The domain of the Sea-fliers is the ocean, their territory the wide world. Some families seem more especially to frequent coasts, others prefer fresh water to the salt sea, but these are mere exceptions. Sweeping continually over the waves, or skimming in the proximity of coasts, they wander over the sea and circle round the globe. By one tie only are they bound to the dry land, and that is, their affection for their offspring. They lay their eggs upon the land, and there watch over their young brood until their wings attain sufficient strength to enable them to join their brethren in the air; the rest of their time is for the most part devoted to the sea, upon which they sometimes repose, or, in exceptional cases, sleep upon the shore. Their flight is longer sustained than that of the Swallows or Swifts, or even of the Humming-birds, for they fly not only during the whole day but often likewise throughout the night. It is to this untiring energy and constant restlessness that we must attribute the boundless range over which particular species spread themselves. Some of them indeed have literally a world-wide distribution, and visit every climate between the equator and the polar seas. Others again, restrict their wanderings, or migrations, or flights, or whatever the reader may choose to call them, either within more or less definite regions, or within certain degrees of latitude; it must, however, be kept in mind that these boundaries are not coasts or countries, but whole continents and oceans. All Sea-fliers are divers, but, owing to the thickness of their plumage, some are less capable than others of plunging far beneath the surface. They generally fly over the waves at a certain altitude, disporting themselves in the sunshine should the weather be fine, or battling with the elements during a storm. While flying, their attention is anxiously bent upon the sea beneath, on the look-out for food, upon which, when discovered, they instantly dart down, some of them with the straightness and rapidity of an arrow, others more leisurely, while some, before they seize their prey, settle down and swim. All are equally rapacious, whether they catch food for themselves or rob other birds of their hard-earned booty, whether they devour living victims or content themselves with feeding, like the Vultures, upon carrion. Whatever the sea presents for their acceptance they greedily swallow. In their mode of breeding these birds very much resemble each other. Their nests are generally placed upon the ground, sometimes in wild mosses and swamps, sometimes on ledges or projections of rocks, and in the holes and excavations of the soil. Occasionally they select precipitous cliffs and mountains, more rarely trees; some lay but one egg, others two, three, or four. All show much attention to their mates and offspring. The young are at once introduced to their future element, where they soon learn to catch their own food.