These birds may be readily distinguished, even when on the wing, by the cuneiform or wedge-shaped tail, and by the dark upper plumage. The bill is also much stouter and hooked at the point, whilst the claws are sharp and curved. Skuas are only exceptionally seen by the ordinary visitor to the sea-side. In the first place, they only breed in our islands in the extreme north or west of Scotland, and in the second place they are decidedly oceanic in their habits, after the nesting season is passed. Occasionally Skuas may be seen on migration, especially in autumn, and along our eastern and southern seaboard; occasionally they are driven shorewards by protracted stormy weather, and under these circumstances have frequently been known to visit inland localities. Odd birds are generally seen, perhaps a party of half a dozen, but on very exceptional occasions large flocks make their appearance—witness the thousands of Pomarine Skuas that visited the coast of Yorkshire during the autumns of the years 1879 and 1880. The Skuas are birds of remarkably powerful flight, displaying marvellous command over themselves in the air, turning and twisting with great speed. These birds are the Raptors of the sea; a terror to the Gulls and Terns; merciless robbers of the hard-won spoil of more weakly species; destroyers even of the eggs and helpless young of other sea birds. All the four species of northern Skuas are visitors to the British Seas, but only two of them are indigenous to our islands. The first of these to be noticed here is the Great Skua, _Stercorarius catarrhactes_, one of the most local of British birds during the breeding season, as its only known nesting places in our area are on Unst and Foula, two small islands of the Shetland Group. Except during the breeding season, the Great Skua is mostly oceanic in its habitat, wandering long distances from land in quest of prey, attending vessels and fishing fleets, only drawing landwards by stress of weather or unusual abundance of food. This Skua is practically omnivorous. During its summer sojourn near and on the land it repeatedly raids the colonies of other sea fowl, to prey upon exposed eggs or unguarded young; it captures the smaller Gulls, notably the Kittiwake: it also picks up any stranded fish or other carrion; and is constantly on the watch to chase any Gull or Tern that catches a fish, following the poor bird with fatal persistency until, terror stricken, it disgorges its food, which is promptly seized by the voracious Skua. The call note of this Skua is very similar to that of the Lesser Black-backed Gull, but when under the excitement of chasing other birds, or of seeking to guard its own domain, the bird utters a loud cry which is likened by many observers to the word _skua_ or _skui_.