Their nests are generally built either upon trees or in fissures of rocks, sometimes upon ledges of cliffs or on the tops of mountains; less frequently on the little islands met with in marshes and swamps. Wherever their breeding-place may be, they are generally indebted to other birds for relieving them of a considerable share of the labour of nest-building, often contenting themselves with adding materials to their deserted domiciles. Some of them lay but a single egg, others two, others four; the eggs are small, of elongated shape, and generally covered with a calcareous deposit which obscures the markings on the shell. Both parents assist in the work of incubation, and some species would seem frequently to produce two broods in the course of the breeding season. Most of these birds feed exclusively upon fishes; some will occasionally devour small quadrupeds, molluscs, and worms, but fishes invariably constitute their ordinary diet. Some of them catch their food by sailing over the water at a little distance above the surface, whence they suddenly plunge into the water beneath; some swim upon the sea like Ducks, and seize their prey in shallow water by means of their long necks; whilst others again, after the manner of the real divers, pursue their prey to a considerable depth. They are terribly destructive to fishes, and are, consequently, everywhere regarded and treated as enemies by all fishermen. * * * * * The TROPIC BIRDS (_Phaëton_), "Children of the Sun," as they were called by Linnæus, always warn the mariner by their presence that he has entered the precincts of the torrid zone, beyond which they seldom stray. The characteristic features of these beautiful creatures are the small size and compact build of their bodies. Their beak is about the same length as the head, and compressed laterally--its upper surface slightly curved, its apex acutely pointed, and its cutting edges finely denticulated. The legs are small in proportion to the size of the body, and the hind toe connected with the inner one by but a very narrow web. The wings are long, and the tail composed of twelve or fourteen quills; the central tail-feathers are remarkably prolonged, and present a structure that is quite peculiar, owing to the laxity and sparseness of their barbs. Their general plumage is thick and delicately tinted. "No one who sees the Tropic Birds for the first time," says Pöpping, "would regard them as inhabitants of the sea, but would rather imagine them to be land birds that had strayed by accident far into the illimitable desert of the ocean. In strength of wing they are unrivalled. Without moving their pinions in the slightest degree, or turning the body, they mount quickly to a great altitude, and there seem to repose upon the air, as if it were a solid support. Only when engaged in fishing, or when they perceive a ship, do they exchange this state of luxurious repose for active exertion. On such