After the strain of the reproductive period, or sometimes at the low ebb of mid-winter, the old feathers drop off, and birds undergo annual moult. The use of this in replacing breakage, and in furnishing a complete machinery for the flight of migration, is very evident; the cause is not yet sufficiently investigated. Moulting obviously presents some analogies with skin-casting and hair-shedding in other animals, and must be associated with some deep-seated constitutional change, such as its connection with the end of the breeding season suggests. Besides this annual growth of new feathers, many birds exhibit double and some triple moulting. The ptarmigan, for instance, changes its suit three times in the year, moulting after breeding into gray, changing this for white as the winter sets in, and acquiring in spring a third and most attractive set of feathers. In association with sexual attraction many male birds seem to undergo a partial moult, as the result of which they acquire those special decorations which are the index of a reproductive climax. Birds usually pair in springtime, but to this rule there are many exceptions. Fertilization is internal, and all birds are oviparous. The number of eggs is often in inverse proportion to the size of the bird. Several, such as the apteryx, lay only one; the doves and birds of prey lay two or three, but the majority of birds many more. It is important to notice that the higher development of birds, as compared with reptiles, is associated with the production of fewer offspring, but at the same time with the enormous increase of parental care and sacrifice. If the young are to be developed within the eggs, the latter must be kept at an approximately constant warmth. In almost all cases this is effected by brooding, the frequent helplessness of the young, the very common arboreal habit, the not infrequent enemies, have necessitated a most varied series of nest-building contrivances. The nest is built before the eggs are ready to be laid, and in most cases the female takes the prominent part in its construction. But both in the building and in the subsequent brooding the male may do his share, or in some cases much more. Each species usually has its own peculiar style and material of construction, though this may be adapted to varying conditions. The nests are usually solitary, more rarely grouped, and very exceptionally (as among cassowaries and ostriches) common property. Rooks, sea-fowl, herons, are familiar examples of breeding communities, while the sociable grosbeak, the republican swallows, and a few others, form even closer associations. The cuckoo and the cowbird have managed by a sort of parasitism to shirk their task, and quite a number of birds lay their eggs in an exceptional manner in the nests of neighbors. The beak is the organ most used in construction, but the pressure of the body may round off the forming nest, and the feet may also be used. How comfortable a nest