The Great Grey Owl builds no nest of its own but breeds in abandoned stick nests left by birds of prey, sometimes on broken tree trunks. The female usually lays 3–5 eggs, but clutch size is strongly governed by the abundance of voles. Good rodent years produce large clutches and high breeding success, whereas in poor years the species may fail to breed altogether. The female is very aggressive at the nest and defends the young vigorously.