Northern Prairie Wildlife Research Center
Cranes can be divided into two groups: migratory and non-migratory. Non-migratory cranes move relatively short distances between breeding and non-breeding areas, and gather in large flocks prior to the onset of the breeding period. Local and seasonal movements of varying lengths are typical of the lower latitude species—the Crowned Cranes, the Blue, Wattled, Brolga, and Sarus Cranes. In most cases, their breeding seasons and hence their movements are tied to, and vary with, the duration and intensity of the local rainy seasons. Local and seasonal movements are also characteristic of southern, non-migratory populations of some of the northern migratory species. This may be seen among populations of the Demoiselle Crane in northern Africa and Sandhill Cranes in Cuba, Florida, and Mississippi.
Such limited and seasonal movements are modest in comparison with the epic migrations of the northern cranes. Some of the migration routes stretch thousands of kilometers, during which the cranes must confront broad deserts, high mountain ranges, and other formidable obstacles. This achievement is even more remarkable for the northernmost species, the Siberian and Sandhill Cranes breeding in the arctic latitudes of Eurasia and North America. Among these populations, the young of the year must in one short growing season gain the size, strength, and endurance to join their adult companions on the long journey south.
Migratory cranes spend several days or weeks at premigration staging areas building up their fat reserves and integrating into life as a flock. Then they commence migration. After feeding for several hours in the early morning (often on a clear day with northwest breezes gusting) they rise into the sky, flap-flying in wide circles, lifted by thermals. After climbing as high as 2000 meters, they stop flapping, extend their wings, assume a “V” formation, and glide southward propelled by gravity and wind. After losing altitude, they repeat the cycle, again spiralling skyward and gliding south. While flying over land, they follow this pattern throughout the day. However, when forced to fly over water, where there are no thermals, they flap-fly in “V” formation. Crane chicks fly close to their parents and during their first migration south learn the migration route. While migrating, cranes call constantly. Their voices can often be heard even before the birds are spotted as tiny spots against the blue sky.
Migration has been studied closely in several species. Whooping Cranes, for example, have been shown to migrate as much as 800 km in a single day, although 300 km is more typical (Howe 1989). A review of Sandhill Crane migration data showed average flight lengths of 267 km/day, with individual flights of up to 740 km (Melvin 1982, Melvin and Temple 1982). Estimated flight speeds in Sandhill Cranes average between 23 and 83 km/h depending on wind speed and direction (Melvin 1982). This range is probably similar to that of most migratory cranes. Several crane migrations stand out as especially impressive. Eurasian Cranes from central Eurasia fly over the Himalayas at altitudes approaching 10,000 meters, while Demoiselle Cranes negotiate the passes in these highest mountains on earth. Other Demoiselles migrate across the wide deserts of the Middle East and northern Africa to wintering grounds in the upper reaches of the Nile basin. Siberian Cranes in the remnant Central population in Eurasia and Lesser Sandhill Cranes breeding in eastern Siberia undertake the longest of all crane migrations, in excess of 5000 km. The Siberian Sandhills move east across the Bering Sea into North America and continue south as far as northern Mexico.
An understanding of crane migratory patterns and behaviors is critically important in assessing the conservation status and needs of the different species. The problems cranes face during migration often constitute the “weak links” in the chain of conservation actions. Even if the cranes are secure in their breeding and wintering areas, they may be vulnerable to habitat changes at traditional staging and resting areas, and often face other dangers associated with human activity along the migration routes. Historically, for example, collisions with utility lines and accidental shooting have been important mortality factors along the Whooping Crane’s narrow migration corridor (Faanes and Johnson 1992, Lewis et al. 1992b).
Because migration is such a critical phase in the annual cycle of cranes, crane biologists have in recent years devoted much time and effort to the study of migration through banding, radio telemetry, and satellite tracking programs. Such studies have been especially important in developing recovery and reintroduction plans for the Siberian and Whooping Cranes. Because knowledge of migration routes is passed along to new generations of cranes by experienced older birds, conservation programs for these most endangered species emphasize the maintenance of existing routes and the development of new techniques for teaching migration. Perhaps most significant, the conservation needs of cranes during migration have necessitated extensive cooperation across national boundaries, sometimes among countries otherwise in conflict with one another (Lewis 1991, Shibaev 1995).