Gruiformes

(Cranes, rails, and relatives)

Class Aves

Order Gruiformes (Cranes, rails, and allies)

Number of families 11

Number of genera, species 82 genera; 210 species

Evolution and systematics

The order Gruiformes (parvclass Passerae, superorder Passerimorphae) has often been described as a sort of taxonomic grab-bag consisting of several avian families with questionable evolutionary ties. In Bustards, Hemipodes and Sandgrouse: Birds of Dry Places (1991), Paul A. Johnsgard wrote, "The traditional order Gruiformes as constituted by Peters (1934) is one that has been rather generally regarded as a collection of seemingly rather disparate and perhaps distantly related forms." W. Meise, the author of the Grzimek's (1968) chapter on Gruiformes, wrote, "A parrot can be immediately recognized, so we can readily understand why all parrots are included in one order, with only one family. This is in direct contrast to the order of cranes. … Hardly any other orderamong birds has so little uniformity."

As of 2002, science recognizes 10 families in this ancient group of birds: Eurypygidae (sunbittern), Otididae (bustards), Gruidae (crowned cranes and typical cranes), Aramidae (limpkin), Heliornithidae (sungrebes and finfoots), Psophiidae (trumpeters), Cariamidae (seriemas), Rhynochetidae (kagu), Rallidae (rails, coots, gallinules), and Mesitornithidae (mesites). One of the Gruiformes families recognized by Meise, the Turnicidae (buttonquails), has since been elevated in some taxonomies to order status (parvclass Turnicae; order Turniciformes), though here it will be discussed as part of the Gruiformes.

Gruiformes have a long evolutionary history. With fossil evidence dating back to the middle Eocene, DNA studies indicate the bustards diverged from the remaining Gruiforme lineage around 77 million years ago. DNA and fossil evidence suggests the trumpeters originated in the late Cretaceous or early Tertiary, 60–70 mya. Fossil crowned cranes date back 50 million years, whereas typical cranes first appear in the fossil record during the Miocene, approximately 24 million years ago. The earliest good fossils of true Rallidae were from the Upper Oligocene and Lower Miocene, 20–30 mya. The flightless, extinct, predatory fossil family Phorusrhacidae, beginning 38 mya with South America's Lower Oligocene, are considered by some to be the distant ancestors of the extant seriemas. The earliest fossil limpkins were found in early Eocene sediments from the state of Wyoming, in the United States, dated approximately 54 mya. Fossil kagu have been found on New Caledonia dating back 4,000 years. No fossil sunbitterns, sungrebes, nor finfoots have been discovered.

Physical characteristics

The Gruiformes reflect great diversity in size, ranging from the diminutive 4.7 in (12 cm) American black rail (Laterallus jamaicensis) to the 5.8 ft (176 cm) Sarus crane (Grus antigone), the tallest of all flying birds. Average sizes for families are: sunbittern (17–19 in; 43–48 cm), bustards (16–47 in; 40–120 cm), cranes (35–69 in; 90–176 cm), limpkin (22–28 in; 56–71 cm), sungrebes and finfoots (10–23 in; 26–59 cm), trumpeters (18–21 in; 45–52 cm), seriemas (28–35 in; 70–90 cm), kagu (22 in; 55 cm), rails (5–25 in; 12–63 cm), mesites (12–13 in; 30–32 cm). The species with the smallest average weight is the 8 oz (20 g) American black rail. At 16 oz (40 g), the inaccessible rail (Atlantisia rogersi) is the smallest flightless bird known to exist. On the other end of the scale, male kori bustards (Ardeotis kori) can weigh up to 7.5 lb (19 kg), and some male great bustards (Otis tarda) have been reported to reach 40 lb (18 kg), putting them on par with the mute swan (Cygnus olor) as the heaviest flying birds.

Plumage coloration is typically earth toned, in shades of black, gray, and brown, and often heavily or cryptically streaked or vermiculated. Several typical cranes are the

exceptions, with mostly white and black plumage accented by red patches on the head or neck. The sunbittern is notable for its exceptional chestnut, black, and buff-yellow "eyespots" on the dorsal side of its wings, which it uses in defensive displays. Many cryptically marked male bustards can erect their feathers in magnificent fashion during courtship displays. Several Gruiforme species have bright red or orange legs, bills, or frontal shields. A few rails, most notably among the gallinules, are greenish or purple. Mesites have feather patches that produce powder down, a feature not found in other Gruiformes.

Bill shapes are somewhat variable and are adapted to the type of food taken. Notable is the limpkin's relatively long, slightly decurved bill that bends to the right at its tip, with crosscutting action from the lower mandible, to aid in feeding upon its primary food, the right-handed apple snail (Pomacea). In contrast, trumpeters have short, chicken-like bills used to forage among the leaves on the floor of the rainforest for vegetable matter and insects. The cranes have narrow, medium-length bills, which in some species are used to probe in moist soil for tubers and invertebrates.

Distribution

Representatives of the Gruiformes may be found on every continent except Antarctica, and on many oceanic islands. Some families are more limited in distribution than others. The monotypic kagu is severely limited, found only on the island New Caledonia. The mesites are restricted to the island of Madagascar. The trumpeters are found in the tropical forests of northern South America, while the seriemas inhabit the grasslands of central and eastern South America. The monotypic sunbittern is found in tropical Central and South America near water. The monotypic limpkin is found in tropical and subtropical Neotropics (the region that extends south, east, and west of the central plain of Mexico). The bustards are distributed in the Old World, with greatest diversity in Africa. The sungrebes are found in the Neotropics; and the finfoots in Africa, and from India to Malaysia. The cranes, many of which are highly migratory, are found worldwide except Antarctica. They have their greatest diversity of species in Asia, and their greatest diversity of genera in Africa. Rails, gallinules, and coots are also distributed worldwide, except for polar regions and waterless deserts, and they are widely distributed on oceanic islands where many species have become flightless.

Habitat and feeding ecology

As in all other aspects of their biology, the habitats (and associated diets) of Gruiformes are quite variable. The families can be roughly ordered from wet-loving to dry-loving groups. The sungrebes and finfoots are primarily aquatic, inhabiting marshes, lakes, and streams, and feeding upon small insects, aquatic animals, and some seeds and leaves. The sunbittern lives near water in dense tropical forests and swamps. There the birds can be seen walking slowly while they stalk insects and small fish or crustaceans. The limpkin is found near wetland areas, such as in marshes or wooded swamps, where the birds feed on apple snails, as well as insects and some seeds. The cranes frequent freshwater and saline wetlands and open upland country, taking a wide variety of seeds, tubers, and other vegetable and animal matter. The rails also live mainly in or near swamps, marshes, and lakes, and eat a wide variety of vegetable and animal foods. The trumpeter species are found in tropical rainforest, where they forage for fruits, berries, seeds, and other plant material on the forest floor. The kagu eats insects, worms, small frogs, and mollusks in its native forests. The mesites are distributed from lush rainforest to dry scrub, taking fruit, seeds, and insects. The seriemas are found in grassland and pampas, where they hunt insects, small reptiles, and mammals, and occasionally take some vegetable matter. Bustards live in open country, including grassland and dry brush and scrub habitats, eating a variety of seeds, small and large insects, and occasionally small animals.

Behavior

Gruiformes are not particularly gregarious, with some exceptions among the bustards, trumpeters, and the cranes. The sunbittern, limpkin, sungrebes, kagu, and rails tend to be solitary, secretive, and even highly territorial residents. The seriemas and mesites are more likely to be found in pairs or small family groups.

Mating systems range from the monogamy of the cranes to the polygyny/promiscuity of the bustards. Cranes lay from one to four eggs, with the norm being two eggs for most species. Both sexes establish a territory, nest-build, incubate the eggs, and feed the precocial chicks. All species of cranes engage in spectacular dances, by leaping, extending their wings, and sometimes head-bobbing. Dances are presumed to create and maintain pair bonds and to synchronize breeding physiology, so both male and female are ready to breed at exactly the same time. Cranes also vocalize with trumpeting duets known as "unison calls," to strengthen the pair bond. Crane pairs often stay with each other year-round and even for life. In contrast, the males of many bustard species gather in traditional "dispersed leks," or display grounds, to attract females. After choosing the male and mating, the female nests, incubates, and rears the young alone without any help from the male. Males copulate with as many females as are receptive.

Although many Gruiformes are strong fliers, as witness the well-known migrations of many crane species, the members of this order are generally reluctant to fly. Gruiformes have evolved more obligate flightless forms than any other avian order. In fact, the rails seem almost evolutionarily predisposed to have evolve flightlessness. More than one-quarter of all known island rails have lost the ability to fly. Their large and energetically expensive avian flight muscles and associated skeletal apparatus have either disappeared or become greatly reduced in these forms; this appears to have come about through arrested development, known as neotony.

Conservation status

The 2000 IUCN Red List of Threatened Species reported on 93 species of Gruiformes. Of these, 22 species were reported Extinct. Another species, the Guam rail (Gallirallus owstoni) is listed as Extinct in the Wild. In an effort to save the species from extinction, several Guam rails were brought into captivity and managed as a global population by the Guam Department of Agriculture Aquatic and Wildlife Resources Division, the United States Fish and Wildlife Service (USFWS), and zoological institutions. Zoos participating in the captive gene pool and conservation programs include member institutions of the American Zoo and Aquarium Association's (AZA) Guam Rail Species Survival Plan® (SSP®) under the umbrella of the AZA's Gruiformes Taxon Advisory Group (TAG).

An additional four Rallidae species are listed as Critically Endangered, and 11 more as Endangered. An additional 30 rallids are globally Vulnerable or at risk. The main causes of extinctions and threats to flightless and island rallids are purposely or accidentally introduced exotic mammalian predators: rats, cats, dogs, mongooses, pigs, snakes, and humans. Habitat destruction plays a lesser role, as humans and their introduced livestock modify wetlands, forests, and grasslands.

The monotypic family Rhynchochetidae (kagu) is listed as Endangered and legally protected in New Caledonia, with CITES Appendix I status. The main reason for the decline appears to be the introduction of dogs to the island in 1774 by Captain Cook. Logging and deforestation are also affecting kagu habitat. The remaining two monotypic families in the order, the Aramidae (limpkin) and the Eurypygidae (sunbittern), are well distributed in the New World and not in any immediate danger.

Among the Otididae (bustards), nine are globally at risk. The great Indian bustard (Ardeotis nigriceps), Bengal florican (Houbaropsis bengalensis), and lesser florican (Sypheotides indica are listed as Endangered, and appear on Appendix I of CITES. Hunting, habitat loss due to agriculture and grazing, and nest failure due to interference from cattle and crows are the main pressures.

The Gruidae (cranes) are severely at risk. Habitat loss due to agriculture, the degradation of wetlands, and direct hunting have caused eight of the 15 crane species to be globally at risk. At greatest risk is the Critically Endangered Siberian crane (Grus leucogeranus). The two Endangered species are the whooping crane (Grus americana) and the Japanese crane (Grus japonensis). Finally, six crane species are globally Vulnerable: Sarus crane, wattled crane (Grus carunculatus), hooded crane (Grus monacha), black-necked crane (Grus nigricollis), blue crane (Grus paradisea), and the white-naped crane (Grus vipio).

Resources

Books

Brock, K. "1998 Guam Rail SSP." In AZA Annual Report on Conservation and Science 1997–1998. Volume 1: Conservation Programs Reports, edited by L. G. Hodskins. Silver Springs, MD: American Zoo and Aquarium Association, 2000.

Feduccia, J. A. The Origin and Evolution of Birds, 2nd ed. New Haven: Yale University Press, 1999.

Johnsgard, P. A. Bustards, Hemipodes and Sandgrouse: Birds of Dry Places. Oxford: Oxford University Press, 1991.

Meine, Curt D., and George W. Archibald, eds. The Cranes: Status Survey and Conservation Action Plan. Gland and Cambridge: IUCN, 1996.

Sibley, C. G., and B. L. Monroe, Jr. Distribution and Taxonomy of Birds of the World. New Haven: Yale University Press, 1990.

Periodicals

American Ornithologists' Union. "42nd Supplement to the Check-List of North American Birds." Auk 117 (2000): 847–858.

Other

IUCN Species Survival Commission. "2000 IUCN Red List of Threatened Species" 2000 (1 April 2002). <http://www.redlist.org/>

UNEP-WCMC."Animals of the World Database." 1 April 2002 (1 April 2002). <http://www.unep-wcmc.org/species/animals/animal_redlist.html>

Charles Eric Siegel, MS