Russel Wallace : Alfred Russell Wallace (sic)
Parkstone, Dorset, October 22, 1897
My dear Violet, In your previous letter you asked me the conundrum, Why does a wagtail wag its tail? That's quite easy, on Darwinian principles. Many birds wag their tails. Some Eastern flycatchers--also black and white--wag their long tails up and down when they alight on the ground or on a branch. Other birds with long tails jerk them up in the air when they alight on a branch. Now these varied motions, like the motions of many butterflies, caterpillars, and many other animals, must have a use to the animal, and the most common, or rather the most probable, use is, either to frighten or to distract an enemy. If a hawk was very hungry and darted down on a wagtail from up in the air, the wagging tail would be seen most distinctly and be aimed at, and thus the bird would be missed or at most a feather torn out of the tail. The bird hunts for food in the open, on the edges of ponds and streams, and would be especially easy to capture, hence the wagging tail has been developed to baffle the enemy.
Copyright: Alfred Russel Wallace Literary Estate.