The red-head is another type. The bill is still nearly of the pick-axe model, the feet not especially different from the flicker’s, the tail rather better adapted to life on a tree-trunk, and the tongue entirely unlike the flicker’s,—not very extensile and heavily clothed near the tip with long, thick, recurved bristles. We infer that though he may climb well, he is not a drilling woodpecker to any great extent, and that his tongue is adapted neither to extracting borers nor to eating ants from their burrows. His habits bear out the inference. He is arboreal, but his food is either vegetable or picked up from the surface, rasped up rather than speared.
The sapsucker presents still another variation. The points to the tail feathers are more acuminate and the tail itself more resembles that of the tree-dwelling woodpeckers in shape; the feet are fitted for clinging to the trunk; the bill, now perfectly straight and no longer smoothly rounded but buttressed by strong angles that spring from the base and run down toward the tip, is the bill of a woodpecker that lives by drilling; but the tongue is wholly unadapted to catching grubs. What kind of food can an arboreal woodpecker with a drilling bill find upon a tree-trunk when his tongue can be extended only a fifth of an inch, and is furnished with a brush of bristles at the end? We have answered that question before: he eats the inner bark of trees and laps up the sap, for which this brushy tip is excellently fitted. It has been observed that the tongue much resembles the tongues of insect-eating birds, which cannot be extended beyond the end of the bill. It is true that the sapsucker catches great numbers of insects, taking them on the wing like a flycatcher. But he also eats nearly as many ants as the flicker, though their tongues are totally unlike. We have made the mistake perhaps of thinking that ants live only underground and can be obtained only by tongues like those of the flicker and the ant-bear, which hunt them there. But ants are abundant on the surface of the ground, and they excavate long tunnels in rotten wood. The black bear is a famous ant-hunter, yet his tongue is like a dog’s and he gets his ants by lapping them up after he has torn open the rotten logs in which they live. This is the way that the sapsucker obtains his ants, and the brush of stiff hairs is a help to him in such work. We see, then, that it is not so much the food as the manner of feeding that explains the form of the tongue.
The downy and the hairy are a step farther along in their development. The fourth toe is longer than the others, a condition that we do not find in any of the woodpeckers not strictly arboreal; the tail is of the improved pattern, holding by a brush of bristles rather than by one stiff point at the end of each feather; the bill is heavier, broader at the base, more heavily ridged, and in every way a stronger tool; and the tongue is highly extensible and of the spear pattern, sharp-pointed and barbed with recurved hooks. Everything about these birds indicates that they are fitted to live on tree-trunks and to dig for borers. This, indeed, is what they do.
But the great logcock and the ivory-billed woodpecker, though of the same type as the other larvæ-eating woodpeckers, are more highly developed along the same line. We notice the great strength of the feet; the claws, as large and as sharp as a cat’s; the enormous weight and strength of the bill, compared with that of the other woodpeckers, which enables them to cut into the hardest wood and even into frozen green timber; and the great development of the tail, which now becomes a strong spring to support and aid the bird in his work.
As we try to group these particulars under general heads, we see that we have observed three things:—
That the structure of a bird is adapted to its kind of life.
That the structure varies by small degrees with the kind of life.
That the kind of life is conditioned largely upon the kind of food and upon the method of procuring it, more particularly the latter.
These are not so much different truths as three aspects of one truth. When we study the first we see why birds are grouped together into orders and families: we study their resemblances. When we observe the second we see why they are divided into species, for we note their differences. But when we consider the third and reflect that birds have the power to choose new kinds of food or new places and means of getting it, we see how it is that there can come to be new kinds of birds, new subspecies and species, springing up from time to time. Wonderful and improbable as it seems, there is more reason to believe than there is to doubt that new kinds of animals and plants are constantly in process of making; that the laws of change are constantly at work, adapting creatures to their surroundings or crushing them out of existence because they will not learn new ways. And it is probable that these differences which we mark in the woodpeckers have been the result of efforts to adapt themselves to a peculiar kind of life where food was abundant; and also that by acquired habits and by acquired tastes for different kinds of foods they will be subject to still further variations in the future.