XII
THE WOODPECKER’S TOOLS: HIS FOOT
We have studied the woodpecker’s bill and have found that it is a very serviceable tool. We shall find that his feet are equally well adapted to their work.
Here is the foot of a woodpecker. Observe how it differs from a chicken’s foot, or a sparrow’s foot. What is it that especially fits it for climbing? Perhaps you will notice that the tarsus is short, and you may be able to explain why it would be a disadvantage for a climbing bird to have long legs, as well as why it is a help for him to have long toes. Toes long and legs short is the rule with the woodpeckers.
I never see a woodpecker’s foot without thinking of an iceman’s nippers with their short handles and long, sharp-toothed jaws. They are designed for similar uses, – to lift heavy weights by laying hold of smooth, flat surfaces. The iceman sets his nippers into the ice and lifts the block; but the bird sets his claws into the tree and lifts his own body.
Suppose the nippers had one short jaw and one long one, would they then take as firm hold as they do with jaws of equal length? In perching birds the hind toe is much the shortest, but they sit balanced upon a limb and have merely to hold themselves in position. The woodpecker climbing a tree-trunk is out of balance; he would fall off unless he had a firm grip; and he could not get this firm hold if his hind toes were not long enough to give his foot a nearly equal spread back and forward. Other birds grasp a limb with the whole under surface of their toes, but the woodpecker when on a smooth, upright tree-trunk nips it only with his toenails. Try with your own hand to hold a stick as large and heavy as you can grasp, and you will see that when you clasp your hand around it as a perching bird takes hold of a perch, it makes little difference that the thumb is shorter than the fingers, but when you try to nip it with your finger tips alone, you must bend your fingers until they are not much longer than your thumb, – that is, a pair of nippers must be equal jawed.
This simple illustration shows why the woodpecker’s foot reaches as far backward as forward. But a sensible objection may be raised, namely, that as there are two hind toes of unequal length, it is by no means certain which is the more necessary.
Scientists tell us that a woodpecker’s foot, though it looks so unlike a chicken’s, is really very much the same. When we ask how one of the front toes disappeared and how the extra hind toe came to be where it is, they tell us that there has been no addition and no loss, but the extra hind toe is only a front toe turned backward. They call it a reversed fourth toe. A bird’s toes are numbered in order starting with the hind toe and going around the inside of the foot to the outer or fourth toe. The hind toe is the thumb, and the others are numbered in the same order as the fingers of our hands. So we see that the woodpecker’s real hind toe is rather small, like that of most birds. It looks very much as if it had been found too small and as if another had turned back to help it do its work. Do you say that a bird cannot turn his toes about in this way? Most cannot, to be sure, but all of the owls can do it. An owl will sit either with two toes forward and two backward, or with three forward and one the other way. The owls have a reversible outer toe, and perhaps the woodpeckers did also before it became permanently reversed.
That this is exactly what had happened is curiously confirmed. There are a few woodpeckers in this country which have but three toes. They are the only North American land birds with less than four toes (though many sea and shore birds have but three). Compare this picture with a four-toed woodpecker’s foot. One toe is gone completely, when or how no one can tell. But in some way the first toe, the thumb, the one we always begin to count from, has disappeared. The one left is the reversed fourth toe, as we know by the number of joints in it. Undoubtedly this woodpecker needed a hind toe, but he must have needed a longer, stronger one than his natural first toe. A toe of the right length was supplied by turning one of the front toes back, and the short hind toe in some way disappeared.
This may seem a roundabout way to show that a woodpecker’s foot is a pair of nippers. First we studied nippers till we found out that they were not good nippers unless they were nearly equal-limbed. Next we studied the woodpecker’s foot to learn about that extra hind toe. Then it occurred to us that four toes were not necessary, since some of our best climbers have but three. What was the essential point? Might it not be a foot equally divided without reference to the number of toes? But that is the principle of a pair of nippers. Then came the question, is there any similarity in their use? Yes, the nippers are used to lift heavy weights, and the woodpecker’s foot is used to lift his heavy body in just the same way, by taking hold of a flat, smooth surface. We conclude that a wide-spread, equally divided, nipping foot would be the best device possible for the woodpecker’s way of living, and we find by examination that every woodpecker shows this type of foot.
There is additional evidence that this is the right explanation. Our only other North American birds that climb on the bark of trees professionally, as we may say, are the brown creepers and the nuthatches. In both these the tarsus is short, as we found it in the woodpeckers, and the hind toe and its claw are fully equal to the middle toe and claw, making an equally divided foot. On the other hand, the foot with two toes forward and two toes backward is confined neither to woodpeckers nor to climbing birds. The parrots, which climb after a fashion, have it; but so do the cuckoos, which do not climb, some of which, like our road-runner, or ground cuckoo of the West, are strictly terrestrial. The “yoking” of the toes may occur by the reversion of the fourth toe, as ordinarily, or of the second toe, as in the trogons; the arrangement appears to be definitely related to the distribution of the tendons that control the toes. But though accounting for the structure may give a clue to its descent, it does not justify its efficiency. The yoke-toed foot is not exclusively a climbing foot. All our families of climbers have at least one representative with but one toe behind, and this clearly proves that the yoke-toed structure is by no means necessary even though it may be an honorable inheritance among climbers. The natural conclusion is that the important point in climbing is not the number nor the arrangement of the toes, but the length of at least one hind toe so as to give an equally divided foot.
There is an interesting point to notice about the woodpeckers. This reversed fourth toe is curiously variable in length. In the flickers, with its claw, it is a little shorter than the middle (third) toe with its claw; in the red-heads and their friends it a little exceeds the middle toe and claw; in the downy and the hairy it is much the longest toe, and in the ivory-billed woodpecker it is abnormally developed. We at once judge that it is some indication of the bird’s manner of life, and we look for it to be largest in the species that live continually upon the trunks of trees, obtaining most of their food by drilling. We expect to see the finest development of drilling bill accompany this enormously developed toe, and we find them both in the ivory-billed woodpecker. In imagination we clearly see the use of it. The great bird, keen in his quest of grubs, sidling hastily round the tree, in an unsteady balance and unsupported by his tail, throws one long hind toe downward to steady himself, hooks the other into the bark above him, and hangs between the two as firmly supported as in his ordinary position. No doubt he does do this, but does it prove the supposition that the heaviest and most arboreal woodpeckers have the greatest development of the fourth toe? Not at all. There is our rare acquaintance the logcock, or pileated woodpecker, a bird nearly as large as the ivory-billed, one of the most persistent of our tree-climbers and more than any other woodpecker I ever observed given to scratching rapidly round and round a tree-trunk, clinging at ease in almost any position except head-downward, and drilling incessantly and at all seasons for grubs; he is a typical woodpecker of the largest size, but his hind toe and claw are, if anything, a trifle shorter than his middle toe with its claw. He throws it out and uses it as we have described, but it has not that disproportion to the other toes which we expected to find as the result of a strictly arboreal life.
What have we proved? We have not shown that the long toe is not more useful than the shorter one, – that is a matter of observation; but we have failed entirely to show that it is so, and this can be done only in one of two ways: either by proving that the logcock’s habits are not what all previous observers have believed them to be, – which would be assuming a great burden of proof; or by demonstrating that his ancestry explains why his feet do not illustrate our theory, – and this, though it is undoubtedly the true solution, could be settled only by a very learned man.
But we have encountered one truth which must always be held in mind in science – that a theory is not proved while a single fact remains rebellious and unsubdued. We might have examined every other woodpecker in the continent but just one; we might have seen that every other one agreed with our theory, as it does; we might have supposed that the explanation was good past doubting; but that one exception – if it was a logcock – would still over-turn the whole theory; and the very facts that we relied upon to strengthen us – its resemblance in size, habits, shape, and color to the ivory-billed woodpecker – have been the strongest possible means of totally demolishing our fine theory. We have learned, if nothing more, that all the facts must be examined and accounted for before an explanation is accepted as indisputable.
XIII
THE WOODPECKER’S TOOLS: HIS TAIL
If we study the woodpecker’s anatomy and observe his broad, strong, highly-arched hip-bones and the heavy, triangular “ploughshare” bone in which the tail feathers are planted, as well as the stiffness and strength of the tail itself, we must conclude that it is not by accident that he uses his tail as a prop. The whole structure shows that the bird was intended “to lean on his tail.” What we wish to discover is how good a tail it is to lean on.
Our first impression is that the woodpecker’s tail might be improved. Why are not the tips of the feathers stiffer? Why is it so rounded? Most of the work seems to fall on the middle feathers, and in some species, as the downy and the hairy woodpeckers, these end in decurved tips so soft and unresisting that they seem quite unfit to give any support. Would it not be better if the woodpecker’s tail had been cut square across and made of feathers equally rigid and ending in short stiff spines? For we see that the woodpecker’s tail is not only weak in its inner feathers, but weaker still in its outer ones, and it is stiff, in most species, only in the upper three fourths of its length.
When we propose a change in nature it is wise to inquire whether our improvement has not been tried before and to learn how it worked. How many kinds of birds have we that use their tails for a support? What are their habits and what sort of tails have they?
Besides the woodpeckers we have but two kinds of land birds that prop themselves with their tails, – the swifts and the creepers. The creeper has a tail very much like the woodpecker’s as it is; while the chimney swift’s is precisely like the woodpecker’s as we thought it ought to be. But we observe that while the creeper’s habits are almost precisely like the woodpecker’s, – so much so that when we first make his acquaintance, some of us will be sure we have discovered a new kind of woodpecker, – the chimney swift has but one habit in common with the woodpecker, that of clinging to an upright surface and propping himself by his tail. If the bird with the tail most like the woodpecker’s has the woodpecker’s habits, is it not a fair inference that this form of tail is better fitted to this way of living than the other would be?
Next, what variations in shapes do we observe among the woodpeckers themselves? The logcock and the ivory-billed woodpecker have the longest tails – because they are the largest birds. When we compare the length of the tails with the length of the birds we are surprised at the results. On measuring sixteen species, representing seven genera, I find that the tail is from three tenths to thirty-five hundredths of the entire length; that it is, in proportion, as long in the flicker as in the ivory-bill, as long in the downy as in the logcock, and longer (in the specimens measured) in the almost wholly terrestrial flicker than in the wholly arboreal logcock. Without much more study all that we can safely infer is that the woodpecker’s tail is not far from one third the length of his whole body measured from the tip of the bill to the tip of the tail. Probably this is the proportion most convenient for his work.
All woodpeckers’ tails agree in one particular: they are rounded at the end. At first sight we would say that some are but slightly rounded and others very deeply graduated; but as nearly as I can determine this is at least partly an optical illusion, explained by the great difference in the shape of the feathers making up the tail, which in some, as the flicker, are very broad and abruptly pointed, and in others taper gradually to the end and are very narrow for their length. The larger birds naturally appear to have longer tails, and the effect of narrow feathers is to make the tails appear longer and more sharply graduated than they really are. This diagram shows the shape of the curve in six species, and indicates that, while the curvature is less than we might expect, it bears some relation to the bird’s way of living; for we see that the strictly arboreal woodpeckers have more pointed tails than the terrestrial species, and that the amount of gradation bears a direct relation to the amount of time spent upon the tree-trunks.
There is a third difference, the shape of the individual feather, to which we shall refer again; but now we wish to examine the uses and meaning of the curved end.
I will show you how to prove this point so that you may be satisfied about it even if you should never see a woodpecker. We will make a little experiment, so simple that even a child can understand it.
First, how many shapes can any bird’s tail have? It may be one of three general patterns, and it can be nothing else unless we combine those patterns. It may be square across the end, it may have the middle feathers longest, or it may have the outer feathers longest. To one of these patterns every form of birds’ tails may be referred; you can invent no other shape.
Let us assume that you know nothing whatever of a woodpecker’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.
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:
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 woodpeckers.
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 logcock’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 woodpecker 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 woodpeckers 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 woodpecker’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 principal 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 acuminate 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 logcock 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 woodpecker the two outer pairs are of no assistance 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 woodpecker 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 woodpecker does so much with his weak body, we can imagine what great birds like the logcock and the ivory-billed woodpecker, 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.
One point we have left unexplained: What is the advantage, if there is any, in the sharper curve to the tails of the arboreal woodpeckers? 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 woodpecker 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 woodpecker’s hole?