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.

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,variesvariesmiddle pair
curved up,middle pairmiddle pairmiddle pair
curved down,allallall
Closed.
one plane,outer pairouter pairmiddle pair
different planes,outer pairouter pairall