On Looking: Eleven Walks With Expert Eyes (17 page)

And then we saw one. A Norwegian rat,
Rattus norvegicus,
peeking over the curb edge. Surprisingly large, balanced on his hind legs, his front raised thoughtfully, he had spotted us first. His nose was in the air—
investigating
us—and his tail was down, a long lean curl of stability.

I stood still. I neither wanted him to come closer nor to run away. Though they live near us, rats are wary of our aversion to them, and are quite happy to avoid actual contact with people. The rat bobbed his head, looking a bit maniacally twitchy. Pigeons head-bob, too, though with more soldierly rigor, in rhythm with their footsteps. Both animals do it to see better: with each bob, they get different vantages on what is in front of them, and they can use that information to estimate depth. Standing still, the rat’s vision is very blurred, compared to ours, although as befits their habitat they can see better at night than we can. By bobbing, he was getting a gauge on how far away I was in order to estimate when he needed to begin to flee.

We were both silent, though I wondered if he was squeaking over my head, as it were. Rats’ primary communication with other rats is along ultrasonic, high-frequency wavelengths above our hearing range: there is a mother-retrieval call by young pups; a “long call,” which accompanies pain or social defeat, and, oddly, ejaculation; and a high chirp that has been interpreted as a laugh, emitted during tickling by humans (in the lab) and during play. They also chatter their teeth, an inadvertent communication of stress. They can certainly hear us. Some rats can distinguish different languages. Resident rats in the basement of an apartment building with a building staff speaking a different language than its residents can tell from their speech which group is possibly threatening to their den, and which will ignore them.

This rat must have decided I was unworrisome, for he soon relaxed into a sit and began wiping his face and ears with his fore-paws. Rats spend up to half their waking hours preening. The ritual keeps their coats covered with their musk, and keeps the parasites at bay. They begin grooming at their heads and proceed downward, so you can get a sense how far along the rat is in his self-shower by seeing what part of his body he has gotten to cleaning.
My rat got no farther: a bicycle rattled by on the street and he dove under a parked car. He was gone—back into the spaces rats inhabit where they can be invisible to us.

An animal behaviorist to the core, Hadidian took an even-handed approach to the animal.

“From a strictly natural history perspective, they’re one of the most poorly understood animals out there.”

“What is there to understand about them?”

“How big are their home ranges? How much socializing do they do? What’s their group structure? . . . We really don’t know what the behavioral repertoire of the rat is.”

Partly because of this, and partly because humans have failed to think one step ahead of the rats, “contemporary rat control seems to vary little from what was practiced in the Middle Ages,” said Hadidian. “The usual consequence of killing rodents . . . is the return, shortly, to the population level that prevailed before, or one slightly higher.”

What we do know is that these rats are only in the city because humans are. Their omnipresence tells us about ourselves. Were we to be tidier, neither rats nor raccoons would keep our company. Both are “opportunistic omnivores”: they eat anything that is available. What good fortune for such an animal to find us humans, who provide in our trash and in our homes an omnivore’s delight. Rats eat edibles—and they eat through lead pipes. They are happy with a small vertebrate meal or an afternoon’s snack of nuts and fruits. Indeed, they are particularly interested in sweet, high-protein, and calorie-rich foods. Like us, they will even eat spicy food, after an initial, wise aversion to the stuff.

Conveniently, we even feed the rats outright. It is the city squirrels who are the intended recipients of the bounty handed out by urban animal-feeders. Every neighborhood has one or many individuals who take it upon themselves to regularly provide
animals—especially squirrels and birds—with bits of bread, nuts, or seeds, tossing the bounty on the ground as they sit nearby, or spreading it Hansel-and-Gretel-like as they walk. But it is the rats, emerging after the squirrels and humans have denned up for the night, who ultimately reap the benefits of this human behavior.

As a result of their diet and our feeding, they live wherever we live. If you are reading this in a city, it is a good bet there is a rat living less than a quarter-mile from where you are sitting—not passing through, not visiting: setting up a den and playing house. Studies of rats in Baltimore found that most rat activity is limited to a single city block or alleyway. As generations are born and move out of the family nest, rat “neighborhoods” are formed, with an area the size of eleven city blocks containing many related rats. We have created the infrastructure that supports them beautifully. Grid layouts are particularly amenable to rat populations, as the rats use the grids to orient themselves. They can map their entire home range through its different smells: the area scrubbed with detergent, the trail left by people passing with dogs, the area where the smokers stand by a building’s side.

For the remainder of our walk, rodent boxes appeared repeatedly in my peripheral vision. I had never seen a rat in one, and I did not then. Rats are wary of new things—
neophobic
—which is at least partly responsible for their ability to elude the many and various attempts to bait and kill them: rats smell a rat in that big, black box. They will sample a new food first, before gulping enough to find it toxic. Later that very night, I spied a rat running to, sniffing, and then veering exactly
around
one. Given the rat’s ability to learn to avoid foods from others’ breath, this rat may have had an encounter with a less savvy rat earlier in the day.

 • • • 

We were again alone on the street, all the animals tucked away. While the subterranean landscape is a popular choice for the urban animal, the city also provides a commodious terrain above-ground. I asked Hadidian how the city we were walking through, New York, looked different to him than D.C. or Baltimore, where he monitors urban wildlife. He did not hesitate.

“Well, it’s much higher. Everything in Washington is twelve stories or less because they don’t want to overwhelm the monumental buildings. What you have here are functional cliffs.”

We both looked up.

As if demonstrating his point, a group of pigeons swooped down from its redbrick clifflike perch over Amsterdam Avenue. Just as it seemed they would nearly land on the cab of an eighteen-wheeler going uptown, they curled upward, then settled down and rounded the corner, out of view.

Biologists do not know exactly what the pigeons—or any birds—are doing on these great swooping flock dives. The birds may be in search of food, avoiding a real or imagined predator, or just stretching their wings. In any event, the “flock-swoop,” as I think of it, is one of the magnificent natural sights of the city. And it is a sight that repeats itself daily, even hourly, in every sector of every city—in high-rent and low-rent districts, over empty lots and between skyscrapers. Even as I write this, my peripheral vision notices motion: out the great long windows of Columbia University’s library the white of the sky highlights a flock of pigeons arcing gracefully south to land on the ledges above the windows.

This kind of bird flock behavior is commonly described in the academic literature as “wheeling and turning,” though even to an amateur eye this hardly captures the dynamics of it. A group
of thousands of European starlings is an ever-changing, amorphous splotch that pulses and throbs, ceaselessly erasing one shape and proposing another over it. Flocks of dozens of pigeons roller-coaster along invisible corridors ten to thirty feet above the street, and wend along and around a curvy, hilly highway we cannot see.

Biologists and others interested in emergent behavior—behavior of a group that is not under the control of any of its members—have identified a number of features common to all these flock-swoops. They are highly synchronized flight patterns that follow certain reliable rules. Individual birds often prompt the flock to take wing, but there is no leader once the flock is in flight. The flock-swooping can happen at any time, but it is more common at dusk or just before sunset, before the birds roost for the evening. Within the flock, the birds stay at least a wingspan apart from each other, though they pack more densely around the periphery than in the center of the group. The flocks themselves are much longer and wider than they are deep: the birds are spread, not layered. When the group maneuvers, it turns in what are called equal-radius paths. That is, if the flock turns left, it is not the result of each individual bird suddenly turning left. Instead, the birds at the front arc only slightly, and wind up being on the right side of the flock. Those birds at the left side wind up at the leading edge. When it is pigeons gliding, they keep a steep angle to the horizontal, which makes them more stable, especially in wind. No wonder they do so well along the breezy valleys of a skyscrapered city.

Watching the birds soar, pitch, and roll, and feeling happier just observing them, it occurred to me that one of the reasons that it is hard to pinpoint the function of this behavior may be that it is functionless. And the most classic functionless behavior, seen in all mammals and most vertebrates, is play. Might these birds be soaring for the mere pleasure of it, a communal recess run to nowhere in particular?

Continue watching this bird play, and the paths that they travel almost start to become visible. When you imagine the city from the bird’s vantage, it really does look like a series of canyons and cliffs. That notion of “functional cliffs” intrigued me, and I pressed Hadidian on it.

“The whole business of cliff ecology is something you can start talking about when you get these structures,” he replied, motioning to a few buildings in the vicinity. “Any building will have what we can call ‘wind shadows’: little places where the wind doesn’t hit. It certainly doesn’t
scour,
the way it does on most structures or surfaces.” The result is that although buildings look like hostile, lifeless zones, the face of a wall can support a whole ecosystem. Natural cliffs, too, have their own microhabitats and microclimates, and they support a huge amount of specialized flora and fauna.

The textbook
cliff
is a tall, steep rock face, with a flat top and maybe a bit of overhanging rock on the top edge. In other words, almost precisely the shape of the classic apartment building, with a vertical face and an eyebrow of cornice at the top. On a natural cliff, there are ten million places for life to bloom. Algae lives on the surface, small plants root in crevices between stones, multiple horizontal ledges collect debris and things that grow in debris—and this then attracts all the animals, invertebrate to vertebrate, that feed on these plants. Similarly, on a building, plenty of opportunistic, specialized plants live in porous stone, in cavities between stones or in broken stone, and at the intersection of brick and marble, or stone and steel. And on the top of the cliff may well be a raptor, perched on a ledge or nesting in a nook.

. . . Or under an air-conditioning unit. Falcons, hawks, and even eagles are again a common sight in the urban sky. They build their aeries in cathedral bell towers; on bridge towers; on, famously, an ornamental stonework ledge on a Fifth Avenue apartment building. Indeed, the animals, plants, and ecology of
buildings—man-made cliffs—show “striking similarities” with those of natural cliffs. Cliff animals include mice, various squirrels, the aforementioned raptors, raccoons, porcupines . . . stop me if these sound familiar. Coyotes have been seen to feed and reproduce in cliff sites—and coyotes are among the most recent urban settler. We do not see many cliff-using black bears, lynx, or mountain goats in the city. Well, not yet.

Indeed, some ecologists have even proposed an “urban cliff hypothesis”: that the urban rats, mice, bats, pigeons, and plants that are so familiar to us evolved from ancient cliff-dwelling rats, mice, bats, pigeons, and plants. Hominid fossils from
Homo erectus
onward have been found in caves at the base of cliffs. As our ancestors moved from caves and rock shelters to shelters of wood, steel, and stone, the animals may have simply moved with us. Humans still live in “concrete and glass versions of their ancestral cliffs, caves, and talus slopes,” Doug Larson, a promoter of this theory, writes. The unlikely habitat of the cliff supported species that had to adapt to their unusual conditions. This adaptiveness turned profitable as the species followed humans, and were flexible enough to dine on our food and live in and on our structures.

The evidence for this hypothesis is multifarious. Pigeons, for example, are well adapted to perch on cliffs or building ledges, as their takeoffs are terrific, explosive affairs, as anyone whose step disturbs a flock of grazing pigeons knows. Their wing muscles are very strong, allowing them to hover and take off nearly vertically, helicopter-like. Rodent remains have been found in cave dwellings dating back tens to hundreds of thousands of years ago. Even the bedbug probably evolved from caves and rocky outcroppings in Africa and Asia, where the nasty things fed on pigeons and bats who lived there; the German cockroach, that ubiquitous city bug, can still be found on the toe of rocky slopes in Africa, where it feeds on fallen cliff detritus. One could even make the case that
we build the structures we do
in order to
reproduce clifflike dwellings. Often our buildings have lots of “subspaces,” which replicate ledges and crevices. We grow plants and trees on terraces—since plant species may thrive in small cliff juttings. We prefer to have our front doors set back and slightly elevated from ground level—just as an entrance to a cliff cave would be. And around us in these urban cliffs is exactly the biodiversity that we look for in nature.