Nor need we wonder at this, for fine nest building requires both strength and delicacy in the bill and feet; and the next group of birds escapes it altogether by finding or making holes in trees and banks, and lining them with moss or leaves. This group is the Climbers, which come, as it were, between the ground birds and birds of active flight, for they clamber swiftly up the trunks and over the branches of trees in search of fruits and insects, seldom going down to the ground, but flitting from tree to tree to find fresh hunting grounds. [Illustration: Fig. 43. The great green Woodpecker.[103]] What is that green object, about as large as a small squirrel, which we see mounting the trunk of one of the elm trees, as we lie resting on the moss in some quiet wood? There it goes, dodging now to this side, now to that, with its head well lifted and its stiff tail bent against the trunk. It is the green woodpecker at his work. His long large feet, with toes divided _in pairs_, two in front and two behind, take firm hold of the tree with their sharp claws; his breast, which is flatter than that of most birds, lies close against the bark, as he mounts by a number of rapid jumps, which are made by pressing his strangely stiff horny tail against the trunk, while he hops forward with both feet, making a slight rustling noise, and moving so fast that it is difficult to see how he does it. Now he pauses; it is to try a suspicious place in the bark, and tapping it with his beak he finds that it gives a hollow sound. This tells him at once that it is rotten, and there is an insect within; and pecking, a hole with rapid blows of his chisel-like bill, he inserts his narrow bill, and darts a long gluey tongue, with barbed tip, into the dark passage, bringing out the intruder, which is swallowed in a moment. A strange tongue this is of the woodpecker, for it has two long bony branches at its roots, and each one is like a bow bent under and round the back of the bird’s head, and as these bows are tightened or slackened by the slender muscles the tongue is drawn in, or thrust out to an extraordinary length. Moreover, it has at its tip a horny covering beset with tiny barbs, and every time it goes back to the mouth these are bathed in gluey slime to catch the next insect it may meet. Nor is the woodpecker obliged always to drill for his food. The tiny ants, as they wander up and down the trees, the beetles and bees settling on the branches--all may fear this gluey weapon, for all alike disappear within the long thin beak.