In the Malayan islands in general, but little care is taken of the rocks and caverns which produce this dainty, and the nests procured are neither so numerous nor so good as they otherwise would be. On Java, where perhaps the birds are fewer, and the nests in general less fine than those to be met with in some of the more Eastern Islands, both the quantity and quality have been considerably improved by European management. To effect this improvement, the caverns which the birds are found to frequent are cleansed by smoking and the burning of sulphur, and the destruction of all the old nests. The cavern is then carefully secured from the approach of man, the birds are left undisturbed to form their nests, and the gathering takes place as soon as it is calculated that the young are fledged. If they are allowed to remain until eggs are again laid in them, they lose their pure colour and transparency, and are no longer of what are termed the first sort. They are sometimes collected so recently after their formation, that time has not been given for the bird to lay or hatch her eggs in them, and these nests are considered as the most superior; but as the practice, if carried to any extent, would prevent the number of the birds from increasing, it is seldom resorted to, where the caverns are in the possession of those who have a permanent interest in their produce. Much of their excellence and peculiar properties, however, depend on the situation of the place in which they are formed. It has often been ascertained, for instance, that the same bird forms a nest of somewhat different quality, according as it constructs it in the deep recesses of an unventilated and damp cavern, or attaches it to a place where the atmosphere is dry, and the air circulates freely. The nature of the different substances also to which they are fixed, seems to have some influence on their properties. The best are procured in the deepest caverns, (the favourite retreat of the birds), where a nitrous dampness continually prevails, and where being formed against the sides of the cavern, they imbibe a nitrous taste, without which they are little esteemed by the Chinese. The principal object of the proprietor of a birds'-nest rock is to preserve sufficient numbers of the swallows, by not gathering the nests too often, or abstracting those of the finer kinds in too great numbers, lest the birds should quit their habitations and emigrate to a more secure and inaccessible retreat. It is not unusual for a European, when he takes a rock under his superintendence, after ridding it of the old nests and fumigating the caverns, to allow the birds to remain undisturbed, two, three, or even more years, in order that they may multiply for his future advantage. When a birds'-nest rock is once brought into proper order, it will bear two gatherings in the year: this is the case with the rocks under the care of the officers of government at _Kárang bólang_.