The Woodpeckers

XI

THE WOODPECKER’S TOOLS: HIS BILL

There is an old saying, “You may know a carpenter by his chips;” but, though chips are seldom long absent when a woodpecker is about, can we call the woodpecker a carpenter? Is he not both in his works and ways of working – with the one exception of the Californian woodpecker – more of a miner?

For the carpenter takes pieces of wood, bit by bit, and joins them together till at last he has built a lofty skeleton or framework for his dwelling, which last of all he covers over and closes in; and the tools he uses are saw and hammer. With these alone he could build his house, though it might be neither very large nor very good. When a carpenter’s house is finished, it is neither a cave nor a hole, but a pavilion built in the open air after the model of a spreading tree, – which frames a roof with its branches and shingles it with overlapping leaves. There is nothing in the woodpecker’s way of building which corresponds to that.

Quite different are the miner’s methods. In the West, where the barren mountain sides stretch up into snowclad summits, on the face of slopes as seamed and gray and verdureless as the wrinkled trunk of an aged oak, I have seen holes where human woodpeckers burrow. The entrance to a mine half-way up a hillside looks strikingly like a woodpecker’s hole and scarcely larger. Nor does the likeness vanish as we think how in their long tunnels inside their mountains of gold and iron and silver the delving miners are picking and prying and picking to lengthen their burrows just as the woodpeckers peck and pry and peck inside their wooden mountain, the tree-trunk. Which shall we call the woodpecker – a carpenter or a miner?

What are the miner’s tools? Pick and drill, are they not? What are the woodpecker’s? The same. Certainly we shall see, if we stop to think, that it is not a chisel that he uses, as we sometimes say. A chisel is a knife driven by blows of a hammer; like a knife its effectiveness depends upon the sharpness and length of its cutting edge. But a woodpecker’s bill is not a cutting tool. It is a wedge, but a wedge working on a different principle from a knife-edge. Look at this one and observe that, though strong and stout, it is not sharp and has no true cutting edge. It is a tapering, square-ended, flat-sided tool, rather six-sided at the base and holding its bevel and angles to the tip. The woodpecker’s bill is a pick, not a chisel. It is used like a pick, being driven home with a heavy blow and getting its efficiency from its own weight and wedge-shape and from the force with which it is impelled. Watch the downy woodpecker at his work and see what sturdy blows he delivers, pausing after each one to aim and drive home another telling stroke. This is pick-axe work. But sometimes he rattles off a succession of taps so short and quick that they blend together in one continuous drumming, too light and quick to be likened to the ponderous swing of the pick-axe. Now he is drilling. The work of a drill is to cut out a small deep hole either by twirling (as in drilling metals) or by tapping (as in drilling stone). The woodpecker drills by the latter method and there is a curious likeness between his bill and the mason’s tools.

Any one who has lived in a granite country knows the deep round holes that stone masons make when they split rock. Did you ever wonder why they are as large at the bottom as at the top? If you remember the shape of a mason’s drill, you will recollect that it looks a little like a stick of home-made molasses candy bitten off when it was just soft enough to stretch a little. The mason’s drill is a round iron rod with a thin, flat end, sharpened on the edge and a little pointed in the centre. In the flattening of the sides and the width across the tip its end resembles that of a typical woodpecker’s bill. The woodpeckers that drill for grubs, especially the largest, the logcock and the ivory-billed woodpecker, have the tip remarkably flattened. The likeness to the drill does not go farther because the woodpecker’s bill is a combination tool; but it is drill-pointed rather than pick-pointed.

What is the advantage of this compressed tip? Can the bird pick as well as he could with a sharp point? The bird and the mason reap the same benefit from this form of tool. A sharp-pointed drill would bind in the hole and could neither be driven ahead nor removed without difficulty, but the sharp-edged tool cuts a hole as wide as the instrument. There is, of course, some difference between working in stone and in wood, but the principle is the same. The mason strikes his drill with his hammer and cuts a crease in the stone; then lifts and turns the drill, cutting a crease in another direction; and so by continually changing the direction of the cuts until they radiate from a centre like the spokes of a wheel, he finally reduces a little circle of stone to a powder fine enough to be blown out of the hole. In drilling for a grub the woodpecker must do much the same thing. He wishes to keep his hole small at the top so as to save work, yet it must be large enough at the bottom to admit the borer when nipped between his mandibles; therefore he needs an instrument that, like a drill or a chisel, will cut a straight-sided hole. Indeed, we might call it a chisel just as well if it were not a double-wedge instead of a single wedge and if it did not move when it is struck instead of being held stationary beneath the blows.

When he is digging his house the woodpecker uses his bill as a pick-axe. When he is digging for grubs he uses it as a drill. Now some species drill very little and some a great deal, according to the number of grubs they feed on; but all dig holes to nest in, – that is, all use their bills as picks but only a few employ them as drills. The flickers, for example, seldom drill for grubs, their food being picked up on the surface or dug from the earth; yet they excavate the deepest, roomiest holes made by any woodpeckers of their size; they use their bills effectively as pick-axes, but seldom, very seldom, as drills. And what do we find? No drill-point – not a truncate, compressed bill fit for drilling, but a sharper, pointed, rounded, curving bill. Notice the ordinary pick-axe and see how much nearer the flicker’s bill than the logcock’s or the ivory-billed woodpecker’s it is. Why is a flicker’s bill better for being curved also? Why do the drilling woodpeckers have a perfectly straight bill? We should find by studying the birds and their food that there is a direct relation between the shape of the bill and the amount of drilling a woodpecker does; that the grub-eating or drilling woodpeckers have a straight bill, for working in small deep holes, while the flickers have a curved bill for prying out chips. And we should note that the flicker’s bill is most like the ordinary bill of perching birds, while the drilling bill, as typified by the logcock’s and the hairy woodpecker’s bills, is a more specialized tool, limited to fewer uses, but more effective within its limits.

There is another detail of the woodpecker’s bills which casts light upon their habits. The species that drill most have their nostrils closely covered by little tufts of stiff feathers, scarcely more than bristles, which turn forward over the nostril. The density and the length of these tufts agree very well with the kind of work the woodpecker does; for in the hairy and the downy, which are continually drilling and raising a dust in rotten wood, they are very thick and noticeable, while in the red-head and the sapsucker they show as scarcely more than a few loose bristles, and in the flicker they barely cover the nostril. This seems a plain provision to keep the dust out of the bird’s lungs; and we might cite as additional evidence the fact that the only other birds of similar tree-pecking habits, the nuthatches and the chickadees, have their nostrils protected in the same way. But we must always be cautious before drawing inferences of this sort to see what may be said on the other side. When we recollect that the crows and ravens and many kinds of finches, among other birds, none of which dig in the bark of trees or raise a dust, have their nostrils as completely covered, we see that we have perhaps discovered a use for these nasal tufts but not the cause of their being there. We must be careful not to mistake cause and accompaniment in our endeavor to explain differences in structure.

Let us see what we have learned and how to interpret it: —

That the woodpecker’s bill is a combination of drill and pick-axe.

That the shape varies with the use to which it is most commonly put.

That the use varies with the food principally eaten; or, what is a step farther back, that the different kinds of food must be sought in different places and by different methods, and therefore require different tools.

Therefore the shape of the woodpecker’s bill has a direct relation to the kind of food he eats. Please notice that we do not assert that it causes him to eat a certain kind of food nor that a certain diet may not have affected the shape of the bill, causing it to be what we now see. Both may be at least partially true, but to prove either or both would need profound study, and all that we have observed is that the shape of the woodpecker’s bill is adapted to his food and that it varies with the kind of food he eats, or, to be more exact, with his ways of procuring it.

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.