Among the interesting subjects which still remain open for search, are the habits and constitution of the hirundo esculenta, the small swallow which forms the edible nests, annually exported in large quantities from Java and the Eastern Islands for the Chinese market. These birds not only abound among the cliffs and caverns of the south coast of the island, but inhabit the fissures and caverns of several of the mountains and hills in the interior of the country. From every observation which has been made on Java, it has been inferred, that the mucilaginous substance, of which the nests are formed, is not, as has been generally supposed, obtained from the ocean. The birds, it is true, generally inhabit the caverns in the vicinity of the sea, as agreeing best with their habits, and affording them the most convenient retreats for attaching their nests to; but several caverns are found inland, at a distance of forty or fifty miles from the sea, containing nests similar to those on the shore. From many of their retreats along the southern coast, they have been observed to take their flight in an inland direction, towards the pools, lakes, and extensive marshes covered with stagnant water, as affording them abundance of their food, which consists of flies, mosquitoes, gnats, and small insects of every description. The sea that washes the foot of the cliffs, where they most abound, is almost always in a state of the most violent agitation, and affords none of those substances which have been supposed to constitute the food of the esculent swallow. Another species of swallow on this island forms a nest, in which grass or moss, &c. are merely agglutinated by a substance, exactly similar to that of which exclusively the edible nests consist. This substance, from whatever part of these regions the nests be derived, is essentially uniform, differing only in the colour, according to the relative age of the nests. It exhibits none of those diversities which might be expected, if it were collected casually (like the mud employed by the martin, and the materials commonly employed in nest-making), and applied to the rocks. If it consisted of the substances usually supposed, it would be putrescent and diversified. Dr. Horsfield thinks that it is an animal elaboration, perhaps a kind of secretion; but to determine its nature accurately, it should be carefully analyzed, the anatomy of the bird should be investigated, and its character and habits watched.