The Woodpeckers Part 4

XII

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

We have studied the woodp.e.c.k.e.r'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 woodp.e.c.k.e.r. 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 woodp.e.c.k.e.rs.

[Ill.u.s.tration: Foot of Woodp.e.c.k.e.r.]

I never see a woodp.e.c.k.e.r'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 woodp.e.c.k.e.r 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 woodp.e.c.k.e.r 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 ill.u.s.tration shows why the woodp.e.c.k.e.r'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.

[Ill.u.s.tration: Diagram of right foot.]

Scientists tell us that a woodp.e.c.k.e.r'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 woodp.e.c.k.e.r'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 woodp.e.c.k.e.rs did also before it became permanently reversed.

[Ill.u.s.tration: Foot of Three-toed Woodp.e.c.k.e.r.]

That this is exactly what had happened is curiously confirmed. There are a few woodp.e.c.k.e.rs 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 sh.o.r.e birds have but three). Compare this picture with a four-toed woodp.e.c.k.e.r'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 woodp.e.c.k.e.r 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 woodp.e.c.k.e.r'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 woodp.e.c.k.e.r'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 woodp.e.c.k.e.r'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 woodp.e.c.k.e.r's way of living, and we find by examination that every woodp.e.c.k.e.r 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 woodp.e.c.k.e.rs, 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 woodp.e.c.k.e.rs nor to climbing birds. The parrots, which climb after a fas.h.i.+on, 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 woodp.e.c.k.e.rs. 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 woodp.e.c.k.e.r 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 woodp.e.c.k.e.r. 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 woodp.e.c.k.e.rs have the greatest development of the fourth toe? Not at all. There is our rare acquaintance the logc.o.c.k, or pileated woodp.e.c.k.e.r, a bird nearly as large as the ivory-billed, one of the most persistent of our tree-climbers and more than any other woodp.e.c.k.e.r 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 woodp.e.c.k.e.r 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 logc.o.c.k's habits are not what all previous observers have believed them to be,--which would be a.s.suming a great burden of proof; or by demonstrating that his ancestry explains why his feet do not ill.u.s.trate 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 woodp.e.c.k.e.r 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 logc.o.c.k--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 woodp.e.c.k.e.r--have been the strongest possible means of totally demolis.h.i.+ng 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 WOODp.e.c.k.e.r'S TOOLS: HIS TAIL

If we study the woodp.e.c.k.e.r'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.

[Ill.u.s.tration: Tail of Hairy Woodp.e.c.k.e.r.]

Our first impression is that the woodp.e.c.k.e.r'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 woodp.e.c.k.e.rs, 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 woodp.e.c.k.e.r's tail had been cut square across and made of feathers equally rigid and ending in short stiff spines? For we see that the woodp.e.c.k.e.r'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?

[Ill.u.s.tration: Tails of Brown Creeper (under surface) and Chimney Swift (upper surface.)]

Besides the woodp.e.c.k.e.rs 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 woodp.e.c.k.e.r's as it is; while the chimney swift's is precisely like the woodp.e.c.k.e.r's as we thought it ought to be.

But we observe that while the creeper's habits are almost precisely like the woodp.e.c.k.e.r's,--so much so that when we first make his acquaintance, some of us will be sure we have discovered a new kind of woodp.e.c.k.e.r,--the chimney swift has but one habit in common with the woodp.e.c.k.e.r, that of clinging to an upright surface and propping himself by his tail. If the bird with the tail most like the woodp.e.c.k.e.r's has the woodp.e.c.k.e.r'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 woodp.e.c.k.e.rs themselves? The logc.o.c.k and the ivory-billed woodp.e.c.k.e.r 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 logc.o.c.k, and longer (in the specimens measured) in the almost wholly terrestrial flicker than in the wholly arboreal logc.o.c.k. Without much more study all that we can safely infer is that the woodp.e.c.k.e.r'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.

[Ill.u.s.tration: Middle tail feathers of Flicker, Ivory-billed Woodp.e.c.k.e.r, and Hairy Woodp.e.c.k.e.r.]

All woodp.e.c.k.e.rs' 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 woodp.e.c.k.e.rs 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.

[Ill.u.s.tration]

Diagram of curvature of tails of Woodp.e.c.k.e.rs. Drawn to scale.

_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 Woodp.e.c.k.e.r.

3. Downy Woodp.e.c.k.e.r.

4. Logc.o.c.k.

5. Central American Ivory-billed Woodp.e.c.k.e.r.

6. North American Ivory-billed Woodp.e.c.k.e.r.

I will show you how to prove this point so that you may be satisfied about it even if you should never see a woodp.e.c.k.e.r. 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.