| Conspicuous ornaments with little apparent cost are ubiquitous in nature, but their function and evolutionary origins are poorly understood. Most evolutionary models predict that ornament expression is associated with a cost and correlated with individual quality. I investigated ornamentation in the Least Auklet (Aethia pusilla), a seabird that displays multiple ornaments and a highly variable degree of ventral mottling. Phylogenetic reconstruction showed that the ancestral state is likely ventrally white for most seabird clades, but only mostly white in auklets. I measured several novel indicators of quality, including measures of take-off performance, and their relations to plumage and ornamentation. Power and acceleration, 0.17 s after take-off were the most repeatable measures (repeatability = 75 % and 72 %, respectively). AIC-based model selection revealed significant interactions in the relationships of two low-cost ornaments (bill knob and auricular plume) with indicators of individual quality (hematocrit, leucocrit, mass/condition, and take-off acceleration). Mass was positively related to both bill knob and auricular plume size in subadults but not in adults, and inversely related to ventral plumage mottling in adults but not in subadults. I rejected the individual recognition hypothesis, which predicts a deviation from a multivariate normal distribution of ornaments that was not found in this study. The birds with less mottling are more conspicuous on land where predation is a constant threat. Predation by Glaucous-winged Gulls (Larus glaucescens) could be a vulerability handicap that ensures the reliability of the plumage as a social signal. Gulls attacked experimentally-whitened model auklets significantly less often than darkened models (P = 0.036), as predicted if mottling is dependent on physical fitness and acts as a signal of escape potential. Also in accordance with prediction, auklets on a different island without aerial predators had significantly lighter plumage (P < 0.001), suggesting a lower cost to conspicuousness in the absence of aerial predators. I did not observe a correlation between take-off acceleration and plumage mottledness, however. My results suggest that ornaments are related to condition in subadults, and that plumage functions as a vulnerability handicap in interactions with gulls. |
