The beautiful tropic birds, whose name implies the limit of their abode--for they are seldom seen but a few degrees south or north of either tropic--hover at such a distance from the nearest land that it is still an enigma where they pass the night--whether they sleep upon the waters, or whether their extraordinary length of wing bears them to some isolated rock. Nothing can be more graceful than their flight. They glide along, most frequently without any motion of their outstretched pinions, but at times this smooth progression is interrupted by sudden jerks. When they see a ship, they never fail to sail round it, and the mariner bound to the equatorial regions hails them as the harbingers of the tropics. The two long straight narrow feathers of which their tail consists, are employed by the natives of the greater part of the South Sea Islands as ornaments of dress, and serve to distinguish the chieftains from the multitude. The esculent swallow (_Colocalia esculenta_)--whose edible nest, formed by a secretion which hardens in the air, is one of the greatest dainties of the Chinese epicure--may almost be considered as a sea-bird, as it chiefly inhabits marine caves in various islands of the Indian Archipelago, and exclusively seeks its food in the teeming waters. [Illustration: ESCULENT SWALLOWS’ NESTS.] The steep sea-walls along the south coast of Java are clothed to the very brink with luxuriant woods, and screw-pines strike everywhere their roots into their sides or look down from the margin of the rock upon the sea below. The surf of ages has worn deep caves into the chalk cliffs, and here the swallow builds her nest. When the sea is most agitated, whole swarms are seen flying about, and purposely seeking the thickest wave-foam, where no doubt they find their food. From a projecting cape, or looking down upon the play of waters, may be seen the mouth of the cave of Gua Rongkop, sometimes completely hidden under the waves, and then again opening its black recesses, into which the swallows vanish, or from which they dart forth with the rapidity of lightning. While at some distance from the coast the blue ocean sleeps in peace, it never ceases to fret and foam against the foot of these mural rocks, where the most beautiful rainbows glisten in the rising vapour.