BIEDS 655 36 eggs in a single nest. Before maturity the product of this nest exceeds in weight their parent at least 20 fold. To provide for these, or but one of them, by internal organs of de- velopment, would be impossible. The nests correspond in their uses to the uterine organs of reproduction of mammalia, and yet more to the marsupial pouches of certain Australian quadrupeds. They serve as external organs indispensable to the development of the im- mature young, from the first appearance of the germ in the egg to a maturity more or less advanced, and varying greatly with the family ; from the ostrich that comes into the world able to shift for itself from the very shell, to the blind and naked offspring of other families that are utterly helpless when first hatched. For this development of the young birds there are two essentials the external receptacle which, though not always with exactness, we call nests, and the applica- tion of a certain nearly fixed or uniform amount of caloric. In nearly all cases the latter is gen- erated by contact with the bodies of the parent birds. In some it is aided by the heat of the sun. In a few instances it is effected by heat derived from vegetable decomposition, or from the sun's rays, without any parental interven- tion after the deposition of the egg. Attempts have been made, with partial success, to clas- sify the various architectural contrivances, or their substitutes, to be found connected with the nesting and incubation of birds. According to the system of Prof. James Ronnie of King's college, London, the entire class are ranged in 12 groups: miners, ground builders, masons, carpenters, platform builders, basket makers, weavers, tailors, felt makers, cementers, dome builders, and parasites. The objections to this arrangement are, that it is imperfect in itself, and that it corresponds to none of the usual systems of ornithological classification. The large number of species which, without being miners or carpenters, invariably occupy for their nests corresponding sites, namely, holes in the earth or hollow trees, have no appro- priate place. Some of these have been improp- erly classed as parasites. Nor is there a well denned place for the large variety of species belonging to every order which resort to the bare ground, making no perceptible nest, or for that remarkable family of Australian birds, the mound builders, which combine something both of the miner and the ground builder. It seldom if ever conforms, in a single family even, with any known classification. Thus, the hawks are platform builders, ground builders, occupants of hollow trees, &c. ; the swallows are miners, cementers, dome builders, masons, &c. The mining birds compose a very large group, belonging to nearly every order, and having no other common peculiarity. They may be divided into two well marked subdi- visions : the true miners, which excavate holes for themselves, in which they construct their nests ; and those which, without mining, occupy 94 VOL. ii. 42 sites precisely similar. Of these a portion are supposed to be parasitic, availing themselves of the labors of others. Among the true miners may be named the common bank swallow, found nearly throughout the habitable globe, the bee-eaters of Europe and Asia, and the whole genus known as storm petrels or mother Carey's chickens ; as also the several genera of puffins, kingfishers, penguins, &c. Among miners only by occupancy may be named the wood wren and the winter wren of North America, the black guillemot, and the burrow- ing owls of North and South America. The last are parasitic miners, occupying invariably holes dug by other annuals. The ground builders include by far the largest group of birds of every order, and nearly of every family, and cannot be defined with exactness. In it must be classed many which build no nest; others that do or do not construct nests, according to circumstances ; those which build on the ground usually, but frequently elsewhere ; some that are usually ground builders, but at times true miners, like the skylark of Europe, &c. The nighthawks and whip-poor-wills of America make no nest, the former depositing their eggs upon the bare earth, always selecting a site corresponding in color to their eggs, the latter selecting dried leaves as better suited to the same purpose of concealment. A very large proportion of the shore birds, waders, gulls, &c., make use of the bare sand, with only a slight excavation for a nest. Others of the same species are more painstaking, and construct well formed nests. The herring gulls usually build a slight nest on the ground, but, after having been repeatedly robbed by eggers, the same birds are known to construct large arid elaborate nests in trees or on precipitous cliffs. The mound builders of Australia (see BRUSH TCEKEY) combine in part the habits of the miners with those of the ground builders, in a manner peculiar to that remarkable family. Among the true ground builders may be cited nearly all the vultures, the entire sub-family of circida or hen-harriers, the zonotrichice or song sparrows of America, nearly all the waders, ducks, geese, swans, gulls, terns, &c., with more or fewer representatives in every order. The birds classed as masons are com- paratively few in number of species. They are so called because they construct their nests, in whole or in part, with walls, coverings, barri- cades, &c., of mud or clay. Of this class the cliff swallow of North America is one of the most remarkable examples. The house swal- lows both of Europe and America, the thrush and blackbird of Europe, the robin and the pewit flycatcher of North America, are among the most familiar examples. The baker bird of South America, the most skilful and remark- able of this class, constructs a nest in the most exposed situations, but at a considerable height, hemispherical, or in the form of a baker's oven. The opening of this nest is lateral, and is twice as high as it is wide, and the interior is divided
