolute asymmetries (|L–R|) of super‐ and suborbital plumes were correlated with ocean climate during the preceding year when the birds would have been moulting, suggesting that fluctuating asymmetry at the population level might be a useful index of environmental stress in this seabird. The spectacular bilaterally expressed facial plumes displayed by Whiskered Auklets provide an interesting test case for questions about asymmetry in sexually and naturally selected traits.