They knock, carpenter, drum. Woodpeckers are the greatest builders in the bird world. No matter whether they are pileated, green or black, they hammer out more cavities than they need themselves, to the benefit of tits, stock doves, pygmy owls, red squirrels and other cavity dwellers. Also for love: to win over mates, woodpeckers drum at the highest frequency. And in their search for the greatest resonance, they sometimes knock on metal!