The wrens of northern North America are small, brown birds, which make up for their drab colors with their complex songs. Active foragers, they generally search for food low in the understory or on the ground, where they use their narrow bills to probe in crevices for insects and spiders. They are usually solitary or in pairs and do not form flocks. Most species lay eggs in enclosed areas, either in cavities or covered nests. Usually the male begins a number of nests, which typically have coarse foundations upon which the actual nest cups may later be built. The female then selects one nest and completes it. Incubation is usually by the female alone, although both parents feed the young. One odd practice of wrens is their tendency to puncture the eggs of other birds, both of their own and other species. One theory to explain this, as well as the multiple nests, is that wrens strive to increase the number of empty nests in their territory. This may protect them from predators that might abandon an area after finding most of the nests empty. Many wrens hold their tails cocked up over their backs. Since many other birds also do this, the posture is by no means diagnostic of wrens.