The "waders," sometimes collectively referred to as the "plover-tribe," are represented in the British Islands by a very long list of species, of which only the commonest are mentioned here. Many, however, are mere casual visitors. Near allies of this "tribe" are the gulls and terns. The peculiarly graceful, elastic flight of these birds surely needs no description. Even town-dwellers know them well. For during the winter months they follow the rivers far inland. Even in grimy London they may be seen in hundreds during the winter months. The black-headed gull is by far the commonest of these winter visitors. But at the same time, to the uninitiated, the name "black-headed" must seem singularly inappropriate; for its head is emphatically _white_. At no time, indeed, is it ever _black_. But keep careful watch of the hosts which throng the river from January, onward, till they depart for their breeding quarters, and you will see them gradually developing a dark patch on each side of the head. And this slowly spreads till the whole head is of a dark sooty brown. Immature birds may be picked out by the presence of brown feathers in the wings, and a black bar across the tip of the tail. Here and there among them, one may see much larger birds of a brownish grey colour, and with black beaks and pale coloured legs, in place of the cherry-red of the beak and legs of the "black-headed" species. These are the immature stages of the greater, and lesser black-backed gulls; or of the herring gull. When fully adult the two first-named have the back and wings of a dark slate colour, the rest of the plumage dazzling white. The beak is pale yellow, with a red spot on the angle of the lower jaw. During flight the wings are also black, but the primaries have white tips. The herring gull has a pale pearl-grey back. With a strange perversity the black-headed gull is commonly called, by the novice, the "kittiwake." This is a totally different bird, rather like a herring-gull in miniature, but with a green beak and short, black legs. Moreover, it is rarely seen inland. It breeds in vast colonies on the ledges of precipitous cliffs along the Scottish coast and the west of Ireland. There are colonies, too, on Lundy, the Scilly Isles, and the Farnes. One other gull must be mentioned here, though it is not common, save in the northern parts of Scotland. But it is a regular winter migrant down the east coast of England during the winter. This is Richardson's skua. You may tell it at once by its dark brown coloration, and long, pointed tail. It gets its living mostly by robbing other gulls, chasing them till they disgorge their latest meal, which is seized in mid-air as it falls sea-ward!