CRANES

CRANES

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Cranes are a family, the Gruidae, of large, long-legged, and long-necked birds in the group Gruiformes. The 15 species of cranes are placed in three genera, Antigone, Balearica, and Grus. Unlike the similar-looking but unrelated herons, cranes fly with necks outstretched, not pulled back. Cranes live on most continents, with the exception of Antarctica and South America.

Some species and populations of cranes migrate over long distances; others do not migrate at all.[2] Cranes are solitary during the breeding season, occurring in pairs, but during the nonbreeding season, most species are gregarious, forming large flocks where their numbers are sufficient.

Most species of cranes have been affected by human activities and are at the least classified as threatened, if not critically endangered.[3] The plight of the whooping cranes of North America inspired some of the first US legislation to protect endangered species.

The plumage of cranes varies by habitat. Species inhabiting vast, open wetlands tend to have more white in their plumage than do species that inhabit smaller wetlands or forested habitats, which tend to be more grey. These white species are also generally larger. The smaller size and colour of the forest species is thought to help them maintain a less conspicuous profile while nesting; two of these species (the common and sandhill cranes) also daub their feathers with mud which some observers suspect helps them to hide while nesting.[citation needed]

Most species of cranes have some areas of bare skin on their faces; the only two exceptions are the blue and demoiselle cranes. This skin is used in communication with other cranes, and can be expanded by contracting and relaxing muscles, and change the intensity of colour. Feathers on the head can be moved and erected in the blue, wattled, and demoiselle cranes for signaling, as well.

Also important to communication is the position and length of the trachea. In the two crowned cranes, the trachea is shorter and only slightly impressed upon the bone of the sternum, whereas the trachea of the other species is longer and penetrates the sternum. In some species, the entire sternum is fused to the bony plates of the trachea, and this helps amplify the crane's calls, allowing them to carry for several kilometres.[4][5]

The fossil record of cranes is incomplete. Apparently, the subfamilies were well distinct by the Late Eocene (around 35 mya). The present genera are apparently some 20 mya old. Biogeography of known fossil and the living taxa of cranes suggests that the group is probably of (Laurasian) Old World origin. The extant diversity at the genus level is centered on (eastern) Africa, although no fossil record exists from there. On the other hand, it is peculiar that numerous fossils of Ciconiiformes are documented from there; these birds presumably shared much of their habitat with cranes back then already. Cranes are sister taxa to Eogruidae, a lineage of flightless birds; as predicted by the fossil record of true cranes, eogruids were native to the Old World. A species of true crane, Antigone cubensis, has similarly become flightless and ratite-like.

The cranes have a cosmopolitan distribution, occurring across most of the world continents. They are absent from Antarctica and, mysteriously, South America. East Asia has the highest crane diversity, with eight species, followed by Africa, which is home to five resident species and wintering populations of a sixth. Australia, Europe, and North America have two regularly occurring species each. Of the four crane genera, Balearica (two species) is restricted to Africa, and Leucogeranus (one species) is restricted to Asia; the other two genera, Grus (including Anthropoides and Bugeranus) and Antigone, are both widespread.[4][7]

Many species of cranes are dependent on wetlands and grasslands, and most species nest in shallow wetlands. Some species nest in wetlands, but move their chicks up onto grasslands or uplands to feed (while returning to wetlands at night), whereas others remain in wetlands for the entirety of the breeding season. Even the demoiselle crane and blue crane, which may nest and feed in grasslands (or even arid grasslands or deserts), require wetlands for roosting at night. The Sarus Crane in south Asia is unique in having a significant breeding population using agricultural fields to breed in areas alongside very high density of humans and intensive farming, largely due to the positive attitudes of farmers towards the cranes.[2] In Australia, the Brolga occurs in the breeding areas of Sarus Cranes in Queensland state, and they achieve sympatry by using different habitats. Sarus Cranes in Queensland largely live in Eucalyptus-dominated riverine, while most Brolgas use non-wooded regional ecosystems that include vast grassland habitats.[8] The only two species that do not always roost in wetlands are the two African crowned cranes (Balearica), which are the only cranes to roost in trees.[4]

Some crane species are sedentary, remaining in the same area throughout the year, while others are highly migratory, traveling thousands of kilometres each year from their breeding sites. A few species like Sarus Cranes have both migratory and sedentary populations, and healthy sedentary populations have a large proportion of cranes that are not territorial, breeding pairs.[9]

The cranes are diurnal birds that vary in their sociality by season and location. During the breeding season, they are territorial and usually remain on their territory all the time. In contrast in the non-breeding season, they tend to be gregarious, forming large flocks to roost, socialize, and in some species feed. Sarus Crane breeding pairs maintain territories throughout the year in south Asia, and non-breeding birds live in flocks that can also be seen throughout the year.[2][9] Large aggregations of cranes likely increase safety for individual cranes when resting and flying and also increase chances for young unmated birds to meet partners.[4]

The cranes consume a wide range of food, both animal and plant matter. When feeding on land, they consume seeds, leaves, nuts and acorns, berries, fruit, insects, worms, snails, small reptiles, mammals, and birds. In wetlands and agriculture fields, roots, rhizomes, tubers, and other parts of emergent plants, other molluscs, small fish, eggs of birds and amphibians are also consumed, as well.[citation needed] The exact composition of the diet varies by location, season, and availability. Within the wide range of items consumed, some patterns are suggested but require specific investigation to confirm; the shorter-billed species usually feed in drier uplands, while the longer-billed species feed in wetlands.[4]

Where more than one species of cranes exists in a locality, each species adopts separate niches to minimise competition. At one important lake in Jiangxi Province in China, the Siberian cranes feed on the mudflats and in shallow water, the white-naped cranes on the wetland borders, the hooded cranes on sedge meadows, and the last two species also feed on the agricultural fields along with the common cranes.[4] In Australia, where Sarus Cranes live alongside Brolgas, they have different diets: Sarus Cranes' diet consisted of diverse vegetation, while Brolga diet spanned a much wider range of trophic levels.[8] Some crane species such as the Common/ Eurasian crane use a kleptoparasitic strategy to recover from temporary reductions in feeding rate, particularly when the rate is below the threshold of intake necessary for survival.[12] Accumulated intake of during daytime shows a typical anti-sigmoid shape, with greatest increases of intake after dawn and before dusk.[13]

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 several years pass before the first successful breeding season. Initial breeding attempts often fail, and in many cases, newer pair bonds dissolve (divorce) after unsuccessful breeding attempts. Pairs that are repeatedly successful at breeding remain together for as long as they continue to do so.[4] In a study of sandhill cranes in Florida, seven of the 22 pairs studied remained together for an 11-year period. Of the pairs that separated, 53% was due to the death of one of the pair, 18% was due to divorce, and the fate of 29% of pairs was unknown.[14] Similar results had been found by acoustic monitoring (sonography/frequency analysis of duet and guard calls) in three breeding areas of common cranes in Germany over 10 years.[15]

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 minimal.[citation needed] Even in areas with a high density of humans, in the absence of directed persecution, species like Sarus Crane maintain territories as small as 5 ha when agricultural crops and landscape conditions are suitable.[2] In contrast, red-crowned crane territories may require 500 hectares, and pairs may defend even larger territories than that, up to several thousand hectares.[citation needed] Territory defence is either acoustic with both birds performing the unison call, or more rarely, physical with attacks usually by the male.[4] Because of this, females are much less likely to retain the territory than males in the event of the death of a partner.[14] Rarely, breeding territorial crane pairs allow a third crane into the territory to form polygynous or polyandrous trios that improves the chances of survival of the pair's chicks.[11] Trios of Sarus cranes were seen largely in marginal habitats and third birds were young suggesting that third cranes would benefit by gaining experience.[11] 781b155fdc