Pigeons, UnfoldedThe Science of an Everyday Bird

A pigeon's head isn't really bobbing. As a pigeon walks, its head holds still in the air while its body moves forward, then the head zips ahead to catch up, and holding still keeps the bird's view of the world steady.³

The movement is too quick for the eye to follow, so it took film to take it apart. In 1978 Frost described the walking pigeon's head movement as two alternating phases: "one where the head is 'locked' in space but moves backward relative to its forward moving body; and another where it is thrust rapidly forward to a new position." The bob that people see is the sum of those two phases, a hold and a thrust, repeated with every step.⁴

An earlier paper in Nature, by Friedman in 1975, had already turned the familiar picture around. "In reality," he wrote, "the walking bird thrusts its head forward, then holds it nearly stationary while the neck retracts and the body catches up." Measured against the bird's own body, the head does swing back and forth. Measured against the pavement, it only ever moves forward, in a series of pauses and sudden jumps.⁵

The point of the pause is clear sight. The researcher Land, quoted by Live Science, put it simply: "Keeping the head still in space during the 'hold' phases means that the image will not be blurred by motion." A camera operator walking with a camera does something similar with a steadying rig. The pigeon does it with its neck, buying a sharp view during each hold and paying for it with a quick blur during each thrust.⁶

The treadmill

If the hold phase exists to keep the view steady, a pigeon that has no view to steady should stop bobbing. Frost tested exactly that. He put pigeons on a treadmill, where the bird walks but its surroundings stay still relative to it, the way a person on a gym treadmill walks without the room moving past. In the study, head-bobbing disappeared. The legs kept stepping, but the head no longer held and thrust.⁷

Frost's conclusion was careful. Because head-bobbing was abolished on the treadmill, he wrote, it is primarily a visual response rather than a response that keeps the bird balanced. Balance had been a reasonable guess: a walking body shifts its weight with every step, and a bobbing head might help to steady it. The treadmill removed the moving world and left the walking, and in the study it was the moving world that the head was answering. The finding belongs to pigeons in that laboratory set-up, and it has held up since.⁸

How still is still?

Later work asked how good the hold really is. Troje and Frost, writing in 2000, found that during the hold phase the head of the pigeon remains locked in space, and that the stabilisation is under visual control, confirming in the study what the treadmill had implied. Yet the head is not perfectly still. A small slip remains, and researchers think that tiny drift may serve as an error signal, the very motion of the image that tells the bird how to correct its head.⁹

In 2017 Theunissen and Troje gave the clearest summary yet. "A walking pigeon, for example, locks its head in space as long as possible and then thrusts it with one sudden, ballistic movement into a new position," they wrote, "where it again becomes motionless while the body catches up." The phrase "as long as possible" is the heart of it. The thrust is not the goal of the movement. It is the price the bird pays for its next still moment.¹⁰

Eyes on the sides

Why should a pigeon care so much about a steady view? Part of the answer lies in where its eyes are. A pigeon's eyes sit on the sides of its head, and BirdNote explains that these laterally placed eyes give a much wider view of the world, including a wide area behind the head. The cost is depth perception: the views of the two eyes overlap only slightly, in a small zone in front of the beak.¹¹

Laboratory measurement has mapped that small zone. McFadden and Reymond, in 1985, described the pigeon's binocular field, the part of the world both eyes see at once, as a narrow oval centred on the line of the beak. Everything else is seen by one eye at a time. A bird built this way sees a great deal of its surroundings at once, which suits an animal that many hunters would like to catch.¹²

Put the two findings together and the walking pigeon looks less comical and more capable. Its wide-set eyes take in the pavement ahead, the sky above and much of what lies behind, and its neck holds that wide picture steady in a run of brief, still frames. The famous bob is a pigeon doing its best to keep looking.¹³

Most birds tip their heads back to drink, but a pigeon sips water up as if it were using a straw, without lifting its head.¹⁴

A Washington State University explainer describes both methods. Most birds scoop water into the lower half of the beak and then tilt their heads back so that it runs down into the throat. Pigeons and doves push the whole beak into the water instead, and muscles in the throat squeeze the water up into the mouth. "It's like slurping up a drink with a straw," the explainer says.¹⁵

The habit belongs to the whole family, not only to the bird on the pavement. The Toronto Zoo, describing the Victoria crowned pigeon, notes that "like all pigeons, they drink by sucking up water instead of tilting their head back to swallow." The comparison is with most birds, not all of them, but among pigeons and doves the straw is the rule. A bird that drinks this way can keep its head down and its eyes on its surroundings for the whole drink.¹⁶

Pigeon or dove?

That family is the Columbidae, and it raises an old question: what separates a pigeon from a dove? The answer is nothing scientific. The RSPB states plainly that there is no scientific distinction between the two, and the British Trust for Ornithology adds that the same species may be called a pigeon in one part of its range and a dove in another. People tend to call the smaller, more delicate species doves and the stouter, more urban ones pigeons.¹⁷

The city pigeon is the clearest example. The RSPB points out that the rock dove and the feral pigeon are actually the same species, Columba livia, so the bird on a sea cliff and the bird under a railway bridge share one scientific name and two common ones. Which name a person uses says more about where they met the bird than about the bird itself.¹⁸

The family is large. The British Trust for Ornithology counts some three hundred species, and the RSPB says over 300, so there are about three hundred kinds in all. Most of them live in the tropics, though members of the family are found worldwide outside the polar regions. Diets follow the climate: most pigeons of cooler regions eat seeds, while a good proportion of tropical species feed on fruit.¹⁹

A pigeon called the dodo

The most famous member of the family is one that no longer exists. The dodo of Mauritius, a big flightless bird, was a pigeon. Genetic studies place the dodo and its relative the Rodrigues solitaire, both extinct, within a group of very diverse pigeons, and those studies show that their closest living relative is the Nicobar pigeon, a colourful bird that can fly.²⁰

The relationship is now repeated in museum and zoo signage alike. The Oxford University Museum of Natural History calls the colourful Nicobar pigeon the dodo's closest living relative today, and the Oregon Zoo says the same of its own Nicobar pigeons. The dodo itself survives only as remains in museum collections. Somewhere in the deep past, the ancestors of a flightless island giant and of the grey bird on the station roof shared a family tree.²¹

The passenger pigeon

A second lost cousin vanished far more recently. Billions of passenger pigeons once flew over the forests of eastern North America, and the species was among the most common birds on the continent. The Cincinnati Zoo records that passenger pigeons were hunted relentlessly for commercial sale and sport, while their forest habitat dwindled. The Natural History Museum in London notes that in just half a century they had disappeared entirely.²²

The end can be dated with unusual precision. The last reliable record of a wild passenger pigeon was a bird shot in Ohio in 1900. The last known individual, a captive female named Martha, died at the Cincinnati Zoo on 1 September 1914, and with her the species was gone. How many passenger pigeons there once were is uncertain, but that a bird so numerous could disappear so completely is not.²³

The straw is not the only habit the family shares. Pigeons and doves as a group also feed their newly hatched young on something no other bird in this book makes: a milk, produced in the crop by both the mother and the father. That is where the next chapter goes.²⁴

Pigeon parents make milk for their chicks. Both the mother and the father pigeon make it in a body part called the crop.²⁵

The word milk is borrowed, and the scientists who study it often keep it in quotation marks. A 2012 paper by Gillespie and colleagues opens with a definition: "Pigeon 'milk' is a substance produced in the crop of both male and female pigeons for the nourishment of their young." The Audubon field guide puts it in the plainest terms, noting that both parents feed the young pigeon milk. It is not the same substance as the milk of a cow or a cat, but the comparison turns out to run deeper than the name.²⁶

Most striking is the fact that the father makes it too. In mammals, only mothers produce milk. In pigeons, the Scientific American Lab Rat blog notes, both female and male pigeons produce it in their crop, so a newly hatched squab can be fed by either parent. Feeding the young is a shared job from the very first day.²⁷

The prolactin connection

The deeper link with mammals is a hormone. In a 1995 review, Horseman and Buntin wrote that prolactin "stimulates the growth and development of specialized epithelial cells lining the cropsac of pigeons and doves," leading to the formation of crop milk, which is fed to the newly hatched squab. The lining of the crop, in other words, grows a layer of special cells, and that growth is the milk.²⁸

Prolactin is also the hormone that regulates milk production in mammals. Gillespie and colleagues make the parallel explicit: "Like mammalian lactation, pigeon 'milk' production is regulated by the lactogenic hormone prolactin." A pigeon on a ledge and a human mother feeding a baby are using the same chemical signal to do something similar, through two very different body parts.²⁹

The hormone does more than build the milk. High prolactin levels while a pair is raising young also support the behaviours that go with feeding: eating more, staying at the nest and bringing food back up for the squab. Exactly how prolactin drives those behaviours is still debated, but the overall picture is of one signal that prepares both the body and the bird's habits for parenthood.³⁰

Pigeon milk in a chicken

How alike are the two milks in what they do? Gillespie's team looked for an answer by feeding pigeon milk to chickens, a convenient model. In the study, chickens fed pigeon milk grew faster and turned their feed into body weight more efficiently than control chickens. They also showed stronger activity of immune-related genes in the gut and had more diverse gut bacteria.³¹

The authors concluded that pigeon milk is functionally similar to mammalian milk, and they offered a hypothesis about its origins: "We hypothesise that pigeon 'lactation' and mammalian lactation evolved independently but resulted in similarly functional products." The scope matters here. The result in the study comes from chickens, not from pigeon chicks, so it shows what pigeon milk can do in a young bird's gut rather than measuring it in a squab.³²

Two parents from the start

The partnership that ends in shared feeding starts with courtship. A courting male rock pigeon spreads his tail, puffs up his chest and struts about, often in circles around the female, repeatedly bowing and cooing. Anyone who has watched a pavement full of pigeons has seen this display. Males may also make a display flight, climbing sharply with loud claps of the wings.³³

Once formed, a pair can stay together for life. The Animal Diversity Web account of the species states that pairs are formed for life and may be formed at any point in the year, and the Audubon guide says that rock pigeons may mate for life. Each partner, the account adds, works cooperatively on most aspects of raising the young.³⁴

Their nest is no showpiece: a simple platform of twigs and grass. A pair may use the same site again and again, adding to the nest each time, so an old site can grow into a substantial pile. Sources disagree about who does the building. The Audubon guide says the female builds with material the male supplies, while the Animal Diversity Web account says the male builds the nest, and this book leaves the question open.³⁵

A clutch is usually two eggs, sometimes one. Incubation is shared by both parents, which take turns sitting, and the Audubon guide gives 16 to 19 days before the eggs hatch, while the Animal Diversity Web account says eggs hatch about 19 days after laying. By the time the first squab breaks out of its shell, both of the birds that kept it warm are ready with milk.³⁶

Baby pigeons stay tucked in the nest until they are almost fully grown, so people hardly ever spot a pigeon that looks like a baby.³⁷

"Where are all the baby pigeons?" is one of the oldest questions people ask about cities. The London Museum answers it directly: "The chicks, called squabs, stay in the nest until almost fully grown – hence the urban myth that you never see their young." The young are there in large numbers. They are simply out of sight until the moment they no longer look young.³⁸

Two habits combine to keep them hidden. The BBC points out that even confident urban pigeons are very secretive about nest sites, hiding them away on rooftops, church towers and in abandoned buildings. Then the squabs stay in the nest, away from human eyes, until they are almost grown and take on an adult appearance. By the time a young pigeon walks onto a pavement, it passes for a grown-up.³⁹

Four weeks out of sight

The stay in the nest is long for a bird of this size. The Audubon guide says young rock pigeons leave the nest at about 25 to 32 days, or later in cold weather, and a poultry extension guide says squabs usually leave at around 26 to 30 days. About four weeks, then, is typical, and for all of that time a city squab is fed by its parents in a place few people ever look.⁴⁰

That timing is what makes the myth. A young pigeon usually stays put until it is almost the size and shape of its parents, so the first time most people see it, nothing about it says baby. The squab years, such as they are, happen entirely out of view.⁴¹

Ledges for cliffs

Where a pigeon hides its nest goes back to its wild ancestor. Wild rock doves nest in crevices and on sheltered ledges of rocky sea cliffs, and in their native range they also nested in inland mountains and gorges. A sheltered shelf of rock, out of the weather and out of reach, is the kind of place the species has always looked for.⁴²

Cities are full of such shelves. The Audubon guide notes that in cities the birds use artificial replacements such as the window ledges of tall buildings, and the RSPB suggests looking under bridges and on roofs, which mimic the birds' natural preference for rocky cliff ledges. The London Museum puts it most vividly: the city's buildings make perfect nest sites, "mimicking the windswept cliffs used by the birds' ancestors."⁴³

One family after another

Hidden nests can be busy ones. A pigeon pair can raise several broods in a year: the Audubon guide says a pair may raise up to five or more, and New York City Pigeon Rescue Central gives an average of four to six times a year. The same group notes that breeding increases dramatically when the food supply increases, so how often a pair nests depends heavily on what there is to eat.⁴⁴

Season is less of a limit than for many birds. The London Museum says pigeons breed no matter the season, and in Britain a few pairs have been recorded nesting even in winter months. A poultry extension guide describes how closely broods can overlap: after the eggs hatch, the female can lay two more eggs, usually in a second nest, and incubate them while the male tends to the two squabs in the first.⁴⁵

The British Trust for Ornithology sums it up: the breeding season is extended, and a pair can lay many clutches in it. Put that beside the four-week stay in the nest and the puzzle of the missing babies dissolves. A city with many pigeons is a city with many squabs, almost all of them sitting quietly on ledges and in roof spaces, waiting to look grown before they step out.⁴⁶

Turn a city pigeon in the light and its neck changes colour. The neck feathers of domestic and feral pigeons shimmer green and purple, and the colours shift as the angle of view changes. The Animal Diversity Web account describes the rock dove's glossy iridescence along its neck and wing feathers, and notes that females tend to show less iridescence than males.⁴⁷

Physicists have measured the shift. Yin and colleagues, in a 2006 study of pigeon neck feathers, reported that "green barbules vary colors from green to purple with the observing angle changed from normal to oblique, while purple barbules from purple to green in an opposite way." A barbule is one of the tiny side branches of a feather. Each kind of barbule, green or purple, slides toward the other colour as the viewer moves.⁴⁸

Colour without paint

Colours that change with angle are a clue that something other than pigment is at work. Pigment colours an object the way paint does, and looks much the same from any side. Yin's team found that each neck barbule has a thin outer layer of keratin around an inner layer, and concluded that the colour comes from "the interference in the top keratin cortex layer, while the structure beyond acts as a poor mirror." This is called structural colour.⁴⁹

Interference is the same effect that paints swirling colours on a soap bubble: light reflecting from the top and bottom of a very thin layer combines, strengthening some colours and cancelling others. What sets a green feather apart from a purple one, the study found, is the thickness of that outer keratin layer. The two colours on one pigeon's neck are made of the same material, cut to two different thicknesses.⁵⁰

A colour beyond violet

The green and purple are only the part of the show that people can see. "HUMANS cannot see the near ultraviolet (UV) wavelengths (320–400 nm) in sunlight," Kreithen and Eisner wrote in Nature in 1978. They went on to show that homing pigeons are sensitive to those wavelengths, capable of responding to UV light at the intensities found in nature. Pigeons can see a band of sunlight that is invisible to us.⁵¹

The discovery had a touch of luck in it. The National Wildlife Federation recounts that a researcher testing pigeons' ability to discriminate colours discovered by accident that the birds can see ultraviolet light. In 1980 Emmerton and Delius reported that pigeons trained to tell colours apart also maintained good discrimination between wavelengths within the ultraviolet range they tested. Later work found that many other birds see ultraviolet too.⁵²

That makes the pigeon's own neck worth a second look. In 2004 McGraw measured the light reflected by iridescent pigeon neck feathers with a spectrometer. Feathers that appear purple to the human eye showed two reflectance peaks in the ultraviolet, as well as peaks in the blue and red. Feathers that appear green showed two ultraviolet peaks as well, among five in all. Both colours carry an ultraviolet component that people miss entirely.⁵³

What the birds make of those hidden colours is a separate question. McGraw suggests that birds may use such ultraviolet colours as signals to one another. That use is a suggestion; the reflectance itself was measured. The safe statement is that a pigeon's neck reflects ultraviolet light and that pigeons can see ultraviolet light, and whether one pigeon reads the other's neck in that light remains to be shown.⁵⁴

Four channels, perhaps five

Human colour vision runs on three kinds of colour channel. Emmerton and Delius tested how finely pigeons could tell one wavelength from another across both the visible and the ultraviolet range, and found several separate points of high sensitivity. From that pattern they concluded that the pigeon's colour system "must be at least tetrachromatic, probably pentachromatic," so scientists think pigeons use at least four channels, and perhaps five.⁵⁵

It was not always thought so. Emmerton and Delius recall that a first study, by Hamilton and Coleman in 1933, had suggested that the pigeon's colour vision was similar to a human's, and that later determinations "definitely corrected that erroneous early impression." Today researchers think the pigeon's colour world is richer than ours, not a copy of it.⁵⁶

So the grey bird on the pavement is grey only to us. Its neck shimmers green and purple from structure rather than pigment, and it reflects ultraviolet light that another pigeon can see. A person looking at a pigeon and a pigeon looking at a pigeon are, in a real sense, looking at two different birds.⁵⁷

Homing pigeons can find their way home from far away, so people used them to carry messages, and a famous story says one pigeon helped save trapped soldiers.⁵⁸

Homing pigeons are domestic rock pigeons, bred and trained to return to their home loft. The Audubon guide notes that trained homing pigeons can return to the loft from long distances away, and the National Museum of the United States Army adds that they find their way home even across terrain the bird does not know. How a pigeon can do that from a place it has never seen has been studied for decades.⁵⁹

Researchers describe how a pigeon can do it from unfamiliar areas as a "map-and-compass" mechanism. A compass alone is not enough: a bird that knows where north is still needs to know where home lies from where it stands. The map tells the pigeon which way it has been moved, and the compass lets it fly in that direction. Each half has been pinned to different senses, and some of them are still argued over.⁶⁰

A compass in the sky

Scientists think the compass part is the sun. Far from home, the British Trust for Ornithology says, pigeons use the sun as a compass and are able to compensate for its movement across the sky. Navigation researchers call this a time-compensated sun compass: the bird can tell direction from the sun's position as long as it allows for the time of day.⁶¹

Time compensation is the clever part. The sun moves across the sky through the day, so a direction taken from the sun at breakfast would be wrong by mid-afternoon. A pigeon that can use the sun all day must, in effect, carry a clock and keep correcting for it, and researchers think the birds do exactly that.⁶²

A map made of smells

The map is harder to pin down. "Forty years ago," wrote Gagliardo in a 2013 review, "Papi and colleagues discovered that anosmic pigeons cannot find their way home when released at unfamiliar locations." Anosmic means unable to smell. Birds that could not smell were lost from unfamiliar places, so scientists think smell is part of the map.⁶³

The olfactory navigation hypothesis explains how scientists think such a map could work. During the first months after fledging, young pigeons memorise the odours carried by the winds at their home loft together with the wind's direction. At a distant release site, the local odours tell the bird which way it has been carried. It is an elegant idea with a gap that researchers admit openly: it is not known which odours the birds rely on.⁶⁴

The magnet question

Pigeons may also sense Earth's magnetism, and this is where the science is most unsettled. In 1971 Keeton glued magnets to the backs of experienced pigeons and released them. Under total overcast, the magnets often left the birds disoriented; under clear skies they did not. One reading is that pigeons fall back on a magnetic sense when the sun is hidden, and scientists are still testing it.⁶⁵

Later results pulled in two directions. In 2004 Mora and colleagues reported that homing pigeons could be trained to tell the presence of a magnetic anomaly from its absence. The hunt then turned to a sensor, and clusters of iron-rich cells in the upper beak looked promising. But in 2012 Treiber and colleagues showed that those cells are macrophages, immune cells, not nerve cells, so how pigeons may sense magnetism is still an open question.⁶⁶

The BBC's summary is honest about the state of play: it is not entirely clear how homing works, with some researchers pointing to the sun and others to magnetoreception. The safest reading of the evidence is that a pigeon may combine several senses, and that which one leads, and when, is still being worked out.⁶⁷

Landmarks near home

Closer to the loft, pigeons switch to something any person would recognise: what the place looks like. In familiar areas, homing pigeons are largely guided by visual landscape cues, following routes that are both stereotyped and idiosyncratic, the same each time for a given bird but different from bird to bird. The British Trust for Ornithology says that near home they seem to use local landmarks.⁶⁸

GPS tracking made those routes visible. In 2004 Biro, Meade and Guilford followed pigeons over familiar country and found that each bird developed its own highly consistent route. Released from new sites off that route, the pigeons were drawn back to it. "This precise route loyalty," the authors wrote, "demonstrates a reliance on familiar landmarks throughout the flight." Near home, the landscape itself is the guide.⁶⁹

People have turned the homing habit into a sport. In pigeon racing, specially bred and trained homing pigeons are released from specific locations and race back to their home lofts. The time each bird takes to cover a carefully measured distance gives its speed, which is compared with the others. Some races are run by local clubs whose members keep their own lofts; in one-loft races, every bird lives and trains in the same loft.⁷⁰

Messages and a legend

Long before racing, homing was a postal service. Armies used homing pigeons because a short note placed in a tiny canister on a bird's leg would travel with it back to its home loft. The most famous of those military birds is the subject of a story that is still told a century later, about a pigeon named Cher Ami and a battalion of trapped soldiers.⁷¹

The setting is real. In October 1918, during the Meuse-Argonne Offensive, a group of U.S. soldiers later called the Lost Battalion was cut off in France's Argonne Forest. Forced to rely on homing pigeons, their commander, Major Charles Whittlesey, sent seven messages over October 3 and 4. The last pigeon to reach the loft arrived wounded, and its message gave the battalion's exact location, which aided their relief.⁷²

In April 1919 an officer of the army's Pigeon Service proclaimed Cher Ami to be the pigeon that saved the Lost Battalion, and the story has been retold ever since. The Smithsonian's research complicates it. What is known is that Cher Ami delivered a vital message despite grievous wounds, but military records cannot say which pigeon carried the key message of October 4. The famous story is partly legend.⁷³

Cher Ami's preserved body is at the Smithsonian, and in 2021 the museum used DNA testing to settle one more detail: the bird was male. Whether or not Cher Ami carried that particular message, the bird did what homing pigeons were trained to do, and did it despite grievous wounds.⁷⁴

In the study, park pigeons kept avoiding a person who had chased them, even when that person wore a different coat, so never chase a pigeon.⁷⁵

The experiment behind that finding, published in 2011 by Belguermi and colleagues, took place in an urban park among ordinary feral pigeons. Two people of similar age and skin colour, wearing coats of different colours, came to feed the birds. During training, the two feeders behaved differently: one was neutral, and the other was hostile and chased the pigeons away. The question in the study was whether the birds would learn which person was which.⁷⁶

They did, and quickly. Within six to nine sessions, the pigeons in the study had learned to avoid the hostile feeder. They kept avoiding that person in later tests, when both feeders behaved neutrally, so the birds were not simply reacting to being chased in the moment. They were remembering who had chased them before.⁷⁷

The decisive test was a change of clothes. In the study, "the pigeons avoided the hostile feeder even when the two feeders exchanged their coats, suggesting that they used stable individual characteristics to differentiate between the experimenter feeders." A coat is the most obvious thing about a person at a distance. The birds looked past it to something about the person that did not change.⁷⁸

The authors call it the first experimental evidence of this level of human discrimination in feral pigeons. It is one experiment in one park, and the result in the study does not say which features the birds used, but it fits a remark from the London Museum: "Pigeons, it seems, are capable of remembering both faces and places." For anyone who feeds or passes pigeons, the practical lesson is the episode's: never chase a pigeon.⁷⁹

Faces in photographs

A laboratory study published the following year asked whether faces alone are enough. Researchers from the University of Lincoln and the University of Vienna first trained pigeons to sort familiar from unfamiliar objects in photographs. Then they showed the birds pairs of human faces, one familiar and one not. In the study, the trained birds recognised and classified the familiar people from their faces alone, while birds without the prior training failed.⁸⁰

The two studies approach the same ability from opposite ends. In the study in the park, the birds were untrained and learned from experience who was a threat; the laboratory birds were trained, and then shown nothing but faces. Neither proves that every pigeon recognises every person, but together they show the ability exists in the species and can be called on in very different settings.⁸¹

Monet and Picasso

Pigeons have also been asked to judge art. In a 1995 experiment, Watanabe, Sakamoto and Wakita trained pigeons with colour slides of paintings by Claude Monet and Pablo Picasso. The birds learned to tell the two painters apart. Then, in the study, they also sorted new paintings by Monet and Picasso that had never been shown to them in training.⁸²

The birds went further than the training asked. In the study, they grouped paintings by Cézanne and Renoir with Monet, and paintings by Braque and Matisse with Picasso. Turning the Monet paintings upside down disrupted their choices, but turning the Picasso paintings upside down did not.⁸³

Counting and spelling

Abstract rules were once thought to be a primate speciality. A 2011 study in Science by Scarf, Hayne and Colombo tested whether pigeons could use an abstract numerical rule. After training, the birds had to put pairs of images in order by the number of items they showed. In the study, the pigeons showed the ability, and their performance was indistinguishable from that of monkeys trained the same way.⁸⁴

In 2016 Scarf and colleagues turned to words. In the study, pigeons were trained to sort real four-letter English words from made-up strings of letters, and each bird learned dozens of words. They then identified real words they had never seen before, using the letter patterns that make words look like words, and were sensitive to how often pairs of letters occur together.⁸⁵

The researchers call this orthographic processing: recognising the visual form of words. It is not reading. The birds in the study had no access to meaning or sound, and the authors note that decoding skills are clearly human-unique, because they are seated in language. What the pigeons showed is that the first step in reading, seeing which strings of letters look like words, can be learned by a bird.⁸⁶

People and pigeons go back a very long way. Writing in 1859, Charles Darwin said that pigeons "have been domesticated for thousands of years in several quarters of the world." Geneticists date the first domestication of the rock pigeon to Neolithic times, and the RSPB says people have bred pigeons for their feathers, eggs, meat and navigation skills for thousands of years.⁸⁷

Written evidence is younger than the partnership itself. The RSPB says the first written evidence of pigeon domestication is from over 5,000 years ago. Darwin, citing the Egyptologist Lepsius, gave the earliest known record as Egypt's fifth dynasty, about 3000 BC, so the oldest written records are about 5,000 years old. Domestication itself may well be older than anything written about it.⁸⁸

Darwin's pigeons

Darwin did not only write about pigeons; he kept them. "Believing that it is always best to study some special group," he wrote in On the Origin of Species, "I have, after deliberation, taken up domestic pigeons. I have kept every breed which I could purchase or obtain." He joined the fancy as well. The London Museum records that he was a member of the Southwark Columbarian Society, one of several pigeon fanciers' clubs in London.⁸⁹

Darwin used his pigeons to make an argument. However different his breeds looked, he was convinced, as he put it, "that all are descended from the rock-pigeon (Columba livia)." If breeders choosing birds over many generations could make such different forms from one wild ancestor, change of that kind was clearly possible.⁹⁰

That was the point the BBC summarises: by artificially selecting in this way, Darwin gathered valuable evidence for his theory of evolution by natural selection. The first chapter of On the Origin of Species leans on fancy pigeons, and the bird that bobs along a station platform is a direct descendant of the same wild species Darwin's breeds came from.⁹¹

Hundreds of breeds

The breeds have only multiplied since. A 2013 genome study by Shapiro and colleagues notes that breeders have selected striking differences in behaviour, voice, skeleton, feather ornaments, colours and colour patterns to establish over 350 breeds. The London Museum lists fantails, pouters and tumblers among the varieties once kept in the city.⁹²

The University of Utah's Genetic Science Learning Center offers a comparison any dog owner will grasp: like dogs, pigeons belong to hundreds of different breeds, which can differ from one another as much as a German shepherd does from a miniature poodle. "Yet all of these breeds are descended from one ancestral species: the wild rock dove," the same source adds.⁹³

How the city got its pigeons

The feral pigeons of towns and cities are part of the same story. The RSPB says they are all descendants of domesticated pigeons that escaped or were released, and geneticists define feral populations as free-living birds descended from escaped domestics. In London, the museum traces the first feral pigeons to birds that escaped from dovecotes and made nests for themselves elsewhere in the city.⁹⁴

Genomes have begun to name some of the escapees. When Shapiro's team compared the genomes of dozens of domestic breeds with feral pigeons, the feral birds grouped with homer and wattle breeds. In the study, that pattern supported the idea that escaped racing homers are probably major contributors to feral populations, and the authors report contributions from a racing breed to North American feral pigeons.⁹⁵

That result has a clear limit: in the study only a small number of feral pigeons were sequenced, so it cannot describe every city's flock. It does fit the rest of this book neatly. Some of the pigeons that never came home to their lofts probably stayed on as city birds, and their descendants are among the pigeons walking the pavement today.⁹⁶

Domestication also explains the bird's range. Wild rock doves are native to Europe, North Africa and southwestern Asia, but people domesticated pigeons and carried them around the world. Feral pigeons now live in cities worldwide, the Animal Diversity Web account says, including throughout North America.⁹⁷

Where are the wild rock doves?

The wild ancestor has paid a price for its domestic descendants' success. The RSPB reports that the rock dove is declining, and that there is some argument about whether any true rock doves are left in the wild. These coastal and mountain nesters survive only where feral pigeons have not yet colonised. Where feral pigeons arrive, the wild and feral birds interbreed, and the feral birds dominate.⁹⁸

The British Trust for Ornithology goes further for its own country, saying it seems unlikely that any true rock doves remain in well-populated countries such as Britain. Those statements concern Britain and places like it, not every coast in the species' native range. Even so, they leave an odd picture: in parts of its own homeland, the wild rock dove has been absorbed by the city pigeon it gave rise to.⁹⁹

A pigeon's natural diet is plain. Rock pigeons eat mostly seeds, foraging mostly by walking on open ground. Away from cities they take waste grain and the seeds of grasses and other plants, sometimes berries or acorns, and occasionally a few earthworms or insects.¹⁰⁰

City life changes the menu. In cities, many pigeons live largely on bread crumbs, popcorn and other junk food provided by people, the Audubon guide says, and the Animal Diversity Web account lists popcorn, cake, peanuts, bread and currants among the foods feral pigeons eat. How much each bird depends on scraps varies from place to place and bird to bird.¹⁰¹

Bread is the food people most often offer, and it is a poor one. The RSPB has said bread is not a good food for birds because it fills them up while offering them little nutritional value. New York City Pigeon Rescue Central is blunter, describing wet bread thrown on the street as virtually nutrition-free. A pigeon's body is built for seeds and grains.¹⁰²

Some places go further and restrict feeding altogether. The London Museum notes that feeding is banned in Trafalgar Square, where Harris hawks have been used to patrol the area. A town-centre ban has been introduced in Boston, England, and the City of Las Vegas says feeding pigeons is prohibited there. Rules differ from place to place and many places have none, so local rules are worth checking first.¹⁰³

A falcon's favourite

Pigeons feed more than people's curiosity. The Audubon guide calls the rock pigeon a favourite prey of the peregrine falcon, and says pigeons support the peregrines that stay around cities. A falcon on a cathedral spire or an office tower is there, in part, because the ledges below are full of pigeons.¹⁰⁴

The peregrine is not the only hunter. The Animal Diversity Web account lists golden eagles, American kestrels and peregrine falcons among the pigeon's predators, and in North America adds opossums, raccoons, great horned owls and eastern screech-owls. The wide view of the first chapter makes more sense against that list: a bird with so many enemies does well to see most of the sky at once.¹⁰⁵

Startle a flock and the escape is audible. When rock pigeons erupt into flight, BirdNote says, some of them may slap their wings together above their bodies in a sharp wing clap. Not every pigeon claps on every take-off, and males also clap during courtship, flying sharply upward near a female in an aerial display.¹⁰⁶

Watching quietly

Seen up close, the ordinary pigeon is not ordinary at all. Its head holds still so that its eyes can see, and those eyes see colours we cannot. Unlike most birds, it sips through its beak like a straw, both of its parents make milk, and its young wait on hidden ledges until they are almost grown. Its domestic cousins can find home across unknown country, by senses that may include magnetism and that scientists are still working out.¹⁰⁷

And in the study in the park, pigeons remembered the person who chased them. That makes the best way to meet a pigeon the simplest. Sit on a bench, keep still and watch quietly: the head that holds and thrusts, the shimmer that comes and goes on the neck, the straw-like sip at a puddle. Never chase a pigeon. It may well remember you.¹⁰⁸