They drill holes in trees and fit acorns into the holes. If the acorn crop fails, however, the birds store pebbles instead. Many nests are marvels of skill. But if a nest is damaged while it is being built, the bird is unable to repair it. The parents have to start over again from the beginning.
Birds show the most apparent intelligence in meeting the problems of getting their food.
A few birds make use of tools in a very human sense. Gulls and ravens break open shellfish by carrying them up to a height and dropping them down on rocks. They have learned that paved roads and stone walls serve the same purpose as the rocks of the seashore. The song thrushes of Europe hammer a snail on a stone until the shell is broken. The woodpecker finches of the Galapagos Islands feed on insects in the ground or in the trunks of trees.