FIG. 317.—CRANES MIGRATING, with leader at point of V-shaped line. ] =Field Study of Migration.=—Three columns may be filled on the blackboard in an unused corner, several months in spring or fall being taken for the work. _First_ column, birds that stay all the year. _Second_ column, birds that come from the south and are seen in summer only. _Third_ column, birds that come from the north and are seen in winter only. Exact dates of arrival and departure and flight overhead should be recorded in notebooks. Many such records will enable American zoologists to trace the migration routes of our birds. [Illustration: FIG. 318.—APTERYX, of New Zealand. Size of a hen, wings and tail rudimentary, feathers hair-like. ] =Moulting.=—How do birds arrange their feathers after they have been ruffled? Do they ever bathe in water? In dust? Dust helps to remove old oil. At what season have birds the brightest feathers? Why? Have you ever seen evidence of the moulting of birds? Describe the moulting process (page 120). [Illustration: FIG. 319.—GOLDEN, SILVER, AND NOBLE PHEASANTS, males. Order? (Key, p. 177.) Ornaments of males, brightest in season of courtship, are due to sexual selection (Figs. 321–7–9, 333). ] [Illustration: FIG. 320. COCKATOO. ] [Illustration: FIG. 321.—BIRD OF PARADISE (Asia). ] =Adaptations for Flying.=—Flight is the most difficult and energy-consuming method of moving found among animals, and careful adjustment is necessary. For balancing, the heaviest muscles are placed at the lower and central portion of the body. These are the flying muscles, and in some birds (humming birds) they make half of the entire weight. Teeth are the densest of animal structures; teeth and the strong chewing muscles required would make the head heavy and balancing difficult; hence the chewing apparatus is transferred to the heavy gizzard near the centre of gravity of the body. The bird’s neck is long and excels all other necks in flexibility, but it is very slender (although apparently heavy), being inclosed in a loose, feathered skin. A cone is the best shape to enable the body to penetrate the air, and a small neck would destroy the conical form. The internal organs are compactly arranged and rest in the cavity of the breast bone. The bellowslike air sacs filled with warm air lighten the bird’s weight. The bones are hollow and very thin. The large tail quills are used by the bird only in guiding its flight up and down, or balancing on a limb. The feet also aid a flying bird in balancing. The wing is so constructed as to present to the air a remarkably large surface compared with the small bony support in the wing skeleton. Are tubes ever resorted to by human architects when lightness combined with strength is desired? Which quills in the wing serve to lengthen it? (Fig. 296.) To broaden it? Is flight more difficult for a bird or for a butterfly? Which of them do