There are six species of Sterna, properly so called, described by British naturalists; and six others, according to McGillivray, nearly resembling them in form, colour, and habit. The true Sterna has the bill straight, slender, compressed, and tapering; it is about the length of the head, with the edge sharp, and the tip elongated and pointed; the upper mandible armate; legs short, slender; anterior toes small; membrane emarginate; wings long and pointed; tail forked. These birds are remarkable for their buoyant, graceful, easy flight, and the soft, loose texture of their plumage. Their prevailing colours are a pale bluish grey or black, and white. The Tern, or Sea Swallow, as it is commonly called, on account of its long pointed wings and forked tail, appears to be, like the Swallow properly so called, a perfect disregarder of rest. They may be seen soaring in the air at a very great height, and then suddenly darting down upon their prey, which their piercing sight has enabled them to descry in the water. Often, too, they may be noticed skimming over the surface of the waves with astonishing rapidity, and seizing in their flight any fish which is imprudent enough to show itself. Their flight over the sea seems incessant, and it is rarely they are seen swimming. When they need rest, they seek some solitary, isolated rock in the ocean. They congregate in flocks more or less numerous, and they manifest so much attachment for individuals of their own species, that, when one of them is wounded by the sportsman's gun, the others surround it, full of grief and sympathy, nor will they leave it until all hope of saving its life is at an end. These birds in their flight give utterance to shrill and piercing calls, which, when produced by numbers together, cause a deafening uproar in the sky. These calls are raised with increased power when they are about to undertake some longer flight than usual. But the time, above all others, when their noise is most discordant and shrill is the breeding season. "On going up to one of their breeding-places," says McGillivray, "which may always be discovered from a distance, as some of the birds will be seen hovering over it, one is sure to be met by several of them, which hasten to remonstrate with the intruder by harsh cries and threatened blows. As you draw nearer, more of them leave their nests; and at length they are all on the wing, wheeling and bounding--now high and now low--at times coming quite close, and increasing their cries, which resemble the syllables 'cree-cree-cree-ae.'" [Illustration: Fig. 106.--The Tern (_Sterna hirundo_, Linn.).]