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Woodpeckers

<h2>XIII</h2> <h3>THE WOODPECKER’S TOOLS: HIS TAIL</h3> 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. <div class="figcenter"> <ANTIMG src="images/fig_015.jpg" alt="Tail of Hairy Woodpecker." title="" /> Tail of Hairy Woodpecker.</div> 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,[87] 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? <div class="figcenter"> <ANTIMG src="images/fig_016.jpg" alt="Tails of Brown Creeper (under surface) and Chimney Swift (upper surface.)" title="" /> Tails of Brown Creeper (under surface) and Chimney Swift (upper surface.)</div> 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[88] 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[89] 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. <div class="figcenter"> <ANTIMG src="images/fig_017.jpg" alt="Middle tail feathers of Flicker, Ivory-billed Woodpecker, and Hairy Woodpecker." title="" /> Middle tail feathers of Flicker, Ivory-billed Woodpecker, and Hairy Woodpecker.</div> 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[90] 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. <div class="figcenter"> <ANTIMG src="images/fig_018.jpg" class="wide1" alt="" title="" /> Diagram of curvature of tails of Woodpeckers. Drawn to scale.</div> <div class="blockquot"> a, a, point of insertion in rump. a, b, outer tail feather. a, c, middle tail feather. If the outer tail feather were of the same length in all cases, the curve at the end of the tail would be represented by the dotted lines. 1. Flicker. 2. Red-headed Woodpecker. 3. Downy Woodpecker. 4. Logcock. 5. Central American Ivory-billed Woodpecker. 6. North American Ivory-billed Woodpecker. </div> 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,[91] 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. <div class="figcenter"> <ANTIMG src="images/fig_019.jpg" alt="Patterns of tails." title="" /> Patterns of tails.</div> [92] 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: <table summary="Using a tail as a prop"> <tr><td>Spread.</td><td>Square end.</td><td>Forked end.</td><td>Round end.</td></tr> <tr><td>one plane,</td><td>varies</td><td>varies</td><td>middle pair</td></tr> <tr><td>curved up,</td><td>middle pair</td><td>middle pair</td><td>middle pair</td></tr> <tr><td>curved down,</td><td>all</td><td>all</td><td>all</td></tr> <tr><td>Closed.</td></tr> <tr><td>one plane,</td><td>outer pair</td><td>outer pair</td><td>middle pair</td></tr> <tr><td>different planes,</td><td>outer pair</td><td>outer pair</td><td>all</td></tr> </table> Which shape brings the most feathers into use [93] 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[94] 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.[95] 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[96] 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[97] 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[98] 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. <div class="figcenter"> <ANTIMG src="images/fig_020.jpg" alt="Under side of middle tail feather of Ivory-billed Woodpecker." title="" /> Under side of middle tail feather of Ivory-billed Woodpecker.</div> 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?[99] <hr /> <div style="break-after:column;"></div>

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