Rock dove

• 1. Introduction

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  The rock dove, Columba livia, gave rise to roughly 1,000 breeds of domestic pigeon. Counting its domestic and feral descendants, as many as 120 million of these birds live around the world. The pigeons familiar to most people are domesticated forms, not the wild bird itself. The wild original is uniformly pale grey with two black bars on each wing, and males and females look much alike. So where did the first rock doves live, and how did a bird of cliffs and caves end up sharing our buildings?

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• 2. A Name Meaning Diver

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  In 1789, the German naturalist Johann Friedrich Gmelin gave the rock dove the name it still carries, Columba livia. He published it in a revised and expanded edition of Systema Naturae, the work of Carl Linnaeus. Gmelin placed the bird in the genus Columba alongside all the other doves and pigeons.

  Columba is the Latin word for pigeon or dove, and its older roots lie in the Ancient Greek kólumbos, meaning a diver. Aristophanes, in Birds, and other writers used kolumbís for the bird. They chose it because of the swimming motion of its flight.

  The second half of the name has a colour behind it. Livia is a Medieval Latin variant of livida, meaning livid or bluish-grey. Theodorus Gaza used that word to translate the Greek péleia, meaning dove, which is itself thought to derive from pellós, meaning dark-coloured.

  Gmelin designated no original material when he described the species. A neotype specimen from Fair Isle in Scotland was therefore chosen to define the species and its nominate subspecies, C. l. livia.

  Common names are messier. The official standard English name, set by the International Ornithological Congress, is rock dove. The bird is also called the common pigeon or rock pigeon, yet the speckled pigeon, Columba guinea, and the pigeons of the genus Petrophassa share the name rock pigeon. Within Columba, the closest relative is the hill pigeon, followed by the snow, speckled and white-collared pigeons.

  Nine subspecies are recognised, from C. l. gymnocycla in Mauritania, Senegal, south Mali and Ghana to C. l. intermedia in south India and Sri Lanka. The nominate form covers west and central Europe and north Africa to central Asia, while C. l. dakhlae is found only in west Egypt.

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• 3. The Wild Plumage

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  A white lower back is the best single clue to a pure rock dove. The nominate subspecies measures 30 to 35 cm in length, with a wingspan of 62 to 68 cm. Wild or feral birds weigh from 238 to 380 g, though overfed domestic birds can exceed that range.

  The head, neck and chest feathers are a dark bluish-grey. Along the neck they carry a glossy iridescence that runs from greenish to reddish-purple. The tail ends in a black band, and the outer web of each tail feather is edged in white. The iris is red-brown, the bill is grey-black with a small off-white cere, and the feet are red to pink. Domesticated and feral birds have a wider eye ring and a much larger cere.

  The adult female is almost identical to the male in outward appearance. Her neck iridescence is less intense and confined to the rear and sides, and young females look duller still.

  When a rock dove circles overhead, its white underwing becomes conspicuous. It is a strong flier, yet it also glides often, holding its wings in a very pronounced V. Its voice is a dovecot coo, quite unlike the phrase of the wood pigeon.

  The subspecies differ in shade and size. C. l. gymnocycla is smaller and much darker than the nominate, almost blackish on the head, rump and underparts, but with a white back. C. l. dakhlae is smaller and much paler. C. l. gaddi is larger and paler than C. l. palaestinae, and C. l. intermedia resembles C. l. neglecta but is darker, with a less contrasting back.

  The skeleton has been described many times. The lower jaw is angled, because the front half sits at 30 degrees to the back half. Beneath the skull, the hyoid skeleton includes an arrowhead-shaped structure called the paraglossum, or entoglossum.

  Pigeon feathers contain two kinds of melanin. Eumelanin generally gives grey or black, while pheomelanin gives reddish-brown, and mixtures of the two make other browns. A study of wild and domestic pigeons found that gene mutations changing the amounts, distribution and proportions of these pigments explain the greater colour variety in domesticated birds. Darker birds may also store more trace metals in their feathers, which could soften the harm from metals that are typically more concentrated in urban areas.

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• 4. Cliffs and Cities

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  Skeletal remains unearthed in Israel show that rock doves lived there at least 300,000 years ago. Fossil evidence suggests the species originated in southern Asia. Its history with humans is so long that its original range cannot be identified exactly.

  The natural resident range covers western and southern Europe and North Africa, extending into South Asia. In the wild, the birds live among rock formations and cliff faces, settling into crevices to nest. Caves, canyons and sea cliffs all serve. They even live in the Sahara, provided an area has rocks, water and some plant matter. They avoid dense vegetation.

  From 1603 onwards, European ships carried rock doves to most of the rest of the world. Including feral birds, the species now has an estimated global extent of occurrence of 10,000,000 km2. Europe alone holds an estimated 17 to 28 million individuals.

  The pure wild type has fared worse. It is endangered in many areas, and the nominate subspecies persists most notably in the far north and west of Scotland, most strongly in the Outer Hebrides.

  Human buildings imitate cliffs remarkably well, so rock doves often live alongside people in a commensal relationship. They gain ample food and nesting spots at no cost to themselves. Skyscrapers, highway overpasses, farm buildings and abandoned buildings with plenty of crevices all suit them. Agricultural settlements are favoured over forested ones. Ideal conditions combine tall buildings, green spaces, easy access to human food and schools. Suburbs far from city centres and with high street density suit them least.

  The modern range of the species is due in large part to humans. As proof of its versatility, one population lives inside a deep well in Tunisia.

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• 5. Flock Life

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  Flocks of 50 to 500 birds gather wherever food is plentiful, though rock doves are often found in pairs during the breeding season. As prey birds, they must stay vigilant. When one pigeon in a flock is disturbed, it takes off with a noisy clapping sound that cues the others to fly. The noise grows louder the faster the bird beats its wings, so it advertises how serious the threat seems.

  Other non-vocal sounds include feet stamping, hisses and beak snapping. Juveniles snap their bills, usually to answer an invasion of the nest. Wings may also be clapped in flight, usually during display fights or after copulation. Foot stamping is done with a particular foot first, which shows that rock doves have footedness, much as humans have handedness. What the stamping is for remains unclear.

  Rock doves are omnivorous but prefer plant matter, chiefly fruit and grains. Feeding groups tend to divide into producers, which scout out food, and scroungers, which follow and eat what the producers find. Groups generally contain more scroungers than producers. Pigeons are naturally granivorous, eating seeds small enough to fit down the gullet, and they sometimes add worms or insect larvae as protein. At the water's edge, a pigeon dips its bill in and drinks continuously. Most birds take small sips and tilt their heads back.

  Peregrine falcons are significant natural predators, and many raptors favour the rock dove. Pigeons are also hunted for food in many of the countries where they are native. The main causes of death in the wild are predators and persecution by humans.

  The body feathers have dense, fluffy bases and sit loosely in the skin, so they drop out easily. When an attacker seizes a pigeon, a mouthful of feathers can distract it long enough for the bird to escape. Tail feathers are also dropped under attack or other traumatic conditions, probably for the same reason.

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• 6. Courtship and Squabs

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  The wild call is a soft, slightly wavering coo, written "crruoo-u". An alarm call, given at the sight of a predator, is a grunt-like oorhh, and there are also a nest call and noises made by juveniles. When a male sings in display, the song is partly sexual and partly threatening. It comes with an inflated throat, fanned tail, strutting and bowing.

  Courtship in feral birds can be watched in urban parks at any time of the year, and wild populations are presumed to behave similarly. The male inflates his crop and puffs up his neck feathers to look larger. He approaches the female at a rapid walking pace, giving repetitive quiet notes, bowing and turning. At first, the female invariably walks or flies a short distance away, and the male follows until she stops. He then keeps bowing, very often making full or half pirouettes in front of her. Next he feeds her by regurgitating food, just as he later does for the young. Mating is very brief, and the male flaps his wings to keep his balance.

  The rock dove is generally monogamous, with two young per brood. The young are called squabs. Current evidence suggests that wild, domestic and feral pigeons mate for life, although the bonds are not unbreakable. The pairs are socially monogamous, and extra-pair matings do occur, often initiated by males.

  The nest is a flimsy platform of straw and sticks, laid on a ledge under cover. Two white eggs are laid, and both parents share incubation, which lasts 17 to 19 days. A newly hatched squab has pale yellow down and a flesh-coloured bill with a dark band. For the first few days, the chicks are fed only on crop milk, also called pigeon milk, which both parents produce in their crops. Every species of pigeon and dove does this.

  Crop milk lets the species breed at any time of the year, with peaks in spring and summer, when food is abundant enough to support egg development. Eggs can be laid up to six times a year. Squabs are altricial, with a fledging period of about 30 days. A rock dove lives 3 to 5 years in the wild and up to 15 years in captivity, though longer-lived birds have been reported.

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• 7. Inner Workings

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  Uric acid is the key to how a rock dove handles the waste from breaking down protein for energy. The process forms amine groups, which the bird secretes in uric acid. This releases nitrogen using only moderate amounts of water. The drawback is the energy cost, because uric acid is chemically complex.

  Water balance is vital, and water is needed to excrete toxins. All columbids, rock doves included, can lower their water intake when food is scarce, and raise it when food is plentiful. Rock doves inhabit arid places, yet research shows that their large flight range lets them reach distant water. Conserving body water is therefore less critical for them.

  Unlike many other birds, rock doves rely on their kidneys rather than a salt gland to keep their internal balance. The kidneys filter blood, reabsorb ions and water, and secrete uric acid. Like mammalian kidneys, they have a medullary region and a cortical region. Bird nephrons cannot directly produce urine that is hyperosmotic to the blood, as mammal nephrons can. Even so, nephrons with a loop of Henle use a countercurrent mechanism, and this lets the collecting duct become hyperosmotic.

  Bird eggs are very sensitive to temperature and humidity. Gas exchange during embryo growth causes water loss, and the embryo can die of desiccation. Parents therefore incubate regularly when conditions change rapidly or turn very dry, though very humid air can inhibit respiration. Pore-like areas in the eggshell let water diffuse in and out. Thinner shells have shorter pores, higher conductance and a larger pore area, and they restrain the embryo less.

  Preening works differently from most birds. Pigeons have either no preen gland or a very rudimentary one, so they use no oil. They spread powder down feathers across the body instead, giving the plumage a soft, silky feel. The powder is akin to talcum powder, and it helps maintain the plumage as the feathers disintegrate.

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• 8. Darwin's Pigeons

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  Charles Darwin put the rock dove at the centre of his discovery of evolution, and it featured in four of his works between 1859 and 1872. He argued that the many hundreds of domestic breeds, despite wide differences in form, could all be traced to the wild rock dove. In essence, he saw human selection of pigeon breeds as analogous to natural selection.

  The breeding itself began long before him. Rock doves have been domesticated for several thousand years, and perhaps as long as 5,000 years ago. Domesticated pigeons serve as homing pigeons, as food and as pets. In the past they were also carrier pigeons, delivering messages in peacetime and in war. Breeders have produced numerous fancy breeds of all sizes, forms and colours, with plumage that can be red, brown, checkered, uniformly coloured or pied.

  Escaped domestic pigeons are the origin of feral pigeons around the world. These birds, also called city doves, city pigeons or street pigeons, live independently and are often unwanted by people. They are commonly described as a public nuisance, a potential reservoir of disease and a cause of property damage. The names Columba livia domestica and Columba livia forma urbana are sometimes given to domestic and feral birds, and both are invalid.

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