Most animals whose lives are spent in the open air and light show more or less color in their coat, but none are more beautifully adorned than birds. The most brilliant examples are to be found in the tropics, and some of the gayest in our colder land, such as the tanagers and humming birds, are strays from large tropical families noted for gaudy attire.
The color we see in plumage may be due to either of two conditions. It may, as is usually the case, be simply coloring matter deposited in the substance of the feathers. But where the plumage gleams with changing rainbow lights, as on the fiery throat patch of the humming bird, on the neck of a dove or on the purple-black coat of the grackle (crow blackbird), these splendid reflections are caused by very minute wrinkles on the feathers, that break up the light. It is the same effect, called "iridescence," as is seen on the mother-of-pearl and on a soap bubble. Blue is usually an effect produced by certain coloring matter not blue underlying a thin covering of feather substance; and when you pound a blue feather into dust that dust will be black or gray—or, at any rate, not blue. Birds of the same group are colored much alike, as a rule.
In some cases the style of colors worn appears to be the best for the safety of the birds of the group by making them hard to see as long as they keep still. Thus most birds whose lives are passed on or near the ground, and which build their nests there, are dull in coloring; they are in danger from more enemies than are tree-dwelling birds, and must be able to hide better. No bird of nocturnal habits is brightly colored. It is mostly among the small, quick-flying species, such as warblers and finches, that we find the gayly dressed ones. They are birds of the sunshine, and usually migratory. In most cases when birds have a plain dress there is little difference in it between the male and the female; but whenever you find a species of bird wearing a gay, ornamental dress, it is almost always the male that sports these fine feathers, while the female and young are clothed in dull yellow, drab or brownish tints. This appears to be another measure of safety. The males can wander about, look out for themselves, and take to flight when danger threatens; but their mates must sit quietly on their nests, and trust for safety for themselves and (what is really more important) their eggs or young mainly to not being seen. In their plain colors they blend into the foliage and the shadows amid which they sit, and so are more likely to escape the sight of prowling foes.
Feathers are not intended to remain permanently; they become worn and faded, or are lost, so that at regular intervals the bird needs a new suit of clothes. Twice a year, therefore, in spring and autumn, they are pushed out by new ones sprouting in the same feather-growing pits. This shedding of the feathers is called "molting," and it is analogous to the shedding of the outer, horny pellicle of its skin by a snake or lizard. Their molting is not very noticeable in land birds, because the feathers drop out little by little; otherwise the poor creatures would be left quite naked, and unable to fly. In most birds the new feathers that come in are the same in pattern and color as those they displace, so that the new plumage differs little if any from season to season; but some birds acquire a new coat for winter that is decidedly different, and sometimes snowy white, making them inconspicuous amid the snow.
The largest and most important feathers in a bird's outfit are those of the wings and tail, by means of which it flies and controls its progress. How birds are able to keep themselves aloft in the air, and move through it at will, is not yet understood. That it requires great strength of wing muscles, and rigid support for them is evident. Therefore we find the head of the arm bone (humerus) fastened by stout ligaments to a great shoulder blade sunk in the flesh beside the fore part of the spine, and also braced in two directions by other interior bones, one of which extends down to join its opposite fellow at the front end of the breast bone, and form the "wishbone" (the united coracoids). This solid bracing by bones and tying by ligaments gives the needed firmness to the wings; and enables their powerful muscles to work them.
How great these muscles are you will know when I tell you that the thick mass of "white meat" in the breast of the fowl carved at your table consists only of the two principal muscles that move the wings when a downward stroke is made. They, in their turn, are attached at the base to the broad surface of the breastbone, or "sternum" and its projecting keel. Beyond the wrist joint stretches a large, misshapen hand, which consists mostly of one great forefinger, in the tough flesh of which the big quills, or outer flight feathers, called "primaries," are rooted. Lying over their bases, when the wing is folded, is a row of somewhat smaller quill feathers called "secondaries." Above those are the small and close "wing coverts."
The tail is very important in guiding and checking a bird in flight, and is useful in various other ways, and may also be extremely ornamental. The tail quills are always in pairs, making an even number of feathers. This results from the reduction to a mere stub of the long clumsy tail worn by the archæopteryx and its fellows. The quills continued to grow in pairs out of the side of the tail as it diminished until all that there is room for (ten or twelve pairs) are now rooted side by side around the edge of the condensed coccygeal bones.
Birds are, as a class, the most active of animals, and their temperature is highest; this means a large consumption of oxygen, and the windpipe is usually capacious, yet the lungs are not large, but are supplemented by another apparatus for aeration. Opening out of the lungs are several pairs of air sacs, amplest in those birds that are much on the wing, which not only occupy spaces between the muscles and organs within the chest, but in many cases extend into the neck and head, and even into the limb bones, which in most birds are hollow.