Columbiformes

(Pigeons, doves, and dodos)

Class Aves

Order Columbiformes

Number of families 2

Number of genera, species 40–45 genera; 300–320 species

Evolution and systematics

This order consists of two families—the pigeons and doves (Columbidae) and the extinct dodos (Raphidae) of the Indian Ocean Mascarene Islands. It is a homogenous group of arboreal and terrestrial birds with several unique anatomical and physiological features. The order is generally agreed to be monophyletic (derived from a single common parent form) and does not appear to have clear affinities with any other. At various times it has been considered close to the game-birds (Galliformes), parrots (Psittaciformes), or shorebirds (Charadriiformes). Anatomically the Charadriiformes appear to be the closest. They have similar features of the palate, nostrils, wings, voice boxes, and skull. They differ, however, in biology, behavior, the type of young, and by the fact that pigeons have a specialized backbone and a long hindtoe. DNA analysis has shown that the Columbiformes have no close relationships with any other birds and their similarities are due to convergence or the retention of primitive characteristics.

Sandgrouse (Pterocliformes) have been often considered to be Columbiformes having similar feather structure and musculature but in other respects they are midway between the Columbiformes and the Galliformes. They are now considered to be closest to the shorebirds.

The order has a somewhat poor fossil history and the oldest species is Columba calcaria from the Miocene of France. Miocene material is also known from Australia and recently from Japan.

Physical characteristics

The Columbidae is the only surviving family in the order. They are generally compact birds with small- to medium-sized heads held on short but graceful necks. The smallest is the Australian diamond dove (Geopelia cuneata) which weighs about 1.1 oz (30 g) and the largest the Victoria crowned-pigeon (Goura victoria) of Papua New Guinea at 3.7–6.6 lb (1.7–3 kg). The sexes are similar with males being marginally larger than females. Generally they possess similar plumage, but some tropical species are sexually dimorphic—the males having quite brilliant plumage. Pigeons are generally very similar in body form although some species resemble quail, partridges, or small turkeys. The unique pheasant pigeon (Otidiphaps nobilis) of Papua New Guinea looks and acts like a pheasant and is considered by local people to be megapode and not a pigeon at all.

The wings are usually broad with rounded tips and pigeons are strong direct flyers. They can glide and often incorporate this into display flights, but they cannot soar. They have proportionally more wing muscle (31–44% of body weight) compared to most other birds. The wings have 11 primary feathers, although the first is very reduced, and 10–15 secondaries and tertials, the two being indistinguishable. Primary feathers are often modified by being variously emarginate (narrowed in their outer sections), but nobody seems to know why they are this shape. Pigeons take off and often fly with a characteristic clapping of wings. The tails are of varying lengths, usually shortish, and there are 12–14 tail feathers (up to 18 in Goura and Otidiphaps).

Body feathers are unique. The shaft is generally flattened, strong, and broad then tapers abruptly to a fine point. There is generally no aftershaft, although small ones may be present on some wing and tail feathers. Body feathers have very dense fluffy bases, are loosely attached into the skin, and drop out easily. This may be a predator avoidance mechanism. If grabbed by a predator large numbers of feathers come out in the attacker's mouth and the bird can make its escape. Pigeons

have no or only rudimentary oil glands, glands possessed by most birds which exude an oil used for preening. Powder downs are dispersed over the entire body. These feathers disintegrate to produce a talcum-like powder that the pigeon uses to maintain plumage condition. This gives a beautiful soft silky feel to pigeon plumage unmatched by most other birds. Two kinds of domestic pigeon produce very peculiar curled "fat quills." These special feathers contain yellow fat that derives from the same cells as powder down.

The generally dull plumage of the domestic pigeon is not representative of the huge range of color schemes the order presents. Combinations of delicate grays, browns, and creams complement the soft plumage. Many species, particularly in the Indo-Pacific, are spectacularly clothed in greens, reds, purples, pinks, blues, and oranges. The gorgeous golden doves of Fiji (genus Ptilinopus) are fiery orange or metallic green and gold. Some pigeons have crests or naked skin around the eyes. Pigeons only have one adult plumage—they do not alternate breeding and nonbreeding plumages and so have a complete post-breeding molt.

Most pigeons have short weak bills, often with a characteristic expanded tip and the nostrils are under a thin plate, which is covered by an often brightly colored, cushion-like structure called a cere. A dove's legs are generally short with small rounded or hexagonal scales at the sides and rear, and it has three front and a long, functional hind toe.

The backbone, usually of 37–39 vertebrae, is very rigid. The vertebrae are strongly amalgamated with the synsacrum (the back plate), and the fused vertical spines on the vertebrae form a low ridge that contrasts abruptly with the coccygeal (tail) vertebrae with their normal spines. The pelvis is peculiarly shaped so as not to form an interpelvic space. The sternum (breast bone) has an internal spine. The skull has small basipeterygoid processes (where the palate joins the brain-case).

Internally the caeca (long appendix like pouches of the gut) are absent or very small. The crop is very large with two lobes. To make room for the crop, the muscles of the syrinx (voice box) are modified being larger on one side. Most species have a thick, muscular gizzard, which, with the help of grit the bird swallows, grinds up hard seeds and fruit. Over one-third of the species in the order—the fruit pigeons (genera Ptilinopus, Ducula, and Lopholaimus)—have thin-walled gizzards and do not ingest grit. These birds are specialist frugivores and their gizzards massage the nutritious flesh off the rainforest fruits they eat—the hard seeds are passed out whole. These birds

act as effective seed dispersers and are common components of the rainforest ecosystems of Indonesia, Melanesia, the Pacific islands, and Australasia.

Distribution and habitat

Pigeons occur in all terrestrial habitats from deserts to forest, although most species are forest dwellers, particularly rainforests. There are no aquatic or wading species. They occur all over the world being absent only from the Arctic, the Antarctic, and very high mountains. The order is generally well distributed on islands, with over 60% of species occurring only on islands away from continental land masses. They have radiated most prolifically in terms of species, plumage, and body form in the tropics, particularly in the Indian and Pacific ocean regions.

Some doves (Columba, Streptopelia) have unique adaptations to control their temperature in very hot conditions. There is a plexus or mass of blood vessels forming a collar inside the neck and around the esophagus. In very hot weather the birds pant, rhythmically bringing the esophagus and the plexus into contact. Heat is transferred from the blood to the moist wall of the gullet where it is lost by evaporation. Some doves have a similar plexus behind the ear coverts that keeps the brain cool.

Behavior

Social behavior varies widely in the order ranging from solitary species to ones that form small to large permanent flocks or are colonial. Most species are gregarious. Many species, even the solitary ones, readily gather at abundant food sources such as fruiting trees or grain crops. Single fruiting trees in tropical rainforests may attract seven or more species of fruit pigeons at the same time. Rock doves (Columba livia), wood pigeons (Columba palumbus), and stock doves (Columba oenas) gather in huge numbers in grain fields—a flock of 100,000 wood pigeons has been recorded in Germany, and the eared dove (Zenaida auriculata) of South America breeds in colonies of up to five million. The extinct passenger pigeon (Ectopistes migratorius) of North America was possibly the most abundant bird on Earth and flocks of hundreds of millions were recorded. One flight of birds in Ontario seen during 1866 was 300 mi long by 1 mi wide (480 km by 1.6 km) and lasted for 14 hours—there were up to perhaps three billion birds!

Pigeons have a fairly consistant set of breeding displays— all species have a range of bows, stretches, and display flights. No species has complex songs and their calls are generally "coos" and "oohs," often quite deep and sonorous. Several species have clicks, soft whistles, and soft grunts, or are almost completely silent.

Feeding ecology and diet

All species in the order eat mostly seeds, fruits, and leaves. Invertebrates are eaten occasionally, but only rarely do they make up a large portion of the diet. An exception is the atoll fruit-dove (Ptilinopus coralensis), which takes a lot of insects and even lizards on the generally barren Pacific coral islands it inhabits.

A supposedly unique feature is a pigeon's capacity to drink by sucking water up directly, but several other birds can do this and pigeons sometimes do not suck—it seems to depend upon how much of a hurry they are in. Drinking was also one of the features that supposedly related pigeons to sandgrouse, but the latter do not drink like pigeons at all.

Reproductive biology

The crop of all pigeons is specialized to an extent unknown in any other birds. This organ has a glandular lining that, during the breeding season and in both sexes, enlarges and produces a soft, nutritious, cheesy secretion called crop milk, which is fed to the chicks. This feature is probably unique to pigeons, although some parrot breeders claim that in the first few hours after hatching young parrots are also fed a crop milk by their parents. Crop milk gives pigeons a distinct advantage in that the chicks need not starve if food is scarce. So long as the parents are fat and healthy the young can continue to receive nutritious food in hard times.

Most pigeons form monogamous pairs at least for one breeding season. They do not put much effort into nest building: a few twigs woven together form a flimsy platform or, for terrestrial species, the egg is laid in a scrape in the ground. Clutch size is usually one or two white, rarely cream or buff, eggs, although some temperate species occasionally have clutches up to four eggs. As a generality the one-egged species tend to be either fruit eaters that live in rainforests and are large or are colonial. Pigeon young, called squabs, are helpless and generally nearly naked with only a few lax yellow, brown, or gray down feathers. Both parents care for the young, which fledge at between 7 and 28 days depending upon the species. Chicks grow very fast and those of some species leave the nest when wing feathers are only half grown. Young of the Torresian imperial-pigeon (Ducula spilorrhoa) are particularly precocial. This pigeon from Australia and New Guinea breeds in colonies in mangroves, usually on off-shore islands. If a person walks through a colony, the tiny chicks leap from their nests and attempt to scramble away. Once the danger is passed, however, they laboriously climb back to the nests using their necks and little stubby wings.

Young pigeons quickly develop a distinct, generally brown juvenal plumage, and start to molt and develop adult plumage before the juvenal plumage itself has fully grown.

The dodos

The family Raphidae appears at first sight quite different from the Columbidae. It consists of three extinct species from the Indian Ocean—the famous dodo (Raphus cucullatus) on Mauritius, the Rodrigues solitaire (Pezophaps solitaria) on Rodrigues, and the solitaire (Raphus solitarius) on Réunion. (The existence of R. solitarius is based on contradictory traveler's accounts. No illustrations or physical remains currently exist, so its taxonomic position is less certain than the other two species in this family.) They were all exterminated in the sixteenth to eighteenth centuries mostly by European sailors visiting the islands and killing the trusting animals en masse for food and fun. The sailors also released pigs, monkeys, and rats, which preyed on eggs and young.

The Raphidae were very large, flightless birds—the Rodrigues solitaire weighed as much as 62 lb (28 kg). Early illustrations and skeletal remains suggest they had large distended bellies, short thick legs, and big heads with massive bills that had a big expanded tip. The dodo's bill was 8 in (20 cm) long and heavily hooked. Their plumage was lax and soft, they had a peculiar short curly tail set high on the back, and they had tiny wings.

While apparently so dissimilar, they were anatomically very close to pigeons. The dodo's big hooked bill is fore-shadowed by the enlarged bill tips of most pigeons. This is obvious in the green pigeons (genus Treron) of Africa and Asia, several of which have enlarged bills. Treron pembaensis from the East African island of Pemba and T. sanctithomae from the West African island of São Tomé have particularly hooked bills. The thick-billed ground-pigeon (Trugon terrestris) of New Guinea has a large bill, heavily expanded at the tip, while the tooth-billed pigeon (Didunculus strigirostris) of Samoa has a bill intermediate in character between pigeons and dodos.

Dodo's feathers were identical to pigeon feathers and the looseness of the plumage is like that seen in young squabs. Indeed the whole appearance of the dodos is very similar to a pigeon squab. Most of their other features are the same as pigeons—the skeletons are similar given the specializations of flightlessness; they laid a single white egg on the ground; they had muscular gizzards and enlarged crops; and, like many pigeons, they were specialized fruit eaters. What is not known is the extent to which physiological and behavioral features were shared. Did they produce crop-milk, for example?

It is now generally agreed that the dodos are an example of neotony or paedomorphosis—the retention of juvenile characters into adulthood. Derived from pigeon-like stock, the dodos lost their wings, developed gigantism, and retained juvenile body form and plumage. Paedomorphosis and gigantism are common in island birds and may be a particular way in which Columbiformes adapt to the harsh life on islands. Like the rails (Rallidae) the strong flying, adaptable Columbiformes have been spectacularly successful in reaching and adapting to island life. Continuing fossil discoveries on Pacific islands reveal that there was a large fauna on these islands, including many pigeons, that was exterminated by Melanesian and Polynesian settlers before Europeans arrived and carried out their own exterminations. Most fascinating of all is the recent discovery on Viti Levu (Fiji) of fossils of a dodo-sized pigeon. Perhaps dodos were not so unique and the ones known of in the Indian Ocean may just have been the last "giant baby pigeons" of a range of island specialists exterminated by man over the last thousand years.

Extinction

Extermination is perhaps the saddest feature of the order. It contains the two most famous avian extinctions attributable to man—the dodos and the passenger pigeon—the former one of the rarer and the latter the most abundant bird ever known. In addition, eight other species and subspecies of pigeons are known to have been exterminated in historical times following European expansion. The toll is even higher when the extinctions brought about by pre-European peoples in the Pacific Oceans are included.

The dodos and to some extent the passenger pigeon, are used as the prime examples of evolution's losers, that there was somehow something wrong with how they lived and fitted into the world—adapt or die. Nothing could be further from the truth. Dodos were beautifully adapted to their island habitats. They simply couldn't adapt to a determined war of attrition waged by people and to predation by introduced species. War of attrition is not an overstatement when applied to the passenger pigeon either—how else would one describe the killing in Michigan during 1874 of 700,000 passenger pigeons a month from one breeding colony alone? The reason for this killing was market hunting and whole train loads of dead pigeons were carried to market in Chicago.

The extinction of dodos is not an example of evolutionary incompetence. It is just another example of blaming the victims. Evolutionarily the dodos, and the Columbiformes in general, are or were some of the best-adapted species on Earth.

Resources

Books

Coates, B.J. The Birds of Papua New Guinea. Vol. I. Alderley, Australia: Dove Publications, 1985.

del Hoyo, J., A. Elliott, and J. Sargatal. Handbook of the Birds of the World. Vol. 4, Sandgrouse to Cuckoos. Barcelona: Birdlife International and Lynx Edicions, 1997.

Frith, H.J. Pigeons and Doves of Australia. Adelaide: Rigby, 1982.

Gibbs, D., E. Barnes, and J. Cox. Pigeons and Doves. New Haven, CT: Yale University Press, 2000.

Goodwin, D. Pigeons and Doves of the World. 3rd ed. Ithaca, NY: British Museum (Natural History) and Cornell University Press, 1983.

Greenway, J.C. Extinct and Vanishing Birds of the World. New York: Dover, 1967.

Higgins, P.J., and S.J.J.F. Davies, eds. Handbook of Australian, New Zealand and Antarctic Birds. Vol. 3, Snipe to Pigeons. Melbourne: RAOU and Oxford University Press, 1996.

Lucas, A.M., and P.R. Stettenheim. Avian Integument Parts I and II. Agriculture Handbook 362. Washington, DC: U.S. Department of Agriculture, 1972.

Schorger, A.W. The Passenger Pigeon: Its Natural History and Extinction. Madison, WI: the author, 1955.

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

Urban, E.K., H.C. Fry, and S. Keith, eds. The Birds of Africa. Vol. II. London: Academic Press, 1986.

Periodicals

Bucher, E.H. "Colonial Breeding of the Eared Dove (Zenaida auriculata) in Northeastern Brazil." Biotropica 14 (1982): 255–261.

Garrod, A.H. "On Some Points in the Anatomy of the Columbae." Proceedings of the Zoological Society of London (1874): 249–259.

Gaunt, S.S.L. "Thermoregulation in Doves: A Novel Oesophogeal Heat Exchanger." Science 210 (1980): 445–447.

Livezey, B.C. "An Ecomorphological Review of the Dodo (Raphus cucullatus) and Solitaire (Pezophaps solitaria), Flightless Columbiformes of the Mascarene Islands." Journal of Zoology London 230 (1993): 247–292.

Mahler, B., and P.L. Tubaro. "Attenuated Outer Primaries in Pigeons and Doves: A Comparative Test Fails to Support the Flight Performance Hypothesis." Condor 103 (2001): 449–454.

Martin, R. "Die Vergleichende Osteologie der Columbiformes unter Besonderer Berücksichtigung von Didunculus strigirostris." Zoologische Jarhbücher Abteilung für Systematik Oekologie und Geographie der Tiere 20 (1904): 167–352.

Mourer-Chauvire, C., R. Bour, S. Ribes, and F. Moutou. "The Avifauna of Réunion Island (Mascarene Islands) at the Time of the Arrival of the First Europeans." Smithsonian Contributions to Paleobiology 89 (1999): 1–38.

Steadman, D.W. "New Species of Gallicolumba and Macropygia (Aves: Columbidae) from Archaeological Sites in Polynesia." Natural History Museum of Los Angeles County (Science Series) 36 (1992): 329–348.

Steadman, D.W. "Prehistoric Extinctions of Pacific Island Birds: Biodiversity Meets Zooarchaeology." Science 267 (1995): 1123–1131.

Worthy, T.H., A.J. Anderson, and R.E. Molnar. "Megafaunal Expression in a Land Without Mammals: The First Fossil Faunas from Terrestrial Deposits in Fiji (Vertebrata: Amphibia, Reptilia, Aves)." Senckenbergiana-Biologica 79 (1999): 237–191.

Francis Hugh

John Crome