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.
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.
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.