Woodpeckers have many adaptations that allow them to perch upright against tree trunks and feed on insects under the bark or within the wood of the tree itself. Further specialization has produced many aberrant forms with different behavior and feeding habits. Most use their strong claws and stiff tail feathers to brace themselves against tree trunks as they climb. The specially adapted skulls of woodpeckers allow them to pound hard on tree trunks to excavate nesting and roosting cavities, to find food, and to communicate and attract mates. A special arrangement of bones and elastic tissues allows woodpeckers to extend their long tongues and extract insect prey from the holes they chisel with their strong, sharp beaks. The principal food of most woodpeckers is insects, especially the larvae of wood-boring beetles. A few woodpeckers feed on ants, nuts, or flying insects. Many also take a small amount of fruit. Most woodpeckers have rounded wings and an undulating flight pattern. The plumage of most is some combination of black and white, though brown is not uncommon. Many, especially males, have small patches of red or yellow on their heads. Although they may appear to damage trees, woodpeckers are generally good for tree health because they feed so heavily on wood-boring beetles. Most woodpecker species are monogamous, and many form long-term pair bonds. The nests are usually lined with nothing but the woodchips created by excavating the nest cavity, which is excavated by both members of the pair. Both sexes incubate the eggs, with males generally taking the night shift. Both sexes also feed and tend the young.