Why do woodpeckers tap with their bill the trees on which they sit?

Because they may disturb the insects concealed within, so as to seize them when they appear.

Why is the white-billed woodpecker called the carpenter's bird?

Because of the great quantity of chips which it makes.

This bird, and the order to which it belongs, are termed peckers, and have a very remarkable structure of the tongue, consisting of two long cartilages, which are placed immediately under the skin, running from behind forwards over the skull, and terminating at the forehead near the root of the bill. These car-tillages, are like springs, by means of which the bird can more readily protrude its worm-shaped tongue, and transfix insects with its horny point. - Blumenbach.