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.
[Illustration: Tail of Hairy Woodpecker.]
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?
[Illustration: Tails of Brown Creeper (under surface) and Chimney Swift
(upper surface.)]
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.
[Illustration: Middle tail feathers of Flicker, Ivory-billed Woodpecker,
and Hairy Woodpecker.]
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.
[Illustration]
Diagram of curvature of tails of Woodpeckers. 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 Woodpecker.
3. Downy Woodpecker.
4. Logcock.
5. Central American Ivory-billed
Woodpecker.
6. North American Ivory-billed
Woodpecker.
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.
[Illustration: 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 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.
[Illustration: Under side of middle tail feather of Ivory-billed
Woodpecker.]
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?
XIV
THE WOODPECKER'S TOOLS: HIS TONGUE
We have seen how the woodpecker spears his grubs: now we will study his
spear.
[Illustration: Tongue of Hairy Woodpecker. (After Lucas.)]
There are many interesting points about a woodpecker's tongue, and they
are not hard to understand. If a woodpecker 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 woodpecker
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, horny, 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?
[Illustration: 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
woodpeckers. 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
knob at the end, marked _a_, bore the horny 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 pass 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, pass 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 pass 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.
[Illustration: Skull of Woodpecker, showing bones of tongue. _a._ Upper
end of windpipe and gullet.]
[Illustration: Hyoids of Sapsucker and Golden-fronted Woodpecker.]
Not all woodpeckers 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 woodpecker 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
principally 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 WOODPECKER IS FITTED FOR HIS OWN KIND OF LIFE
We have studied the woodpeckers 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
woodpecker,--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 principally 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 woodpecker 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 acuminate and the tail itself more resembles that of
the tree-dwelling woodpeckers in shape; the feet are fitted for clinging
to the trunk; the bill, now perfectly straight and no longer smoothly
rounded but buttressed by strong angles that spring from the base and
run down toward the tip, is the bill of a woodpecker that lives by
drilling; but the tongue is wholly unadapted to catching grubs. What
kind of food can an arboreal woodpecker 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.