Cranes are perennially monogamous breeders, establishing long-term pair bonds that may last the lifetime of the birds. Pair bonds begin to form in the second or third years of life, but it may be several years before the first successful breeding season is achieved. Initial breeding attempts often fail, and in many cases newer pair bonds will dissolve after unsuccessful breeding attempts. Pairs that are repeatedly successful at breeding will remain together. In a study of Sandhill Cranes in Florida, seven out of the 22 pairs studied remained together for an 11-year period. Of the pairs that separated, death of one of the pair was the primary cause (53%), whilst 18% parted after unsuccessful breeding attempts, reasons for the parting of the remaining 29% of pairs is unknown. Similar results has been found by acoustic monitoring (sonography / frequency analysis of duet and guard calls) in 3 breeding areas of Common Cranes in Germany over 10 years.
Cranes are territorial and generally seasonal breeders. Seasonality varies both between and within species, dependent on local conditions. Migratory species begin breeding upon reaching their summer breeding grounds, between April and June. The breeding season of tropical species, however, is usually timed to coincide with the wet or monsoon seasons. Territory sizes also vary depending on location. Tropical species can maintain very small territories, for example, Sarus Cranes in India can breed on territories as small as one hectare where the area is of sufficient quality and disturbance by humans is minimized.
In contrast Red-Crowned crane territories may require 500 hectares, and pairs may defend even larger territories than that, up to several thousand hectares. Territory defense is usually performed by the male.Because of this females are much less likely to retain the territory than males in the event of the death of a partner.