As with other cranes, the Whooping Crane has a low reproductive rate compared to most birds. This has contributed to its slow recovery. The birds begin pair bonding during their second or third winter; the process may take up to three winters while the birds associate in flocks of sub-adults. The birds remain paired until one dies, after which the other usually re-mates quickly. Pair bonding, which is repeated annually, consists of walking, calling, and dancing in unison; the dance includes spectacular vertical leaps with neck arched and wings flapping. Breeding usually begins at four years of age. Experienced pairs arrive on the breeding grounds in and around Wood Buffalo National Park, on the border of Alberta and the Northwest Territories, in late April, returning to their previous territories, which they defend vigorously against young pairs arriving later. The nest is a heap of vegetation with a shallow depression on top in which two eggs are laid. Both birds incubate, primarily the male in daytime and the female at night. The precocial young hatch in about a month and are brooded at night and during bad weather for several weeks, initially in the nest but subsequently elsewhere within the territory. The young are fed entirely by the parents at first and gradually learn to feed for themselves, though they continue to beg for months afterward. The chicks can fly well by about three months of age and reach adult size at about eight months. They migrate south with their parents the first time and remain with them through the first winter, usually returning with them to the breeding grounds. Young birds usually spend their first few summers near their natal area before beginning to breed, and migrate and winter together.