Animals that play important roles in the lives of others in an ecological community are known as keystone species. Many woodpeckers are keystones, as some birds, mammals and invertebrates benefit from the cavities that they create. Indeed, some species can be highly dependent upon woodpeckers for nesting or denning sites. Birds and mammals that do not excavate holes themselves, but use those made by others for nesting or roosting, are called secondary cavity-users. In some cases the local populations of secondary-cavity using birds can even be determined by the availability of woodpecker holes. Birds that use woodpecker cavities include titmice, flycatchers, hirundines, ducks, kestrels, owls, doves, parrots, trogons and even large species of toucans and hornbills. Photo: Tawny Owl Strix aluco in a Black Woodpecker hole in Hungary (Laszlo Becsy).