Heterospecific mating preferences for a feather ornament in least auklets

Auklets (Alcidae, Aethiini) include five species of small, socially monogamous, sexually monomorphic seabirds that display a variety of feather and bare-part ornaments during the breeding season. Previous experimental work on two auklet species has demonstrated that some ornaments are likely to be favored by sexual selection because mutual male and female mating preferences benefit individuals with the most elaborate expression of these traits. In this study we experimentally investigated whether naturally crestless least auklets Aethia pusilla have a mating preference for forehead crests similar to the most prominent ornament of two other species, crested A. cristatella and whiskered auklets A. pygmaea. Our objective was to investigate the function of this ornament as a species-recognition mechanism or as a product of one or more of three proposed sexual selection models that address the origin of elaborate traits and preferences. During the experiment, least auklets reacted to realistic models equipped with artificial forehead crests with approximately an order of magnitude more frequent sexual displays and greater interest, consistent with the idea that they have a mating preference for crests, even though they do not naturally express this ornament. This heterospecific preference also favored large crest size. These results refute the possibility that least auklet forehead ornamentation alone determines species recognition at present. Among models of sexual selection considered, the results are consistent with the sensory exploitation model, although this could not be established unequivocally because a viability indicator or Fisherian mechanism could have been involved if least auklets had an ancestor with a forehead crest.