A Feathered River Across the Sky

by Joel Greenberg

  Far from water, hardships could also await pigeons. An early April storm hit Sullivan County, Pennsylvania, in 1868 and dumped a foot of snow on the same day that the pigeons arrived. A large tract of beech trees nearby was often used as a nesting ground during springs following a mast year, as this was. Ordinarily, the snow would disappear in a day or two, but when temperatures stayed low for two weeks, pigeon forage remained buried. A few birds sought sustenance in sumac seeds, but most were unable to find any food at all. Birds too weak to fly and then corpses by the many thousands soon covered the entire area.56

  Another cause of mortality was brought about by the pigeons themselves. The huge masses of pigeons provided protection from predators and helped locate food sources. However, the sheer volume of their neighbors resulted in the death of large numbers of birds. Just think of the roosting and nesting colonies brimming with pigeons. Then imagine the destruction that would ensue when tree limbs, or at times entire trees, snapped and plummeted to the ground, crushing hundreds if not thousands of birds. When flocks descended to drink, at times the birds that landed first would drown under the weight of newcomers.

  Predators took their share as well. Animals devoid of wings or the social structure and technology of Homo sapiens could not follow the big flocks, but other predatory mammals in the vicinity of a gathering helped themselves to the temporary buffet. Martins, mink, weasels, and raccoons would climb trees to procure eggs, squabs, and adults. Although I have not seen gray foxes specifically mentioned, they routinely scamper up trees and might well have preyed on pigeons. Black bears, bobcats, and mountain lions, all proficient climbers, joined the landlubbers, such as wolves and skunks, in seeking the dead or flightless birds that accumulated on the ground.57

  Barred and great horned owls (and perhaps other species of owls as well) took pigeons at night. During the day, vultures would eat dead and wounded adults, as well as the vulnerable squabs displaced from their nests. The principal diurnal predators, however, were the two groups of bird-eating hawks. Five species of hawks native to eastern North America rely almost exclusively on birds, and each of them would go after pigeons if given the chance. But the fortunes of the three largest, the northern goshawk, Cooper’s hawk, and peregrine falcon, were likely tied to that of the pigeons. The northern goshawk and Cooper’s hawk (as well as the smaller sharp-shinned hawk) are accipiters, with short, broad wings and long tails that endow them with superior maneuverability. They relentlessly pursue their targets, following every dodge and cut as their prey desperately try to escape. Should the victim seek the shelter of a tangled brush pile, the hawk is apt to follow. Goshawks are the most brazen and unrelenting of the continent’s avian predators. One story has it that as a chicken frantically tried to elude a pursuing goshawk, it sought security under the skirts of a startled woman, whose agitation no doubt soared when the goshawk followed close behind. On another occasion, a goshawk pursued a hen through an open doorway into a kitchen and caught the fowl even as the tenant and his daughter stared in amazement a few feet away. Audubon wrote of a goshawk as it made predatory forays above the trees and encountered a flock of pigeons: “He immediately gives chase, soon overtakes them, and forcing his way into the very center of the flock … then you see him emerging with a bird in his talons, and diving towards the depth of the forest to feed.”58

  Goshawks used to summer or breed as far south as northern Pennsylvania and southern Wisconsin. No data exist directly linking the events, but the pigeons disappeared and goshawks as nesting birds retreated much farther north. Today, they are but uncommon-to-rare fall or winter visitors to these former haunts. As for Cooper’s hawks, there seems to be less specific information. In recognition of its predilection for the pigeons, southerners called the Cooper’s “the great pigeon hawk.” Ohio ornithologist Milton Trautman connected the concentration of pigeons with the former abundance of the hawk. Old market hunters told him that Cooper’s hawks had declined in numbers between 1860 and 1921. The first thirty or forty years of that period encompass the decades that saw the great collapse in pigeon numbers.59

  Should a group of passenger pigeons seek to avoid the skulking accipiters and take to the open sky, they risked contending with the equally formidable peregrine falcon. With long, pointed wings and a thin tail, these raptors coursed through the air like bullets: a peregrine can attain 180 miles per hour in a dive. Wherever this cosmopolitan species occurs, it preys extensively on some species of pigeon. Avian ecologist Stanley Temple has worked with peregrine falcons throughout his career and speculates that passenger pigeons figured prominently in their diet, particularly when the eastern forests were still intact. “Falcons would have found pigeons flying above the canopy to be among the easiest of prey,” he told me. “They were the perfect size and of the perfect behavior.” Even a peregrine with closed eyes dashing through a compact mass of pigeons at breakneck speed could hardly miss. (Of course the wrong kind of hit could result in grave harm to the hawk.) Once the forests were opened up, however, peregrines enjoyed easier access to a much greater variety of prey, and Dr. Temple thinks this prevented a steep decline in peregrines following the decimation of the pigeons. But he has no doubt that there was a strong connection between the species.60

  The nineteenth-century ornithologist George Bird Grinnell tells of a peregrine falcon in hot pursuit of a passenger pigeon across the South Dakota plains. To elude its tormentor, the pigeon used Grinnell and his party as cover, even landing on the back of a horse. These maneuvers succeeded in confusing the falcon, for it lost sight of its quarry and had to ascend to re-evaluate the scene. The pigeon, perched on the horse, could not see the falcon for a few moments. It then looked up, saw the raptor hovering, and made the mistake of launching forth in what proved a fruitless race against its faster adversary.61

  Mast anchored the forested realm of the passenger pigeon, ecosystems providing sustenance for a range of creatures including filbert weevils (Curculio), blue jays, wild turkeys, and ninety species of mammals. Of the mammals, white-tailed deer are preeminent in how they impact the woods, and ecologists consider them to be a keystone species. So would, arguably, have been passenger pigeons: the two species would surely have affected each other, and the extinction of the bird no doubt altered the life of the deer. Other important mammals include mice, squirrels, and black bears. In poor mast years these species would decline or be forced to move on to other areas. Eastern gray squirrels in the 1800s were known to embark on great movements by the thousands. Black bears would also emigrate on a lesser scale. But when the nuts were thick, the local animals would prosper, although they had to compete with those vast flocks of pesky interlopers: passenger pigeons. This would later be the case with domestic swine, which in many parts of the country were allowed to forage in the forests, saving the expense of having to fatten them up on grain.62

  It is impossible that billions of passenger pigeons did not affect the ecosystems that provided them with sustenance and quarters. Nor is it possible that their elimination in such a short time did not also impact those forests. Unfortunately, no one took much notice when the study of living passenger pigeons might have shed light on the many connections that had evolved over the history of the species. Only in hindsight have scientists attempted to explore the subject.

  Imagine a show with two or three major characters and numerous ancillary roles. The principal actors would play off each other, as well as the lesser characters, who would in turn form relationships among themselves. Events trigger consequences that rattle around through the story, to be revealed at unexpected times or places. Now remove one of the stars and watch how events unfold. If you had never seen the original but only experienced the second version, you could only guess how that missing character would have affected the remaining players. In the case of the passenger pigeon, it is probably the best you can do. And if you subtract yet another member of the original cast (mast), the American chestnut, and add a major villain, the nonnative and highly destructive gyps
y moth, the effort of reconstruction becomes even more difficult and speculative.63

  The eastern forests started taking form when the icy cloak of the last glacial incursion melted about ten to fifteen thousand years ago. Vegetation began to reclaim the now bare ground. Trees pushed northward at rates and in patterns preserved by the records written in fossilized pollen. A seeming anomaly of that record is that the nut-bearing trees, with heavy, rounded fruit, expanded at the same pace as maples, elms, and pines, endowed with lighter fruit, some of which had wings that could ride the wind. (Another riddle involved the patchy distribution of beech, a species absent from large areas in between populations.) Two possible explanations were offered: either the hard-mast trees enjoyed earlier seed production and thus had more generations to compensate for shorter increments of expansion, or the seeds were moved by animals. Of course, these are not mutually exclusive.

  Various calculations revealed, however, that the large seeds somehow moved a greater distance per generation than did the light seeds. This left animals as the likely source of the power that enabled these fruits to expand their range. The flaw here, though, is that unlike humans with their Johnny Appleseeds who deliberately relocate living plants, the continent’s indigenous wildlife tended to collect seeds for food, thus exposing the material to the destructive forces of the gastrointestinal tract. It is true that species such as eastern gray squirrels and fox squirrels are known to cache nuts underground, and although most are consumed, anywhere from 25 to 40 percent escape later detection and often germinate. Ecologist Sara Webb concluded, however, that passenger pigeons provided a much more likely form of transport allowing the spread of mast trees. While the vast majority of the nuts consumed by pigeons would be digested, it might take up to twelve hours for such armored fruit to be broken down. During that time, some number of mobile pigeons would die. Given the billions of birds, and the millennia that transpired, enough seeds could germinate to create forests.64

  Once these forests were established, passenger pigeons continued exerting an influence on their components and form, particularly as they promoted white oaks over red oaks. As stated earlier, the birds preferred the acorns of the red oak group to those of the white oaks. Heavy predation in this way favored the white oaks and probably contributed, along with other factors, to their becoming the dominant oaks in many parts of the eastern United States. Indeed, many forests of Ohio, Pennsylvania, New England, and New York consisted almost exclusively of white oaks. Conversely, the elimination of the pigeons may well have enabled red oaks to spread at the expense of the whites.65

  The pigeons impacted the landscape in other ways, too, by imposing a regimen of disturbances through their nesting and roosting activities. We have already seen how the masses of birds would break branches and even topple trees, thereby opening up the canopy and creating tinder. Further, in areas used by the birds over time, excrement would blanket the ground like snow, chemically changing the soil through the deposition of nitrates and salt. Little, if any, understory survived, and the trees themselves became stressed. In another scenario, the canopy gaps would have aided the establishment of trees such as white oaks or red oaks that are inhibited by too much shade.66

  These events would often act together in magnifying their total effects. If a tornado had recently visited a forest before its occupation by pigeons, damage by the birds might have been less than if the timing of the disturbances had been reversed, but still greater than had only one disaster arrived. But likely the most profound alterations brought about through pigeon-generated damage were an increase in the frequency and intensity of fires. Exposing the forest floor to light helped dry it out, and breaking branches created fuel to feed the flames. Fire plays a major role in what kind of vegetation thrives in an area. At the heavy end of the burning continuum, grasslands prevail, as grasses and many flowering plants have extensive root systems that can survive the immolation that is fatal to trees. On land that is less often burned, the thicker-barked oaks such as white and burr do well, although black oaks also prosper under such fire regimens. Where fires rarely reach, the red oaks, beech, and maples might predominate.67

  Ecologist Reed Noss has suggested that passenger pigeon activity might have been important in the formation and maintenance of yet another ecosystem, the canebrakes, “one of the most enigmatic grasslands of the south.” Canebrakes is the term used to describe the dense tracts of American bamboo or giant cane (Arundinaria gigantea), which can reach forty feet in height. They have decreased markedly to the point where they now rarely exist in large enough expanses to constitute a significant ecosystem. Their decline may well have led to the likely extinction of the Bachman’s warbler and severe reductions in populations of Swainson’s warblers.68

  Canebrakes concentrated on rich alluvial soils and required a disturbance regime that kept them from being overwhelmed by trees. The soil quality proved to be excellent for agriculture, and Indians cut the cane to make room for their crops. But when the sites were abandoned, they would often be recolonized by the cane. This pattern of human disturbance, along with frequent flooding and anthropogenic fires, helped maintain the cane. But Noss points out that even before humans arrived in the American south there was enough cane to support red pandas, whose occurrence is documented in the fossil record. Therefore, other factors must have been present to promote the growth of the cane. Passenger pigeons, by opening the canopy, enriching the soil with their droppings, and increasing the frequency of fires, might well have been one such factor. In support of this idea, when white immigration ended native agriculture, reduced fires, lessened flooding, and destroyed passenger pigeons, canebrakes retreated as well.69

  Beyond their impact on the forests and their relationships with certain hawks, the pigeons have been linked with several other species. Although intriguing and well conceived, these connections are based mostly on conjecture. One such connection involves the American burying beetle (Nicrophorus americanus). This once-widespread scavenger was reduced to a range consisting of two islands off Rhode Island (one a colony deliberately introduced for conservation purposes) and a few counties in eastern Oklahoma. The populations in between disappeared over fifty years beginning in the 1930s. This collapse has been called “one of the most disastrous declines of an insect’s range ever to be recorded.” In searching for explanations behind this, some scientists have looked at the ideal-size food for this one-inch beetle and wondered which among them have also suffered precipitous decreases. Two candidates have emerged: greater prairie chickens and passenger pigeons. While the beetle maintained its range long after the pigeon was gone, the loss of the two birds might have forced the beetle to rely on other carrion sources that over time also declined for varying reasons.70

  A special kind of relationship that impacts people is that between mast, mice, deer, black-legged ticks (Ixodes scapularis), and the bacterium Borrelia burgdorferi, which causes Lyme disease. A bane to those who spend time out of doors, Lyme disease first manifests itself through fever, headache, fatigue, and a rash, but in advanced stages can affect the heart, joints, and central nervous system. I used to feel smug that being a nature nerd I was not likely to contract the virulent new diseases that began emerging in the 1970s. But then came Lyme and West Nile (carried by birds and spread to people primarily by Culex mosquitoes), and it is no longer even safe hanging out at tamarack bogs at dawn.

  When mast is heavy during the autumn, white-footed mouse (Peromyscus leucopus) populations soar the following summer. Mice are the principal carriers of the organism that causes Lyme disease, so when mice are abundant, the ticks have increased opportunities to take a slug of mouse blood loaded with the bacterium. Understanding this fully requires looking at the four-stage life cycle of the black-legged tick. First, there is the egg, which just lies there in the leaf litter after being laid in spring or early summer. The larvae emerge in summer, usually free of the disease. After one blood meal from a host they molt into the nymph, which is when the tick is mo
st likely to become infected. The odds that will happen increase if the host is a white-footed mouse, the premier reservoir of the disease. Ticks at the adult stage are the most finicky, strongly preferring white-tailed deer. Sometime in the fall, the adult will take one last sip from the deer it is riding, then drop off onto the ground to overwinter. The absence of mast prompts the deer to move around, thus spreading ticks to wherever it happens to be when the hangers-on decide to let go. Ornithologist David Blockstein has suggested that had passenger pigeons remained in large numbers, they would likely have reduced the forage available to mice, thereby keeping in check their populations and limiting the spread of Lyme disease. But nothing in nature is simple, and this idea has a flip side. Mice are also major predators of gypsy moths, so in outcompeting the mice for food, the pigeons would also have aided the spread of the moths. This would in turn have reduced oaks and other trees, thereby making fewer forests suitable to the pigeons.71