Northern Prairie Wildlife Research Center
Cranes are generally monogamous. Mated birds stay together throughout the year, and typically remain paired until one bird dies. The age of sexual maturity has seldom been studied in wild cranes, but individuals in most species probably begin to establish pairs in their second or third years. Pair bonds form within flocks of non-breeding birds, or outside of the breeding season within mixed flocks. Many, if not most, pairs fail to breed successfully in their initial attempts. Among Sandhill Cranes, whose breeding biology has been most extensively studied, it has been shown that pairs that are unsuccessful in their first attempts to breed often dissolve, while successful pairs remain together (Nesbitt 1989). A strong pair bond is maintained as long as the pair successfully reproduces. However, if breeding efforts continually fail, the pair bond weakens and new mates are eventually taken. Most studies indicate that individuals do not successfully reproduce until they are between four and eight years old (Drewien 1973, Kuyt and Goossen 1987). Other species may share this general pattern.
Securing a breeding territory is a prerequisite of reproduction, and in areas where all available territories are occupied, young birds may need to wait to breed. Cranes of the northern temperate and arctic zones begin to establish breeding territories soon after their arrival from the wintering grounds, usually between mid-April and mid-June. The breeding seasons of cranes in the tropical and subtropical zones are much more variable, but generally coincide (and vary) with local rainy seasons. The breeding seasons of the Brolga and Sarus Cranes are closely associated with the distinct monsoons of southeast Asia and Australia. By contrast, Sarus Cranes in India and Wattled and Grey Crowned Cranes in southern Africa may breed throughout the year, although breeding usually peaks in response to localized conditions. Such variability is evident even within species. Sandhill Cranes breeding in Alaska produce almost all of their eggs in June. The breeding season for Florida Sandhill Cranes extends from December to June, with most eggs produced from February to April (Johnsgard 1983).
Breeding densities and territory sizes are poorly known for most cranes, but in some species are apparently quite variable in response to local conditions. In India, for example, nesting territories as small as one hectare are sufficient for Sarus Cranes if the quality of the water and vegetation is adequate and human disturbance is minimized (Gole 1989b, 1991b). In Cambodia, by contrast, Eastern Sarus Cranes establish extensive nesting territories in remote and isolated wetlands (Barzen 1994). Similarly, Eurasian and Sandhill Cranes have both adapted to the intensification of human settlement by establishing breeding territories in smaller, less natural wetlands. Once territories are established, pairs defend them through unison calls, threat postures, and attacks. The male is primarily responsible for defense, while the female is more involved in domestic affairs.
New pairs engage in long bouts of dancing before attempting copulation, whereas established pairs copulate with facility and without tension. The copulatory sequence can be initiated by either sex. One member of the pair will elevate its bill, arch slightly forward, and emit a low purr-like call. If the mate reciprocates with similar behavior, one (usually the male) will circle the other with exaggerated steps. The female then spreads her wings and the male approaches. With wings flapping he jumps on her back, and crouches. The female elevates her tail as the males lowers his, and their cloaca meet. The male then jumps forward over his mate’s head and performs threat displays for a few seconds. Both members of the pair then engage in a long sequence of preening. Cranes copulate for several weeks in advance of laying, usually before sunrise, although copulation can occur at any time during the daylight hours.
Both sexes participate in nest building. They select a secluded spot within their territory and unison call from that spot. Walking away from that selected spot, they toss nesting materials (mainly the stems and leaves of sedges, cattails, and other wetland plants) behind them over their shoulders. They return to the nest site and pull in the materials within their reach before walking slowly away from the nest and throwing additional materials behind them. As they repeat this sequence many times, large quantities of nesting material accumulate at the low platform nest, while a “moat” of water forms around the platform.
This nest-building behavior holds for those species that nest within wetlands. Two species—the Blue and Demoiselle Cranes—nest on dry ground. In both cases, eggs are usually laid directly on the ground. The pair may gather together some small stones or vegetation to provide protection and camouflage, but otherwise the nests show little if any preparation. In some portions of their range, Sandhill Cranes also nest on dry sites. On rare occasion, Grey Crowned Cranes will nest in trees (and have even been observed using the abandoned nests of other large tree-nesting species).
Cranes almost invariably lay two eggs. Exceptions are the Crowned Cranes, which regularly lay three and sometimes four eggs, and the Wattled Crane, which usually lays only one egg. In the Crowned Cranes, incubation begins after the clutch is complete. In the Gruinae species, incubation begins after the first egg is laid. Incubation in cranes averages between 28 and 32 days in most species (Johnsgard 1983). Wattled Cranes, at 33-36 days, have the longest incubation period. Species breeding in higher latitudes and at higher elevations tend to have the shortest incubation periods (<30 days). The female usually incubates at night; during the day the sexes exchange incubation duties several times. The non-incubating member of the pair usually flies to a favorite feeding area far from the nest. Crowned Crane chicks hatch synchronously, while Gruinae chicks hatch asynchronously. Wattled and Siberian Cranes have been observed to leave the nest after only the first egg has hatched, and typically only a single chick is raised in these species. In other species, the parents frequently rear two chicks, but one of these soon becomes dominant. If food is scarce, the weaker chick often dies.
The extent to which multiple clutching can or does occur in cranes is little known. Repeat clutches have, however, been reported for Grey Crowned, Blue, Wattled, Sandhill, Eurasian, White-naped, and Red-crowned Cranes. Florida Sandhill Cranes have been observed laying third, and in one case fourth, clutches (Mirande et al. in press b). Cranes of the northern latitudes, including the Whooping and Siberian Cranes, experience such short growing seasons that even if they did produce second clutches, it would be difficult to fledge the young in time to undertake migration. In captivity, however, females of these and other species have been induced to lay repeatedly through the removal of the eggs, either one at a time or as clutches. In this manner, females have regularly produced up to ten or more eggs in a single breeding season.
Adults care for their chicks continuously through the pre-fledging period. The length of the fledging periods varies widely among the cranes (Johnsgard 1983). Fledging periods are shortest in species that inhabit upland areas (Grey Crowned, Black Crowned, Demoiselle, Blue) and that nest in the high arctic (Siberian and Lesser Sandhills). In general, chicks of these species fledge at between 50 and 90 days. On average, the Demoiselle Crane has the shortest fledging period, usually between 55 and 60 days. Cranes that inhabit permanent wetlands in warmer climates (Wattled, Sarus, Brolga) have the longest prefledging periods. Chicks of these species usually fledge at between 85 and 100 or more days. The Wattled Crane has the longest fledging period at about 90-130 days.
The productivity of a given crane population can be measured in several ways, but is most easily determined by counting the number of juveniles in the flocks during the non-breeding period. In general, about 10-15% of a healthy population will consist of non-breeding juveniles.
Juvenile cranes remain with their parents throughout the non-breeding period. At the conclusion of the non-breeding period, juveniles either voluntarily leave their parents or are driven off by the adults after the family returns to the breeding territory. Unpaired juvenile birds gather in non-breeding flocks and are often nomadic throughout the breeding period. By the end of their second year, juvenile birds have usually initiated their own attempts to form pair bonds.