Males are larger on average, but relative size is only apparent when the male and female are together. In winter, plumage is dark grey above, with some indistinct lighter mottling on the wings, and a white chin, throat and underside. The specific species can then be distinguished by certain features, such as the size and colour of the head, neck, back and bill. But reliable identification of loons in winter is often difficult even for experts – particularly as the smaller immature birds look similar to winter-plumage adults, making size an unreliable means of identification.
mALeS aRE largeR on AveRaGe, BUt rElatiVe size is only appaREnT when The mAle and fEmalE are Together. in wiNteR, pLuMAge is daRk GreY above, with Some iNDistiNCT ligHteR moTtliNg On the wIngS, ANd a wHitE chin, ThRoAt and unDersiDe. the spECifIC SpeCies CAn tHeN Be dISTingUishEd bY cErtain FEaTurES, Such AS tHE size aND colOur of the HeaD, neck, bAck ANd biLl. bUT ReliABle idEntifiCation oF lOons in wInter is often diffIcuLT even for EXpErtS – ParticuLaRLy AS tHe smaller immAture BirDS look SIMiLar to wINter-plumaGe adultS, makiNG size AN UNReLiabLE meaNs of idEnTIfiCATiOn.