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 by dint of diligent search—the kingfisher forms a couch of the undigested spiny fish bones which she ejects in pellets from her own stomach. Other birds, such as the woodpeckers, hew holes in living trees, even when the timber is of considerable hardness, and therein establish their nursery. Some of the swifts secrete from their salivary glands a fluid which rapidly hardens as it dries on exposure to the air into a substance resembling isinglass, and thus furnish the “edible birds’ nests” that are the delight of Chinese epicures. In the architecture of nearly all the passerine birds, too, some salivary secretion seems to play an important part. By its aid they are enabled to moisten and bend the otherwise refractory twigs and straws, and glue them to their place. Spiders’ webs also are employed with great advantage for the purpose last mentioned, but perhaps chiefly to attach fragments of moss and lichen so as to render the whole structure less obvious to the eye of the spoiler. The tailor-bird deliberately spins a thread of cotton and therewith stitches together the edges of a pair of leaves to make a receptacle for its nest. Beautiful, too, is the felt fabricated of fur or hairs by the various species of titmouse, while many birds ingeniously weave into a compact mass both animal and vegetable fibres, forming an admirable non-conducting medium which guards the eggs from the extremes of temperature outside. Such a structure may be open and cup-shaped, supported from below as that of the chaffinch and goldfinch, domed like that of the wren and bottle-titmouse, slung hammock-wise as in the case of the golden-crested wren and the orioles, or suspended by a single cord as with certain grosbeaks and humming-birds.

Certain warblers (Aedon and Thamnobia) invariably lay a piece of snake’s slough in their nests—to repel, it has been suggested, marauding lizards who may thereby fear the neighbourhood of a deadly enemy. The clay-built edifices of the swallow and martin are known to everybody, and the nuthatch plasters up the gaping mouth of its nest-hole till only a postern large enough for entrance and exit, but easy of defence, is left. In South America the oven-birds (Furnariidae) construct on the branches of trees globular ovens, so to speak, of mud, wherein the eggs are laid and the young hatched. The flamingo erects in the marshes it frequents a mound of earth sometimes 2 ft. in height, with a cavity atop. The females of the hornbills submit to incarceration during this interesting period, the males immuring them by a barrier of mud, leaving only a small window to admit air and food.

But though in a general way the dictates of hereditary instinct are rigidly observed by birds, in many species a remarkable degree of elasticity is exhibited, or the rule of habit is rudely broken. Thus the falcon, whose ordinary eyry is on the beetling cliff, will for the convenience of procuring prey condescend to lay its eggs on the ground in a marsh, or appropriate the nest of some other bird in a tree. The golden eagle, too, remarkably adapts itself to circumstances, now rearing its young on a precipitous ledge, now on the arm of an ancient monarch of the forest and again on a treeless plain, making a humble home amid grass and herbage. Herons will breed according to circumstances in an open fen, on sea-banks or (as is most usual) on lofty trees. Such changes are easy to understand. The instinct of finding food for the family is predominant, and where most food is there will the feeders be gathered together. This explains, in all likelihood, the associated bands of ospreys or fish-hawks, which in North America breed (or used to breed) in large companies where sustenance is plentiful, though in the Old World the same species brooks not the society of aught but its mate. Birds there are of eminently social predilections. In Europe, apart from sea-fowls—whose congregations are universal and known to all—only the heron, the fieldfare and the rook habitually flock during the breeding season; but in other parts of the world many birds unite in company at that time, and in none possibly is this habit so strongly developed as in the anis of the neotropical region, the republican swallow of North America and the sociable grosbeak of South Africa, which last joins nest to nest until the tree is said to break down under the accumulated weight of the common edifice.

In the strongest contrast to these amiable qualities is the parasitic nature of the cuckoos of the Old World and the cowbirds of the New. The egg of the parasite is introduced into the nest of the dupe, and after the necessary incubation by the fond fool of a foster-mother the interloper successfully counterfeits the heirs, who perish miserably, victims of his superior strength. The whole process has been often watched, but the reflective naturalist will pause to ask how such a state of things came about, and there is not much to satisfy his inquiry. Certain it is that some birds whether by mistake or stupidity do not infrequently lay their eggs in the nests of others. It is within the knowledge of many that pheasants’ eggs and partridges’ eggs are often laid in the same nest, and gulls’ eggs have been found in the nests of eider-ducks and vice versa; a redstart and a pied flycatcher will lay their eggs in the same convenient hole—the forest being rather deficient in such accommodation; an owl and a duck will resort to the same nest-box, set up by a scheming woodsman for his own advantage; and the starling, which constantly dispossesses the green woodpecker, sometimes discovers that the rightful heir of the domicile has to be brought up by the intruding tenant. In all such cases it is not possible to say which species is so constituted as to obtain the mastery, but it is not difficult to conceive that in the course of ages that which was driven from its home might thrive through the fostering of its young by the invader, and thus the abandonment of domestic habits and duties might become a direct gain to the evicted householder.

Nests and Coloration.—The correlation between nests and the coloration of the birds has been investigated by A. R. Wallace. Accordingly he divides birds into two main groups, first those in which the sexes are alike and of conspicuous or showy colours, and which nidificate in a covered site; secondly, those in which the males are showy and the females sombre, and which use open sites for their nests. The many exceptions to these generalizations caused J. A. Allen (Bull. Nutt. Orn. Club, 1878) to write an adverse criticism. C. Dixon (H. Seebohm’s Hist. Brit. Birds, ii., 1884, introduction) has reviewed the question from Wallace’s point of view. He established the following categories.

1. Birds in which the plumage of the male is bright and conspicuous in colour, and that of the female dull and sombre, and which nidificate in open sites. In these very common cases, the female alone incubates, and obviously derives protection from its inconspicuous plumage.

2. Birds in which the plumage of both sexes is showy or brilliant in colour, and which nidificate in open nests. This group forms one of those exceptions which at first sight appear seriously to affect the validity of Wallace’s theory. In most of the cases, however, the birds, as, for instance, crows, gulls, herons, are either well able to defend themselves and their nests or, as, for instance, the sandpipers, they seek safety for themselves in flight, relying upon the protective tints of their eggs or young.

3. Birds in which the male is less brilliant than the female, and which nidificate in open nests. Such birds are exceedingly few, e.g. the Phalaropes, the common cassowary, the emu, a carrion hawk (Milvago leucurus) from the Falkland Islands, an Australian tree-creeper (Climacteris erythrops) and an Australian goatsucker (Eurystopodus albigularis). In all these cases the male performs the duty of incubation. The male tinamous do the same, although they do not differ from their mates, but the conspicuously coloured male ostrich takes this duty upon himself during the night.

4. Birds in which both sexes are brightly coloured, and which rear their young in holes or covered nests. For instance, the gaudy coloured rollers, bee-eaters, kingfishers, the hoopoe, hornbills. toucans, parrots, tits, the sheldrake and many others.

5. Birds in which both sexes are dull in colour, and which build covered nests from motives of safety other than concealment. For example, the swifts (Cypselus), the sand-martin (Cotyle riparia), wrens, dippers and owls.

6. Birds in which the female is duller in colour than the male, and which nidificate in covered nests; e.g. the redstart (Ruticilla phoenicura), the pied flycatcher (Muscicapa atricapilla), rock-thrushes (Monticola), chats (Saxicola) and robin-chats (Thamnobia), and birds of the genus Malurus. In some of these cases the showy male bird assists in incubation, the kind of nest allowing him to do so with safety.

Similar difficulties beset the generalizations concerning the correlation of the colour of the eggs and the exposed or hidden condition of the nest. The eggs of most birds which breed in holes, or even in covered nests, are white, but the number of exceptions is so great that no general rule can be laid down to this effect. Conversely the number of birds which lay purely white eggs in open nests, e.g. pigeons, is also large. The eggs of owls are always white,