Varying even more than the manner and power of flight among different birds are the vocal utterances, the cries and calls and singing. By their calls and songs alone many birds may be identified although they remain unseen. The field-student of birds comes to know them by their songs; knows what birds they are; knows what they are doing or not doing; knows what time in their life-season it is, whether they are mating, or brooding, or preparing to migrate; knows whether they are frightened, or self-confident, whether in distress or happy. Little urging and suggestion are needed to induce the student to attend to the songs. But the naturalist should not only hear and enjoy them, but by observation and the recording of repeated observations, he should come to understand the significance of the calls and songs. As to how these sounds are made, attention has already been called (see p. 338) to the voice-organ or syrinx. The condition of this organ varies much in birds, as would be expected from the differing character of vocal utterances. Dissections will make these differences apparent. =Nesting and care of young.=--Among the birds' most interesting instincts and habits are those domestic ones which include mating, nest-building, and care of the young. Birds' eggs and birds' nests are always attractive objects of search and collection for boys, and most boys have a considerable personal knowledge of the domestic habits of the commoner summer birds of their region. With this interest and unsystematized knowledge as a basis the teacher should be able to get from the class much excellent field-work and personal observation. The first thing to undertake in this study is the gathering of data regarding the character of the nests of different species, their situation, the time of nesting, the participation or non-participation of the male in nest-building, etc.; also the number of eggs, their size and color markings, the length of incubation, the help or lack of help of the male in brooding, etc. In connection with this gathering of data in the field by note-taking, sketching, and photographing, nests and eggs can be collected (see directions on page 469). Let only one clutch of eggs of each species be taken for the common high-school collection, and if more than one nest is desired take used and deserted nests. When the nestlings are hatched, the bringing of food, the defence of the home, and the teaching of the young to fly should all be observed and noted.