The Homing Instinct

by Bernd Heinrich

  On the time scale we need to consult to arrange our affairs, it was only “yesterday,” in 1878, that the last of the immense communal nesting places of the passenger (from the French word passager, “to pass by”) pigeon were recorded. It was near Petoskey, Michigan, and this nesting area encompassed an immense expanse—over four hundred square kilometers. Before that, observers had described flocks that literally darkened the skies and took days to pass. The flocks encompassed billions of pigeons. Passenger pigeons were described “beyond number and imagination.” John James Audubon as well as Alexander Wilson, “The Father of American Ornithology,” told of one breeding place near Shelbyville, Kentucky, that in 1806 was several kilometers in breadth and sixty-five kilometers long. Wilson encountered a flock from another nesting place ninety-five kilometers from this one that, from its breadth, flight speed, and duration, he estimated to be 385 kilometers long and one and a half kilometers wide and to contain 2,230,272,000 pigeons. In 1813, as Audubon was leaving his house at Henderson “on the banks of the Ohio” to go to Louisville, Kentucky, he saw a flock that was so thick that “light of noonday was obscured as by an eclipse.” The birds passed in undiminished numbers for three days in succession. At the breeding and roosting places, limbs and trees crashed to the ground from the sheer weight of their numbers.

  The passenger pigeon swarms moved up and down the continent from at least Nova Scotia to Florida, and they were reputed to nest for most of the year wherever and whenever they found food. They were long-lived, said to live to twenty-four years in captivity, and Audubon estimated that their numbers doubled and quadrupled each year, so the idea of them going extinct was unthinkable. They were the most common bird in America, but sadly, as with the Rocky Mountain locust, no scientific studies of even the most basic sort were made of them that might now help to explain their demise. “Overhunting” is a commonsense assumption, as is deforestation, but I believe these are poor or at least simplistic explanations that do not touch the root of the problem, which concerns their biology.

  The best account of the passenger pigeon’s probably critical home life was given in the Chautauquan by “the last Pottawottomi [sic] chief of the Pokaton band” as quoted by Edward Howe Forbush. Chief Pokaton had been camping in mid-May 1850 on the headwaters of the Manistee River in Michigan when he awoke one morning to hear “a gurgling, rumbling sound, as though an army of horses laden with sleigh bells was advancing through the deep forest toward me.” Soon he concluded that instead it was the “distant thunder” of an approaching storm, “yet the morning was clear, calm, and beautiful. Nearer and nearer came the strange commingling sounds of sleigh bells, mixed with the rumbling of an approaching storm.” Then he beheld moving toward him “in an unbroken front millions of pigeons” that “passed like a cloud through the branches” and also surrounded him and even landed on him. They were mating and preparing to nest. He states that this was an event he had long hoped to witness and so he sat down to watch carefully.

  In the course of the day after a great mass of birds passed by him, the trees were filled with birds sitting in pairs that gently fluttered their half-closed wings and uttered the bell-like wooing notes that he had mistaken for the ringing of bells in the distance. But “on the third day after this, the chattering ceased and all were busy carrying sticks with which they were building nests in the same crotches of the limbs they had occupied as pairs the day before.” Their nests were finished and eggs laid by the morning of the fourth day, and the hens sat on the nest while the males left to feed and to return by 10:00 a.m., when the females left the nest and the males took over the incubation. In midafternoon, the females returned and again resumed their second bout of incubation while the males left once more to return at sundown. (The closely related and very common mourning dove has the same routine.)

  Chief Pokaton found the nesting grounds strewn with eggshells, “convincing [him] that the young had hatched,” on the eleventh day after the eggs were laid. He likely refers to the beginning of hatching of the colony—most pigeons incubate at least two weeks. Nevertheless, to me the report of the apparently instant beginning of nesting after settling and the resulting synchrony in breeding of such a huge number of birds is truly remarkable, and I think it may be noteworthy and relevant because it provides contrasts with the mourning dove. Mourning doves are also gregarious; they roost together in small groups, and I see them also in small groups at my bird feeder in the spring at nesting time. But they draw the line in gregariousness when it comes to nesting; they nest independently of one another. Pokaton noted that the pigeon parents fed their young for thirteen days, which is similar to mourning doves. The pigeons then left their young to re-nest, again like mourning doves. These observations show that the pigeons differed from the still-extant mourning dove mainly in their extreme orientation to each other in their nesting in dense colonies.

  Almost everything in the passenger pigeons’ lives revolved about themselves, and as Chief Pokaton recorded, the vast horde nested synchronously, as one. Could passenger pigeons have nested in small groups, or in isolation? Were the vast numbers as such the critical environmental cue for triggering their reproductive behavior? We do not know. However, as I thought about this I recalled a lecture I heard at UCLA by Daniel S. Lehrman just before he died in 1972.

  Lehrman was a pioneering behavioral physiologist working in the lab (the only place where he could have made the following discoveries) with the ring dove, Streptopelia risoria. What he showed then made a huge impression on me. It was this: adult physiology and behavior in birds, and in this case in the dove, are highly sensitive to sensory stimuli. Lehrman showed that if a caged dove saw a courting male, hormones were released from her brain that started to kick in her reproductive cycle, and in each stage of that cycle (from seeing twigs to build a nest, to feeling eggs on the belly to incubate the eggs, to seeing the young to start producing the crop milk to feed them) it was stimulus perception rather than pure instinct as such that guided development which affected behavior. This is not a novel concept, as it was well known from the pioneering work by Wigglesworth with his bedbugs, and the previously mentioned developmental switch to migratory morphs in locusts is an equally dramatic example. However, its direct experimental proof in a dove underlies the fact that the stimuli as such are arbitrary but are strict with respect to a species and the specifics of its environment. In the passenger pigeons, that critical stimulus was likely, as in the locusts, the crowd.

  Pokaton noted that, again as we know for mourning doves, both sexes secreted crop milk or curd with which they fed their young until they were ready to fly. He reported that they stuffed them with mast “until their crops exceeded their bodies in size,” and within two days after their stuffing they became “a mass of fat” and the parent birds then drove them from the nest. A difference from the mourning dove, though, was that the passenger pigeon usually laid only one egg.

  As in other birds, brood reduction generally means there has been a food reduction. The adult pigeons were reputed never to gather the nuts and acorns in the nest vicinity, leaving that mast for the young to feed on when they came out of the nest. A reasonable scenario therefore is that the fattening up of the squabs just before fledging, until they became “the mass of fat” that Chief Pokaton described, could have been a critical adaptation related to food depletion around the colony; the young birds needed a large buffer of stored energy before they could engage in long-distance travel to feed themselves as a counter-strategy to extreme aggregation. So, if there was a cost to aggregating, the colonies should not have increased to what seemed to be nearly infinite size. Evolution does not generally produce anything “extra” unless it is a byproduct of something else that is useful. Why then did they aggregate so enormously?

  The pigeons could have banded together for a variety of advantages, including the sharing of information about locations of rich but patchy food resources. But once aggregated, the communally breeding flocks would have attracted predators
. Yet, in the vast numbers that the pigeons achieved, they would have swamped the ability of local predators to make a dent in them. Native Americans from kilometers around probably camped and feasted at the colonies, so long as they were not intruding on the territories of neighboring tribes. It was only after the white man came that the birds were wiped out. But why did one dove become extinct while the other very similar one, which is a popular game bird still hunted all over the American continent, remain as common as ever?

  Naturalist John Burroughs’s 1877 book Birds and Poets provides what I think may be clues. He writes:

  Few spectacles please me more than to see these birds sweeping across the sky, and few sounds are more agreeable to my ear than their lively piping and calling in the spring woods. They come in such multitudes, they people the whole air; they cover townships, and make the solitary places gay as with a festival. The naked woods are suddenly blue as with fluttering ribbons and scarfs, and vocal as with the voices of children. Their arrival is always unexpected. We know April will bring the robins and May the bobolinks, but we do not know that either they or any other month will bring the passenger pigeons. Sometimes years elapse and scarcely a flock is seen. Then, of a sudden, some March or April they come pouring over the horizon from the south or southwest, and for a few days the land is alive with them.

  Burroughs’s observations illuminate a critical part of the pigeons’ adaptive syndrome. If the birds nested repeatedly at the same place (as many sea birds do), predators would be able to gather and to multiply, and to make inroads on the flock. By staying on the move, by arriving quickly and then again leaving quickly, even as large flock size attracts predators, the pigeons stayed a step ahead and the predators could not accumulate. But if so, what did them in anyway?

  Burroughs continues: “The whole race seems to be collected in a few swarms or assemblages. Indeed, I have sometimes thought there was only one such in the United States.” He went on to note that

  scouting and foraging squads are not unusual, and every few years we see larger bodies of them, but rarely indeed do we witness the vast spectacle of the whole vast tribe in motion. Sometimes we hear of them in Virginia, or Kentucky and Tennessee; then in Ohio or Pennsylvania; then in New York; then in Canada or Michigan or Missouri. They are followed from point to point, from State to State, by human sharks, who catch them and shoot them for market.

  The nineteenth-century ornithologist E. H. Forbush from Massachusetts wrote:

  Schooners were loaded in bulk with them on the Hudson River for the New York market, and later, as cities grew along the shores of the Great Lakes, vessels were loaded with them there, but all this slaughter had no perceptible effect on the numbers of the Pigeons in the West until railroads were built. . . . Every great market from St. Louis to Boston received hundreds or thousands of barrels of Pigeons practically every season. The New York market at times took a hundred barrels a day. . . . Often a single western town near the nesting-grounds shipped millions of pigeons to the market during the nesting season, as shown by the shipping records.

  The simple truth is that the pigeons’ nesting aggregations were both too large to escape detection and also not temporary enough to evade modern communication and transportation. Forbush writes that every nesting ground became known and was then “besieged by a host of people as soon as it was discovered, many of them professional pigeoners, armed with the most effective engines of slaughter known.”

  Here, in a nutshell, is the explanation of why the pigeons’ superb adaptation hastened their demise. It was not a flaw originally, because for a long time it worked superbly to protect them. But in the end the large scale of their adaptive “perfection” resulted in overwhelming predation. The pigeon had no home boundaries over which to spread itself and continued to orient only to itself, so it could be everywhere, even to the end.

  Their social sense was so strong that it drew the new predator, technologically equipped humans, from afar. It made them not only easy targets, but also easily duped. The commercial pigeon harvesters used huge nets in which they caught thousands at a time. They did this by taking a few flock mates, sewing their eyes shut, and attaching them to a perch. These “stool pigeons” then fluttered in place, and the flock came down to attend to them, and so they were caught and slaughtered by the thousands at once.

  The passenger pigeons’ progenitors, derived from a common ancestor with the mourning dove, had given up their ancestral homes, which the doves still keep. To the pigeons, the only “home” they knew was in the crowd, and now they had become victims of it. Second, and perhaps more important, even with their guns, nets, and pigs, the human predators could have eaten all they wanted individually, with little or no effect on the pigeon population. However, “the market” extended the exploitation beyond any boundaries, practically to “global” proportions limited only by the length of rail lines. And then, when the crowd was no more, the remainder could not recoup, because they needed the crowd as a stimulus in order to nest.

  The passenger pigeon’s adaptation was like the schooling adaptation in herring encountering the bubble-net-enveloping hunting strategy of a baleen whale. The only real difference may have been that in this instance the whole population of “fish” was concentrated into the same school. Human communication, efficient long-range means of travel by roads and railroad, and also the lack of territorial boundaries of human predators had tipped the scales to make their adaptation their doom. Even though the pigeons did periodically shift their roost locations, they could never give up their habit of staying with the flock and trying to keep up with it, which had always been their defense.

  The last passenger pigeon died in the Cincinnati Zoo on September 1, 1914. But today their close cousin (with which they were often confused), the common and very familiar mourning dove, is as common as ever if not more so. It is a favorite of hunters and is one of the most widespread birds of North America. It may be numerically not far from what the passenger pigeon was once, but because it lives dispersed over the entire continent, its numbers don’t seem spectacular.

  Mourning dove (left), rock dove / common pigeon (right), and the passenger pigeon (center)

  But there is another pigeon, the rock dove, Columba livia, that, in contrast to the now-extinct passenger pigeon, thrives specifically because of us and with us, living in almost all cities of the world. There are 290 species of pigeons/doves worldwide, but only this one has taken to living primarily in our cities, the world over. It lives underfoot in busy train stations and city streets and nests on window ledges, bridges, underpasses, and in abandoned buildings. It is sometimes hated because it leaves droppings, and admired and loved by others for its homing ability. Why would one pigeon make a pact with humans for at least already fifteen hundred years, and maybe much longer, and recently even experience a population explosion because of us, and the other on contact with humans, in practically a heartbeat, go extinct?

  One answer—the main one—again concerns home-making. Almost all members of the Columbidae make flimsy nests and have two pure-white eggs per clutch. Nests of most species are on the ground or in trees. But the rock dove, a native of the Mediterranean region and North Africa, traditionally made its home on cliffs. It “sees” most of our human dwellings as almost perfectly designed, safe home sites. Human-grown grain and even food scraps, too, mean that the rock dove never experiences a total break in the gravy train.

  The rock dove was introduced to America in the early 1600s and immediately took to the streets. It now lives over the whole continent. It is a bird at home on almost all city blocks everywhere, on high rises, tenements, and train stations, nesting on pseudo rock ledges such as steel girders, window ledges and cornices, highway underpasses and bridges. Gustav Kramer, one of the premier students of pigeon homing in the 1940s, would have agreed that they nest on predator-safe nest sites—he died on a cliff while trying to reach a pigeon nest. With a stream of food and perfect home sites that are almost predator free, what mor
e could a bird ask for? Not much.

  When I think of the pigeon’s amazing life, the first image that pops up in my mind is my recent visit in a European train station with thousands of people hurrying along the corridors, multiple shops in a huge cavern with the rumbling of trains and loudspeakers blaring, glaring lights, sky blocked from view by a huge steel-reinforced canopy, escalators running up and down, and trains coming and going. I don’t know where it was. I think it was Berlin, but a similar scene of pigeons walking around literally underfoot among the bustling throng of people could have been in Boston, Barcelona, Paris, Moscow, or Rome. These birds are not pets. They are wild birds, as wild as any passenger pigeon ever was. They are social like most pigeons, but they are not dependent on a crowd. They are free. Are we?