Let us a.s.sume that you know nothing whatever of a woodp.e.c.k.e.r's tail except that it has ten feathers, is used as a prop, and is held at an angle of thirty or forty degrees with the tree-trunk. Now, take three strips of paper of the same width and length, and of any size not inconveniently small. Fold them all down the centre. Cut one square across; cut one with a rounded end and the third with a forked end, making them of any shape you please so long as the three papers are of the same length. To give our models a fair test they must be of the same width and length. Next, pin a sheet of paper of any size you please into the form of a cylinder and stand it on end to represent a tree-trunk.

Then fit the patterns to the tree-trunk and see which is the form that would give the most support.

[Ill.u.s.tration: Patterns of tails.]

But first, in how many ways is it possible for a bird to use his tail as a prop? He may of course hold it open or closed; and the open tail may be held in a single plane, "spread flat," as we say; or curved up at the edges, like a crow blackbird's; or curved down at the edges. And the closed tail may be held in a single plane; or, by dropping each pair of feathers a little, in several planes. Thus we see there are five positions in which each shape may be held against the cylinder of paper.

Try each one against it, holding it first in the open positions and then after folding the paper like a bird's tail with the outer feathers underneath, in the closed positions. The size of the model tree-trunk and the shape you cut your curves will make the results vary a little, but you will be surprised to observe, if your models are not too small, how many times you will get the same answers. Note the number and position of the pairs that touch:

_Spread._ _Square end._ _Forked end._ _Round end._

one plane, varies varies middle pair curved up, middle pair middle pair middle pair curved down, all all all _Closed._ one plane, outer pair outer pair middle pair different planes, outer pair outer pair all

Which shape brings the most feathers into use in all positions? Which positions bring most feathers into use? We see at once that the rounded end has a decided advantage, that the middle pair of feathers is used in all possible positions, that the pair next outside is the next important, and that the spread tail curving downward at the edges and the closed tail in different planes are the two shapes which give the best support. There is therefore a reason for the rounded end which we said was the rule among the woodp.e.c.k.e.rs.

Our little experiment is what we call a _deduction_. It shows us what we ought to expect under certain imaginary conditions. But it does not show us what actually exists, so there often comes a time when our deductions are faulty because Nature has done some unexpected thing, as when we found the single exception of the logc.o.c.k's foot upsetting a fine theory of ours. A deduction must always be compared with facts, and is worth little or nothing if a single fact of the series we are studying is not explained by it. This time all the facts do agree; for I had, before we made our experiment, examined the tails of every species of woodp.e.c.k.e.r ever found in North America, and there was no exception to the rounded end. I had already drawn my conclusion that this form was better adapted to life on a tree-trunk than the square or the forked tail would be, reasoning by a different process called _induction_. An induction examines many, and, if possible, all the facts before drawing any conclusion; a deduction examines the facts after the conclusion is reached. There is no hard-and-fast line between the two kinds of reasoning, but we may say that a _deduction is reasoning out a guess and an induction is guessing out a reason_. Deductions are easier and quicker; inductions are surer, and in preparing them we often make other discoveries.

The rounded tail is no doubt the best; but we have yet to decide whether the sharper curve is more advantageous than the lesser curve, as we thought probable from our observations. And there is still another deduction from our experiment which we did not make. If in the rounded tail the middle pairs of feathers do most of the work, and if use increases the size and efficiency of a part, which is almost an axiom in science, we should expect to find the middle tail feathers not only strongest in all woodp.e.c.k.e.rs but also strongest in increasing ratio in the species that use them most. To determine this we must study the use of the tail and the structure and shape of the individual tail feathers.

We should remark, perhaps, that the woodp.e.c.k.e.r's tail is always composed of twelve feathers--ten pointed rectrices and two tiny abortive feathers so short and so hidden that no attention is paid to them. The ten princ.i.p.al feathers are arranged in corresponding pairs numbered from the outside to the centre as first, second, third, fourth, and fifth pairs.

In the flickers all ten feathers have wide vanes and are similar in everything but the shape; all are more or less pointed. The flicker's tail looks and feels very much like that of any other bird except that the shafts are stiffer and the vanes contract to an ac.u.minate tip. But as we take up the other species we notice a change, not only in the shape of the feathers but much more in their texture and in the difference between the various pairs. While in the flicker four pairs out of five are pointed and all are rigid, in the downy and the hairy three pairs out of five seem to be too soft to give any support, the sharp points have disappeared, and the tail has lost much of its stiffness. The two middle pairs of feathers are the only ones capable of doing much work and they are wavering and infirm at the tips where we should expect them to be strongest. In the logc.o.c.k it is about the same,--two pairs are apparently unfit for work, one pair is infirm, and the two middle pairs are compelled to give all the support, except the little contributed by the third pair. In the ivory-billed woodp.e.c.k.e.r the two outer pairs are of no a.s.sistance and the three central ones do the work, and here again we find the base of the rectrices rigid and inflexible and the last fourth of their length weak and yielding. But what a difference in the individual feather! It is well able to do all the work; for, except for that weak tip which we cannot now explain, it is one of the toughest and strongest feathers to be found. The shaft is broad and flat, as elastic as a watch-spring; it looks like a band of burnished steel as it runs down between the vanes. And the vanes themselves are of a very curious pattern. They curl under at the edges so that we do not see their whole width, and the barbs crowd so thickly upon each other that they over-lie until they present an edge three or four broad. Indeed, the under side of one of these tail feathers reminds one of nothing so much as of the under side of a star-fish's arm with its two long lines of ambulacral suckers on each side of a central groove, so thickly do the spiny vanes of these strong rectrices over ride and crowd together. These spines lay hold of the bark of the tree, rank after rank, hundreds of bristling points that cannot be dislodged except by a forward motion of the bird or by lifting the tail. Compared with this, the spiny points on the flicker's tail were a poor invention.

This device, which takes hold like a wool card, or a wire hair-brush, cannot slip from place. We begin to see, too, the use of that weak and flexible tip; it is to press down upon the tree-trunk a flat surface sufficiently large to hold hundreds of these little spiny points against the bark. The ivory-bill braces against this with the stiff upper part of the shaft and has a support that will not slip. The upper part of the shaft acts like a spring also, and adds tremendous force to the blow of the bill. Watch a hairy woodp.e.c.k.e.r when hard at work and see how his legs and tail form a triangular base by bracing against each other, and how his blow is delivered, not with the head alone, but with the whole body, swinging from the hips, the apex of the triangle on which he rests. He swings like a man wielding a sledge hammer, and to the strength of his neck adds the weight of his body, the spring of his tail, and the momentum of a blow delivered from a greater height. When the little hairy woodp.e.c.k.e.r does so much with his weak body, we can imagine what great birds like the logc.o.c.k and the ivory-billed woodp.e.c.k.e.r, with their tremendous beaks, their huge claws, their springy tails, and their great physical strength can do. They are magnificent birds, the terror of all the grubs that hide in tree-trunks.

[Ill.u.s.tration: Under side of middle tail feather of Ivory-billed Woodp.e.c.k.e.r.]

One point we have left unexplained: What is the advantage, if there is any, in the sharper curve to the tails of the arboreal woodp.e.c.k.e.rs? It is a simple question. The curve is caused by the unequal length of the tail feathers; each tail feather is a prop, and by their inequality they become props of different lengths. Now ask any carpenter which will best support a tottering wall--props all of the same length set at the same angle, or props of different lengths set at different angles? His answer will help you to solve the problem. But if a little is good, why are not all the pairs used as props? Partly, perhaps, because the woodp.e.c.k.e.r is always crowded for houseroom, and while he must have tail enough, he cannot afford to have any which he does not use. Did you ever think what an inconvenience any tail at all must be in a woodp.e.c.k.e.r's hole?

XIV

THE WOODp.e.c.k.e.r'S TOOLS: HIS TONGUE

We have seen how the woodp.e.c.k.e.r spears his grubs: now we will study his spear.

[Ill.u.s.tration: Tongue of Hairy Woodp.e.c.k.e.r. (After Lucas.)]

There are many interesting points about a woodp.e.c.k.e.r's tongue, and they are not hard to understand. If a woodp.e.c.k.e.r would kindly let us take hold of his tongue and pull it out to its full extent we should be afraid we were "spoiling his machinery," for the tongue can be drawn out almost incredibly--between two and three inches in a hairy woodp.e.c.k.e.r and more in a flicker. A strange-looking object it is, much resembling an angle-worm in form, color, and feeling; for it is round, soft, and sticky, except at the flat, h.o.r.n.y, bayonet-pointed tip, and as it lies in the mouth it is wrinkled like the wrist of a loose glove; but it grows smaller and smoother the more we pull it out. Evidently we are only drawing it into its skin. But where does so much tongue come from?

Does it stretch like a piece of elastic cord? Or is a part hidden somewhere? And if so, where is it kept?

[Ill.u.s.tration: Tongue-bones of Flicker. (After Lucas.)]

_a._ Cerato-hyals, fused and short.

_b._ Basi-hyal, long, slender.

_c._ Cerato-branchials.

_d._ Epibranchials.

Basi-branchial is wanting.

These questions are answered by studying the bones of the tongue, for without bones it could not be guided as swiftly and surely as it is.

Indeed, all tongues have bones in them, as you will discover by cutting carefully the slices near the root of an ox-tongue; but no other creature has such long and elaborate tongue-bones as some of the woodp.e.c.k.e.rs. They are the slenderest and most delicate little bony rods, joined end to end, but not really hinged nor needing to be, because they are so elastic. Here are the bones of a flicker's tongue. The little k.n.o.b at the end, marked _a_, bore the h.o.r.n.y point of the tongue and directed it; the straight shaft marked _b_ was inside the round part of the tongue as it lay within the bird's mouth; but what was done with these two long branches, fully three quarters of the entire length of the bones? They are too sharply curved to pa.s.s down the bird's throat, and, not being jointed, they cannot be doubled back in his mouth. They were tucked away very neatly and curiously. As the hyoid or tongue-bone lies in the mouth its branches diverge just in front of the gullet, and, traveling along the inner sides of the fork of the lower jaw, pa.s.s up over the top of the skull, looking in their sheath of muscles like two tiny whipcords. But still the bones are too long by perhaps half an inch for the place they occupy, and the ends must be neatly disposed of.

Usually both pa.s.s to the right nasal opening and along the hollow of the upper mandible. Very rarely they may curl down around the eyeball in a spiral spring. So when the flicker thrusts out his tongue he feels the pull in the end of his nose, for the tip of the tongue being run out, the long slender bones are drawn out of their hiding-places, down over the skull until they lie flat along the roof of his mouth. As soon as he wishes to shut his bill, back fly the little bones guided by their hollow sheaths of elastic muscle into their hiding-place in the top of the bill. The muscular covering is a part of the same soft envelope that we saw lying in wrinkles at the root of the tongue. It covers the whole length of the little bones just as the woven outside covers an elastic cord.

[Ill.u.s.tration: Skull of Woodp.e.c.k.e.r, showing bones of tongue. _a._ Upper end of windpipe and gullet.]

[Ill.u.s.tration: Hyoids of Sapsucker and Golden-fronted Woodp.e.c.k.e.r.]

Not all woodp.e.c.k.e.rs have tongues precisely like this. The sapsucker's is the shortest of any, and reaches barely beyond the hinge of the jaws. In the Lewis's woodp.e.c.k.e.r and others of his genus the branches of the hyoid extend part-way up the back of the skull but in the kinds that live princ.i.p.ally upon borers they are very long and resemble the flicker's in arrangement. The only other North American birds that have a tongue built upon this plan are the hummingbirds, in which also it is extensile. The flicker, in proportion to his size, has the longest tongue of any bird known.

XV

HOW EACH WOODp.e.c.k.e.r IS FITTED FOR HIS OWN KIND OF LIFE

We have studied the woodp.e.c.k.e.rs at some length: first, what all of them do; next, what some that are peculiar in their ways do; lastly, how each is fitted for a particular kind of life. At first we were inclined to think they were all alike; but now we begin to see that there are very real differences between them,--in tails, feet, bills, and tongues, and at the same time in their food and habits.

The flicker's tail is less sharply curved than that of any other woodp.e.c.k.e.r,--a sign that he is probably not exclusively a tree-dweller; his bill is curved and rounded, a pick-axe rather than a drill,--an indication that he does not dig for grubs; his feet do not tell us much; but his long extensile tongue shows that, whatever he feeds upon, he seeks it in holes. We find a tongue like this in no other bird, but among mammals the aard-vark, the ant-bear, and the pangolins are all similarly equipped, and all live on ants which they extract from their mounds and burrows in hundreds by means of these round, sticky, and extensile tongues. This is precisely the way the flicker gets his living. He lives princ.i.p.ally upon the ground or near it, pecks very little except when digging his nest, and feeds largely upon ants, thrusting his head into the ant-hills and drawing out the ants glued to his tongue rather than speared by it. As he has been known to eat three thousand ants for a meal, we see how much easier this is than spearing them one by one.

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 woodp.e.c.k.e.r 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 ac.u.minate and the tail itself more resembles that of the tree-dwelling woodp.e.c.k.e.rs in shape; the feet are fitted for clinging to the trunk; the bill, now perfectly straight and no longer smoothly rounded but b.u.t.tressed by strong angles that spring from the base and run down toward the tip, is the bill of a woodp.e.c.k.e.r that lives by drilling; but the tongue is wholly unadapted to catching grubs. What kind of food can an arboreal woodp.e.c.k.e.r 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 woodp.e.c.k.e.rs 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 logc.o.c.k and the ivory-billed woodp.e.c.k.e.r, though of the same type as the other larvae-eating woodp.e.c.k.e.rs, 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 woodp.e.c.k.e.rs, 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 woodp.e.c.k.e.rs 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.

XVI

THE ARGUMENT FROM DESIGN