Few birds are more thoroughly marine in their haunts and their habits than those which are included in the present chapter. They are inseparably associated with the sea; they form one of the most interesting features of marine life, whether in summer, when they crowd in countless hosts at their breeding stations upon the cliffs and islands, or in winter, when they spread themselves far and wide over the waste of waters. From whatever point of view we study them, they are intensely interesting birds. The Auks, as they are collectively termed, form the small yet well-defined family Alcidæ. Although the Auks are a specialised group, systematists pretty generally agree in associating them more or less closely with the Divers, the Grebes, the Gulls, and the Limicolæ. Auks are web-footed birds, with no hind toe, with the legs placed far back, and the bill subject to great variation in size, and in some species presenting considerable change in appearance according to season. All the Auks have comparatively short and narrow wings; in the recently extinct Great Auk these were incapable of supporting the bird in the air; and the tail is remarkably short, in some species being scarcely perceptible under ordinary circumstances. The Auks are exclusively confined to the north temperate and polar regions of the Northern Hemisphere: and by far the greater number of species inhabit the Northern Pacific. They number some thirty species. The prevailing colours of the Auks are black and white; none of them are showy birds; but some species are remarkable for their eccentric nuptial plumes, and for the brilliancy of colour of the bill. The Auks are thoroughly aquatic, and not adapted in any way for a terrestrial existence. They swim well, dive with marvellous skill, and save during the incubation period, pass most of their time on the sea. None of the species are remarkable for any great migration flights; as a rule they wander little from their high northern homes. They are all gregarious birds, breeding in companies wherever possible. Some species undergo but little change in their appearance between summer or winter plumage; others are more remarkable in this respect. During the breeding period some species resort to lofty cliffs washed by the sea; others burrow into the ground. Many species make no nest whatever, but others form slight structures in which to deposit their eggs. The young of the Auks are hatched covered with down, assuming their first plumage in a few weeks. Adult Auks moult in September; the difference in the colour of the plumage peculiar to the pairing season, apparently being entirely due to a change in the hue quite irrespective of a moult. The complete change from white to brownish-black observed prior to the breeding season on the necks and heads of Guillemots and Razorbills is very curious and interesting. According to the observations of Herr Gätke, the shafts of the feathers