USGS - science for a changing world

Northern Prairie Wildlife Research Center

  Home About NPWRC Our Science Staff Employment Contacts Common Questions About the Site

The Cranes

Status Survey and Conservation Action Plan

An Overview of Crane Biology: Morphology


GIF-Heads of all cranes
ROW 1: Black Crowned Crane (Balearica pavanina), Grey
Crowned Crane (Balearica regulorum), Demoiselle Crane
(Anthropoides virgo) ROW 2: Wattled Crane (Bugeranus
carunculatus
), Blue Crane (Anthropoides paradiseus) ROW 3:
Siberian Crane (Grus leucogeranus), Sandhill Crane (Grus
canadensis
), Sarus Crane (Grus antigone) ROW 4: Brolga
(Grus rubicundus), White-naped Crane (Grus vipio) ROW 5:
Hooded Crane (Grus monachus), Eurasian Crane (Grus grus),
Whooping Crane (Grus americana) ROW 6: Black-necked
Crane (Grus nigricollis), Red-crowned Crane (Grus
japonensis
)

Cranes are large to very large birds with long necks and legs, streamlined bodies, and long rounded wings. In the field, they are readily recognized by their imposing size and graceful proportions. Cranes are among the world’s tallest birds, ranging in length from 90 to more than 150 cm. The smallest is the Demoiselle Crane. The Sarus Crane is the tallest. The Indian subspecies of the Sarus, which can stand as high as 175 cm, is the world’s tallest flying bird. The Red-crowned Crane is the heaviest crane, weighing up to 11 kg when fat deposits peak in the autumn. Male and female cranes of all species are identical in their external features, although males are usually somewhat larger than females (Johnsgard 1983). Compared to the other tall wetland birds, cranes generally have longer legs and hold their necks straighter than day-herons; larger bodies than egrets; and longer legs, lighter bodies, and proportionately smaller bills than storks.

Distinctive features within the family reflect the varied evolutionary history and ecological niches of the different species. The long, prehensile hind toe (hallux) of Crowned Cranes allows them to roost in trees. Demoiselle Cranes and Blue Cranes have short, bustard-like toes adapted for rapid running in their grassland habitats. The relatively short bills of these cranes allow them to forage more efficiently for seeds, insects, and other food items in upland habitats. All the other cranes display adaptations to more aquatic conditions: elongated necks and bills, long bare legs, and broader feet. Siberian Cranes, the most aquatic of all cranes, have the longest bill and toes—adaptations for probing and walking in mud. Brolgas, which use salt marshes and other saline wetlands more extensively than the other species, have specialized salt glands near their eyes, through which they are able to secrete concentrated salts.

The length and position of the trachea are critical features of crane anatomy, and shape the distinctive voices of the various cranes (Niemeier 1983). The non-Gruinae cranes have shorter tracheas that are impressed slightly against the sternum. In the Gruinae cranes, the trachea actually penetrates the sternum to varying degrees. In Siberian and Wattled Cranes the trachea makes a slight indentation on the sternum, an indentation twice as deep as that found in the Anthropoides cranes. With the exception of the Siberian Crane, the trachea of all Grus species coils on the vertical plane within the sternum. In the Brolga, Sandhill, and Sarus Cranes, the coiled trachea fills most of the anterior half of the sternum, while in the White-naped, Eurasian, Whooping, Hooded, Black-necked, and Red-crowned Cranes the trachea penetrates the entire sternum. The bony rings of the trachea fuse with the sternum to create thin plates. When cranes vocalize, the plates vibrate. This amplifies the cranes’ calls, which can carry several kilometers (Gaunt et al. 1987).

Crane eggs are ovule-pointed and in most species heavily pigmented. Cranes inhabiting tropical and subtropical areas lay either light bluish eggs (Crowned Cranes) or white eggs (Sarus, Brolga). Species inhabiting the coldest regions—Siberian, Black-necked, and Lesser Sandhill Cranes—produce darker eggs. This tendency for eggs to be light-colored in warmer climates and dark in colder climates is probably an adaptation to environmental conditions, allowing eggs to reflect heat in the former case and to absorb heat in the latter. Red-crowned Cranes lay both white and pigmented eggs, an indication that the species may have evolved under warmer climatic circumstances (G. Archibald pers. obs.).

The chicks of most crane species are predominantly brown. The exceptions are the Demoiselle, Blue, and Brolga Cranes, whose chicks are silver-grey. Chicks lose their egg tooth within a few days of hatching. The initial down is replaced by a second down that is in turn replaced by feathers (Kashentseva 1988, 1995; Kashentseva and Tsvetkova 1995). The rate of chick growth is astonishing, especially among chicks of the northernmost species. Legs grow rapidly during the first six weeks, followed by more rapid development of the wings.

Juvenile Demoiselle, Blue, Wattled, and Brolga Cranes are predominantly grey at the time of fledging (possibly affording camouflage in upland habitats). Juveniles of all the other species are russet brown, providing cryptic coloration as a defense against predation. This is of particular importance to Siberian, Whooping, and Red-crowned Crane chicks, which are destined to be primarily white as adults. During the second year of growth, adult plumage gradually replaces the juvenile plumage. By the end of their second year, many juvenile cranes are difficult to distinguish from adults.

The varied features of the heads of the fifteen species are distinctive and diagnostic (see Figure 1.2). Crowned Cranes have elaborate tawny crests, bare cheeks, and a gular wattle. Unlike any of the Typical Cranes, mated Crowned Cranes preen one another’s head plumage. Demoiselle Cranes and Blue Cranes have completely feathered heads, and during display can elongate the plumes on the sides of the head. This is especially pronounced in the Blue Crane, and gives this species its unusual “cobra-like” appearance. Wattled and Black-necked Cranes are similarly able to raise the feathers on the sides of their heads.

All cranes except for the Blue and Demoiselle have bare red skin patches on their heads. Wattled and Siberian Cranes have the red skin on the front of the face. The red skin extends down the upper mandible to the nares, and in the Wattled Crane extends further down the front of the two fleshy dewlaps suspended from its cheeks. The Siberian Crane can expand the dorsal portion of its comb backward when displaying, and the Wattled Crane can extend its wattles downward. The red comb of the White-naped Crane covers the face to a point behind the ear. In the Brolga the red skin surrounds the back of the head, and in the Sarus Crane it covers the side and back of the head and continues down the neck several centimeters. In sharp contrast, the red comb in the remaining species is on the top of the head and expands down the back of the head during display.

The cranes that dwell in vast open wetlands, where the pressure from terrestrial predators is relatively low, are either entirely white (Siberian, Whooping, Red-crowned) or partially white (White-naped, Wattled), and are generally larger in size. Their size and bright white plumage makes these cranes conspicuous to conspecifics, and presumably facilitates defense of the breeding territory. The cranes that nest in smaller and/or forested wetlands are generally smaller and colored various shades of grey. Their size and plumage color may help these cranes to hide on their nests. At the onset of the breeding season, Sandhill Cranes and Eurasian Cranes paint their feathers with mud, staining them russet brown. Cranes painted in this way are much more difficult to see on their nests than unpainted cranes. At the onset of its breeding period, the Siberian Crane paints dark mud on the base of its neck, but this behavior is part of the species’ sexual display rather than a camouflaging exercise.

Cranes have ten functional primary feathers (most species have a vestigial eleventh) and from eighteen to twenty-five secondaries. With the exception of the Red-crowned Cranes, which has white primary flight feathers, the primaries of all the cranes—including the mostly white Siberian and Whooping Cranes—are black or dark grey. The dark pigment apparently strengthens the structure of the feathers, thereby improving their effectiveness on long migrations. Red-crowned Cranes may once have been predominantly non-migratory (as they are today in northern Japan) and could “afford” to sacrifice durability for display. The inner secondaries of many species are elongated, and when the wings are folded produce the impression of a prominent “tail” or “bustle.” This is especially pronounced in the Blue, Demoiselle, and Wattled Cranes. In most species, adults molt annually during the postbreeding period. The main flight feathers are lost at this time, rendering the birds flightless. Molting patterns, however, vary among and within species. The wing molt in Brolgas, Demoiselle, and Crowned Cranes occurs gradually, so that these species do not actually experience an extended flightless period.

Cranes take flight with a running start, usually into the wind, quickly gaining speed before lifting into the air with a push of the wings. When flap-flying, cranes flick their wings with a distinctive rhythm, pushing deliberately on the downstroke and rising rapidly on the upstroke. This rhythm is especially apparent when cranes are disturbed or otherwise eager to gain altitude quickly. Cranes, like storks, flamingos, geese, and swans (and unlike the large herons) fly with their necks extended straight forward. With their long legs trailing directly behind them along a single axis with their bills, necks, and bodies, cranes in flight present an elegant silhouette, resembling perhaps most closely that of the flamingos. In cold weather, cranes sometimes pull their legs in against their bodies. When landing, cranes approach the ground with their head semi-erect, wings extended, and legs dangling. They descend with wings and tail spread out and down, and with a final flapping of the wings alight in a normal standing stance.


Previous Section--Evolution and Classification
Return to Contents
Next Section--An Overview of Crane Biology: Distribution and Habitat

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://www.npwrc.usgs.gov/resource/birds/cranes/morph.htm
Page Contact Information: Webmaster
Page Last Modified: Friday, 01-Feb-2013 19:06:30 EST
Reston, VA [vaww54]