Population differentiation in whiskered auklets Aethia pygmaea: do diurnal and nocturnal colonies differ in genetics, morphometry and acoustics?

Despite a great number of empirical studies, the mechanisms of population differentiation and the factors that influence this process, particularly in seabirds, remain insufficiently understood. Here we analyzed population structure in the whiskered auklet Aethia pygmaea, a previously poorly studied alcid species with unusual differentiation in colony attendance rhythms (i.e. diurnal in the Sea of Okhotsk vs nocturnal in the Bering Sea), and examined the influence of it on intraspecific differentiation. For this study, we analyzed morphometric measurements, acoustic variables, mitochondrial control region fragment and five microsatellite loci from nine whiskered auklet colonies spanning the breeding range. Previous research has shown a clinal morphometric variation in this species. We build on this analysis by adding auklets from more colonies, for the first time analyzing vocalizations from different colonies and genetic structure of this species. Our data supports a clinal variation in morphometric and acoustic characters with the largest size and the lowest call frequency in western birds, and the smallest size and highest call frequency in the east. We also found two mitochondrial lineages – whiskered auklets from colonies in the Sea of Okhotsk and the Commander Is. (Bering Sea) and from the Aleutian Is. (Bering Sea), that were presumably formed during Sangamonian interglacial period (115 000–130 000 years ago). Genetic clusters found did not reflect differences in colony attendance rhythms, suggesting that they were shaped by other factors (e.g. differences in predator pressure) and are unlikely to have participated in the formation of population structure. Colony fidelity, mobility of birds, proximity of foraging grounds and location of colonies in relation to seasonal ice pack, seem to be more likely determinants of population differentiation.