On Looking: Eleven Walks With Eyes Part 5

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 racc.o.o.ns 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 pa.s.sing 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 pa.s.sing 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 Was.h.i.+ngton 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 skysc.r.a.pers. 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 skysc.r.a.pered 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 cla.s.sic 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 cla.s.sic 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, racc.o.o.ns, 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 h.o.m.o 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 gla.s.s 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 c.o.c.kroach, 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 "subs.p.a.ces," 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.

As we rounded the final corner on our walk, a group of loafing pigeons, startled by a pa.s.sing bike, lofted upward. Until that moment I had not realized what was so odd about our urban-wildlife tour. In part, it had held more traces of wildlife than actual wildlife. But more than that, even those animals we saw were remarkably quiet. Mute, even: any sounds made by birds in flight were largely lost among the sounds of the city. When these pigeons took off, though, we heard their wings slapping the air and their bodies. Until then, they were silent-movie stars, padding along not twenty feet from us in complete silence. Pigeons are typically far from silent. Males coo as they woo, generating a large round warm noise while they puff their chests, spread their tail feathers, and try to look mate-worthy. When eating, pigeons hammer their beaks against the ground, messily spraying food around them. Their long nails sc.r.a.pe the ground audibly as they walk. But our pigeons stepped lightly, cooed psychically, and m.u.f.fled their pecking. And we had only to look around us to see other stars of this spontaneous silent film: above our heads, dried and curled leaves noiselessly rustled on a towering oak; to our right, apparently weightless squirrels leapt from a stone wall to a tree trunk. All make sound, and all were close enough to be heard, but we were not bothered by not hearing them.

And just as in silent films, without sound the scene we saw was suspended in time, the action having no clear beginning or clear end. I watched a dog across the street venture forward, unhurried, noiseless. It felt like a little peek at infinity.

I asked Hadidian if he had any predictions about what the next animal to move into the big cities might be.

He smiled and was silent for a minute. When he spoke, he began slowly, almost cautiously, then quickly built into an outpouring of tumbling sentences.

"The real question is what animals will truly come to adapt to cities and accept the urban environment for the opportunities it presents, and it certainly does present a lot of opportunities-food, shelter . . . I'm sure you know about the phenomenon called the Heat Island effect?" (I did not.)5 Hadidian continued all the same. "So an animal could subsist in a more northerly lat.i.tude than the species might usually be found." Thus we see the mockingbird, a warmth-loving bird, in the Northeast; the beaver and Canada geese are, as we all know, so well adapted to human presence that they are considered "problems."

"I guess it depends on the city. I mean, twenty years ago people didn't think that javelina would be colonizing Tucson, Arizona."

"What is that?"

"A peccary? The wild pig."

"They are in Tucson?"

"Yeah."

"Seriously?"

"Yeah."

"On the street?"

"Well not by day, but yes. They're in backyards. They come to water." Wild pigs, in search of a good drink in the desert, have lived in Tucson, a city of half a million people, for twenty years. In some cities of Germany, wild boar-feral hogs-are common sights on the streets. A wild boar in New York City would surprise me, I have got to admit.

"Wouldn't coyotes have surprised you?" Hadidian reminded me of New York's alarmed, overblown reaction to the arrival of a handful of coyotes, Canis latrans, in the city parks over the last few years. Some Chicagoans are surprised to learn that there is a well-established group of coyotes living in the city proper. Night dwellers, the animals may grow up, mate, reproduce, and die unseen by the human nine-to-fivers. "It's eye-opening to realize. They shelter in, like, shrubs by the post office." Hadidian pointed to a few bushes packed into a small s.p.a.ce beside the sidewalk. "You could have a coyote (nesting) in a place like this. People would walk by all day long, never look, never see it."

I hung back behind Hadidian to take a closer look. No coyote. As far as I could tell.

When I grew up in the foothills of Colorado, canids around our house were not so surprising. But then again, in my early childhood we never saw elk, a five-hundred-pound animal which is now common enough in Boulder that the animal has its own street signs. The city of Bristol, England, has foxes like we have stray cats. When Hadidian began studying urban wildlife twenty-five years ago, even deer were not around.

My question was unanswered. Maybe there is no profit in predicting the next urban animal. Maybe we just have to wait for it, and keep an eye out. But if you are interested in hurrying up the process, take a cue from Hadidian's racc.o.o.n tale. Plant a persimmon tree in your city and see what shows up.

1 Lotor is Latin for "washer," alluding to their habit of dipping food in water before eating.

2 It is not a crazy crash of screeches, flas.h.i.+ng lights, and possibly predatory or confrontational creatures approaching me: it is a subway car approaching the station.

3 When I have played this video in my psychology cla.s.ses, students feel confident of their final numbers, but most of them count incorrectly, a phenomenon I cannot explain as expectation's responsibility.

4 This is true for monkeys who have been tested in this game, too. And they respond faster than humans in every trial.

5 I have since learned: the mean temperature of a city with a million residents can be up to 5.4 degrees warmer than the suburb outside the city-up to 22 degrees warmer on some evenings.

"We must always say what we see,

but above all and more difficult, we must always see what we see."

(Le Corbusier).

A Nice Place (to Walk).

"Moving aside to let someone pa.s.s, I was nearly seated in a small alcove along a building-perhaps a place to sit, but it was lined with spikes. I did not sit there."

I was late to meet Fred Kent. Google Maps pinpointed his office as being at Twenty-fourth and Broadway but I arrived there and it was not. I chastised myself, remembering the convoluted mathematics that figuring out the cross street for a Broadway address in New York City takes. Surely I should not have expected the ever-seeing but uncaring Google Maps to be up to it.

Until recently, the NYC street address algorithm was printed in the front of those print-age relics, phone books, alongside numbers for the local emergency room, the FBI office, and instructions on how to perform the Heimlich maneuver. This seemed apt, for the algorithm answered an urgent need: the translation of an arbitrary number to its location in s.p.a.ce. For Broadway, one needed to take the building number, drop the final digit, divide by two, and then subtract either 29, 25, or 31 from that figure, depending on the initial address. The pleasure of completing this calculation was reliable, and each figuring brought forth in my head an image of the intersection that was my now-known destination. Knowing the calculus for any given street was a marker that one was a true Manhattanite, just as the realization that on side streets the odd-numbered addresses were on the north marked one as a sufficiently long-term resident to have had a couple of odds and evens among one's past apartments.

Today, phone books appear more often in buildings' recycling bins-still encased in plastic wrap-than by a telephone, and one emotionlessly asks Google Maps to bring up a bird's-eye view of a building's location. Well, on this day it failed me, and I found myself a mile uptown of the Project for Public s.p.a.ces, where I was to meet its president, Kent.

I emailed an apology, fruitlessly phoned, then briefly jogged, and finally taxied to the correct address downtown. When I burst in, the office was still but for the sound of computer keypads being tickled in the distance. I spotted Fred Kent chatting amiably with someone across the room. He waved off my apology. "Time doesn't matter," he calmly welcomed me.

What does matter to Fred Kent is s.p.a.ce: how urban s.p.a.ce is used or not used; usable or inhospitable. Kent founded PPS thirty-five years ago after working with the urban sociologist William "Holly" Whyte, a masterful observer of the behavior of people in cities. In the 1970s, Whyte and a posse of young volunteers set out to determine how the design of the city-in particular, New York City-worked or failed to work for urban dwellers. His group placed cameras atop buildings and light posts (quite unusual for the time), set out with clipboards and observation sheets, and watched.1 They watched where people sat and how they negotiated walking by one another. They noted who loitered, who flirted. They captured the dynamics of bus-stop queuing; they even recorded a day in the life of a trash can on Lexington Avenue. Though I admit to being curious about that trash can's day, I was walking with Kent to try to see, through his eyes, the theater of the sidewalk, played out by the people who find themselves on it.

Despite leading PPS for four decades, Kent is more public than presidential in his bearing. His height forces the individual of average alt.i.tude to strain to look him in the eyes, but he wears a perpetual almost-grin that puts one at ease. On the day we met, he was pleasantly rumpled, in a way that bespoke attention to things more important than whether one's s.h.i.+rt is properly creased. Unlike the majority of New Yorkers who do their best to avoid looking like tourists, Kent carried a camera with him and began searching for places to point it as soon as we stepped out the door.

He immediately found something. We were barely a half block into our walk before I had to maneuver around a vendor's huge food cart. Kent stopped outright-not to begrudge it, but to admire it. Just as the food cart is an adjunct to the city sidewalk, this cart seemed to have its own adjuncts: protrusions, displays, and containers that widened its girth. Kent snapped a picture.

"This is not just a little cart . . . that's the Cadillac of carts," he said admiringly, later adding, "The vendors add a lot [to the city], because they tend to slow you down."

Before I could protest-after all, weren't vendors a nuisance at every moment except when you wanted a salty pretzel?-Kent segued to his next a.s.sessment: "Oh, that's a bad window." I followed his gaze to a modest shop window featuring clothing designs set back from the street. "It's too recessed and it should really be farther forward-so you spend a little more time there."

We were on one of my least-favorite blocks: Broadway in the Village. To someone who enjoys walking in the city, this street made me second-guess my hobby. Its sidewalks are constantly busy with slow-moving pedestrians clutching recent purchases and looking at the storefronts, up in the air, and anywhere but where they are going. The storefronts that attract their attention are ubiquitous and cluttered-to my eye, visually messy. I must have furrowed my brow at Kent, because he smiled and set to explaining himself by gesturing to a crowd of loiterers outside the clothes shop's entrance.

"See this? People stand right at the entranceway, right at the traffic flow, so then you slow down even more."

I looked at the folks checking their phones and leaning against the building's wall, adopting the poses of the unhurried and idle. There were enough of them that they had begun to crowd the sidewalk, and both pedestrians and people entering the store were obliged to slalom around them. And then I realized what Kent saw.

"You view slowing down as positive," I offered.

Kent answered without hesitation: "Sure, yup. Yes! It's social; it's kind of getting a sense of something. That's what a city is."

To Kent, the density of shops on Broadway was ideal. A good urban experience, in the Whyte spirit, was one that encourages us to slow down and loiter. I tend to see a surfeit of slow walkers and loiterers as hindering my progress on a rushed morning. These same people were viewed by Kent as essential const.i.tuents of the urban landscape.

Already, on that block of Broadway with Fred Kent, I was starkly reminded of the very simple truth that there are many ways to look at the same event. So, too, might we look at pedestrians not just with tolerance, or with acknowledgment of their role in making the city rich, but also as impressive collaborators in an unlikely sidewalk dance. As Kent and I stopped at a corner, a large group of people cl.u.s.tered around us, all of us waiting to cross the street. Car traffic steadily puttered past. The light turned and both our side and the opposite sidewalk exhaled large groups of people onto the street. Kent lit up: "It's a platoon!"

A platoon, to researchers in the field of pedestrian movement a.n.a.lysis, is a large collection of bunched-up, unaffiliated walkers. Fifth Avenue in Midtown serves as the prototypic platoon generator: the walking signals running up and down the avenue are not synchronized to allow the normal walker to make the light for block after block; they are synced for car traffic. Given the pace the average person walks in the city-well south of two meters a second-pedestrians get stopped at nearly every light.2 Once the light turns, two large platoons set forth toward each other-and toward what is called a "likely conflict."

It is not that cities have not studied or planned for pedestrian movement. Indeed, I spent a happy afternoon with a book that details just what we can expect to find on our sidewalks. The charming Highway Capacity Manual (HCM) is a product of the Transportation Research Board (TRB), one of the private, acronymed national advisory committees trying to a.n.a.lyze, a.s.sess, and model all elements of contemporary civilized life. Charged with describing not just highway activity, but that of the pedestrians and others who might affect how those thoroughfares run, the HCM characterizes six stages of sidewalk traffic. Their description essentially ranges from "fancy-free walking" to "feeling oppressed in a crowd." At one end, "level A," movement is open and unimpeded. You have whole blocks of sidewalk to yourself-at least 130 square feet. You can, the manual cheers, "basically move in (your) desired path" without having to alter your course for anyone. At level B we take the leap into "impeded": there are others in sight, and you may need to choose a side of the sidewalk, but you can still walk at whatever speed you want. Then comes level C, constrained: though you are walking normally, you are not alone. If some of the other pedestrians are walking toward you, unspecified "minor conflicts" might occur. D, from constrained to crowded: some of these others are in groups, and you can no longer walk at the speed you want or pa.s.s people with ease. At this point you are down to only a paltry 15 square feet for yourself. This is the highest "tolerable" flow rate for the design of pedestrian walkways. Congested, level E: there are what could only be described as a whole lot of people. Everyone slows. You might even have to slow to a shuffle. Don't bother trying to forge a perpendicular path to a group this size. By F you are packed in tight. Your speed is probably down to less than a foot a second. If you shuffle, you're glad for it. Essentially, you are waiting in a queue, and a queue in which contact is frequent and unavoidable. There is, in the lingo, high jam density.

Back on Broadway, our own south-walking platoon weaved smoothly through the north-walking platoon, some forty persons strong, with nary a bang, a b.u.mp, or a jostle, defying the HCM odds.

"We don't b.u.mp people," Kent shouted over the crowd to me. He was reporting our experience at that moment-we were indeed unb.u.mped-but he was also reporting what urban sociologists have discovered by eavesdropping on walker behavior. They were impressed: urban pedestrian behavior is quick and fluid-all the more impressive for being largely unconscious. Together we are doing a cooperative dance, a kind of pedestrian jig, without even knowing we are dancing.

When we walk in a heavily trafficked city, we adapt to being but a wee fish in a big pond by subtly adjusting our behavior in parallel with those around us. Fish happen to be a good model for our behavior: research on fish "traffic" management has led to the formulation of the few simple rules they follow to avoid congestion while moving together with hundreds or thousands of other fish. The same rules explain the remarkable synchrony of behavior in flocks of birds, as John Hadidian and I saw, as well as swarms of locusts and army ants, and ma.s.s migrations of wildebeest, whales, and turtles. Schools and herds execute impressively sharp turns; flocks gracefully swoop, soar, roll; and all groups pulse effortlessly around obstacles. Millions of army ants move together across the forest floor foraging for food, but their paths are never marked by the crowd-stopping congestion you see on a typical interstate highway at ten minutes past five in the evening. These group behaviors are especially impressive when we remember that some of these animals are exceedingly simple neurologically-insects, for instance, have no brain to speak of. While birds are much bigger brained, bigness-of-brain does not actually seem necessary for the behavior. Instead, all of these animals rely on three simple rules. The rules are these: First, Avoid b.u.mping into others (while staying comfortably close). What counts as "comfortably close"-an animal's "personal" s.p.a.ce-will vary by species; what is similar for all animals is that if you follow only this one rule, it forces you to attend and react to the behavior of those in your vicinity. And that is the essence of what is called swarm intelligence: everyone must make movements that are sensitive to everyone else. The second rule: Follow whoever is in front of you. "Whoever" need not know where she is going: she may herself be following another. And so on and so on, until you reach the very head of the pack. Even there, the animal at the leading edge is neither leader nor sovereign. In flocks and schools, the role of leader is constantly changing hands. For only a moment will she determine the group's direction. The final rule: Keep up with those next to you. Everyone must speed or slow with attention to those around them. This seems like an impossible calculation, until you realize how little effort you have to pay to walk next to someone else down the street, never once considering how you will be able to keep at the same pace.

These rules of "avoidance," "alignment," and "attraction"-keeping apart while staying together-are sufficient to explain all herd, school, flock, and swarm behavior. Artificial intelligence scientists have created animations of mindless "boids" programmed with just these rules: their behavior matches that of swooping sparrows and swarming ants.

And big-brained-and-busy human pedestrians. Sidewalk walkers follow the same rules. We try to avoid b.u.mping, like other animals, though we do want to stay more or less together. We tend to follow others, and this leads us to form natural walking routes that become well peopled with people. While we do not settle exactly in someone else's slipstream like fish do, we hover, preferring to look over the shoulder of the person in front of us instead of ducking right behind him. On a sidewalk, this tendency sets up ever-widening-and-narrowing channels of walkers headed in the same direction.

One element Kent and I encountered that the swarm management teams do not is the simple fact of other swarms: there are always people coming the other way. Here the urban pedestrian has a special skill.

"It's interesting how people from the suburbs get on subways," Kent mused. "They come in as though they're SUVs," and they are immediately distinctive from the "native" walkers. They barge ahead, but this is not the way to smooth traffic flow, and crowds of visitors then clog a route or entrance. "We, who know the city, can kind of . . ."-and here Kent mimed a small movement out of the way of an oncoming walker.

He was doing the "step and slide." If sidewalk traffic is dense and collision seems imminent, we pull this two-step pedestrian-dance move. While striding forward, the walker turns ever-so-slightly to the side, leading with his shoulder instead of his nose to turn the step into a side-step. We twist our torsos, pull in our bellies, and generally avoid all but the mildest brushes of other people (and if we do brush against someone else, we keep our hands close to our body and our faces turned away from one another).

This commonplace maneuver was identified after researchers watched untold hours of people walking past one another. Some of the more daring researchers also studied it by doing the walking themselves. They set out onto ever-busy Forty-second Street between Fifth and Sixth Avenues in Midtown Manhattan and walked back and forth, acting naturally but for intentionally not step-sliding out of anyone's path. The people they inevitably knocked into clearly felt that the researchers had committed a small pedestrian violation by not doing their part to avoid the collision. In the seventies, when some of this research was done, the most common result was a mild imprecation or an indignant, Whatsa matter-ya blind? Even without knowing exactly what the rules are in order to avoid b.u.mping each other, we sense at once when someone else is not following those rules.

Urban pedestrians make other small adjustments to others near us. When crossing paths with another walker, one party slows his pace just enough-maybe only for a fifth of a second-to enable both to keep the direction of their route unaltered. If someone behind us approaches quickly, we slide slightly to the left or right to give them s.p.a.ce to pa.s.s. As Kent and I saw on the street, sometimes these accommodation behaviors are less subtle. Walking aside each other and approaching us on a part of a narrowed sidewalk was a couple holding hands. Expecting some full-body contact with them, I braced myself and continued forward. Unprompted, the male of the couple swung his partner in front of him, dancelike, to pa.s.s us single-file. Another man walking alone behind them stepped lightly into a tree pit to avoid us, leaving an imprint of his shoe in the dirt.

Even those who stop right in the middle of a sidewalk are accommodated. Whyte's cameras noticed that people in the city (not just tourists) tended to stop to chat smack in the center of the flow of pedestrian traffic. People in the kind of conversation that obliges one to slow to a full stop-greeting a long-unseen friend, doing the final rounds of good-byes, or responding to someone's surprising or serious comment by stopping outright-wound up squarely in others' way. Oddly, the "others"-the walkers inconvenienced-navigated smoothly around them, shoals of fish opening and closing around a jutting reef. Perhaps the cameras were too distant to catch any imprecation muttered under their breath. More likely, the urban pedestrian, moving toward his destination, simply steers around stoppers as he would nonhuman obstacles: no one chastises the lamppost for being in his way.

One reason all of our step-sliding, pedestrian-jigging works is that we are regularly looking-ahead and at each other. We do not just look to see who is there; we constantly, steadily look to calculate how we need to move relative to those around us. We regularly turn our heads back and forth, to the left and right, surrept.i.tiously peeking at who is behind us or to our sides. When our heads face forward, we survey the scene ahead of us. Our eyes make small saccades. Within a long oval projecting forward from our feet to about four sidewalk squares ahead, we quickly note the direction and pace of anyone headed our way. We also glance at others' faces, which tell us if they are likewise looking forward into their own long ovals (and whether they are reacting to something surprising or alarming that might be behind us). There is information in the angle of others' eyes and the turn of their head. Most of the time, people are looking where they are going: gazing straight ahead. But they begin actually inclining toward their destination when it is in sight. Should someone seem to peer over to the doorway of the building down the block, more likely than not, he will walk there directly. Or just follow his head: we all make antic.i.p.atory head movements when we are going to turn a corner. Our heads lead our bodies by eight degrees and as much as seven steps, as though all in a hurry to get around the bend. Watch a walker's head and you can predict his path down to a single step. We learn this without anyone teaching us, and without knowing we know it.

The importance of this "looking" in the success of the dance comes into play with the relatively new species of pedestrian on the street: phone talkers. Their conversational habits change the dynamic of the flowing shoal. No longer is each fish aware, in a deep, old-brain way, of where everyone is around him. The phone talkers are no longer even using their fish brains: they have turned all their attention to engaging with the person on the phone. They block out their sense of someone walking too close; they fail to look into their walking ovals and step-slide out of the way. They no longer follow the rules that make walking on a crowded sidewalk go smoothly: they do not align themselves (they swerve); they do not avoid (they b.u.mp); and they do not slip behind and between others (they blunder). They stop minding the social convention to stay to the right, and weave across lanes of traffic. Texters are as bad or worse: they fail to even move their heads before turning, since they are slumped over to monitor their texting thumbs. I fantasize that the phone talker's route, if reconstructed and synchronized with their conversation, would reveal the organization of the chat: straightforward questions-and-answers matching straight, forward walking; sidetracking and topic-changing marked by weaving and divergent walking.

Notably, not all of our crowd behavior mirrors the animal swarms. Mormon crickets and desert locusts seem to cooperate marvelously, march-stepping in the same direction in caravans miles long. But they are not just cooperative; they are also cannibalistic. Cooperative, streaming swarm movement can also be generated, it turns out, if you are trying to eat the animal in front of you while avoiding being eaten by the animal behind you. Thus the double-takes seen when a foot race comes through a neighborhood not expecting it: whether the racers are running together or away from one another is not obvious from the simple fact of their speedy running.

Some desert locusts also have a gregarious side, which is useful in swarming. And there is a neuropsychological mechanism that may explain what prompts the lowly locust to seek company. Their gregarious behavior correlates with a huge increase in the amount of serotonin in the central nervous system. In humans, serotonin is involved in many behaviors, including moving in a group. Some of the most common contemporary antidepressants modulate the action of serotonin by increasing the amount of it lying around in our brains. One could speculate that a rise in serotonin also allows us to be sensitive to-and, for some of us, to feel rewarded by-moving smoothly in a swarm of our own species.

Emerging from our platoon on the next corner, I looked over at Kent. His head was just turning to look across at the other, westerly side of the street. I took that as a request. With a small adjustment of my path we crossed the street together without exchanging a word.

I should note: we crossed the street-but not at its corner. Every good New Yorker makes herself known in foreign cities by doing what we just spontaneously did: we jaywalked, crossing the street in its middle. As we were crossing, I looked up into the cab of a truck turning onto the street, aimed right at us. In response, the driver chose not to hit us and slowed to a stop.

A couple of cla.s.sic street-scene events had just occurred: one historical, one psychological. Historically, we continued the proud urban tradition of walking wheresoever we pleased. Jaywalking was first used a century ago to describe the behavior of a pedestrian unaccustomed and naive (a jay being a silly person) about how to walk safely in a city. It was among many terms of mild opprobrium used about bad walkers. A New York Times article from 1924 includes "the veerers who come up sharply in the wind and give no signal," "the runners who dash to a goal and then dash back again," "the retroactive, moving crabwise," "left ends and b.u.t.ters," "the plodder," and "those who flee and turn swiftly" among those who deserved equal blame for the chaos of the streets and sidewalks at the time. "Jay driver," though suggested by many jaywalkers who saw the speedy driving of the new auto menace on the streets as the real hazard, never caught on.

Jaywalking is a civic traffic violation, but I happily do it. I rationalize my behavior by noting that crossing the street against the traffic light makes me pay more attention to what I am doing, rather than mindlessly following the traffic signals. And that is the psychological component: sharing attention. Kent agreed: "You're actually safer because you're making judgments based on eye contact."

Our jaywalking during this walk might have been the most eye contact I would make on the street all day, in fact. In the city, eye contact is carefully wielded, as I saw when my son stared at a dest.i.tute man limping toward us. Moving along a sidewalk on a summer's day, full to D-crowd levels, eye contact is fleeting and reserved for estimating others' walking paths. To stare-to look continuously into the eyes of someone else-is laden with meaning. Between pa.s.sersby, the intent can be provocative or salacious: I hate you or I want you. But it also may be, as it is between driver and walker, path-directing. By holding someone's gaze, you actually control his movement to some extent, obliging him to move around you. One psychologist I walked with described a quasi-mind-control game she used to play with unsuspecting fellow bus riders. She would try to "seat" people on the bus by making eye contact. "n.o.body likes to be looked at, so they keep walking" as long as they noticed her looking at them. If, then, she turned away, "that's when they sat." The rule against maintaining eye contact with strangers makes any eye contact powerful. Conversely, in a context where eye contact is supposed to be made, one person can, in theory, "move" the other by looking just to his left-forcing him to adjust himself until he can make eye contact again. I tell my undergraduate students to test this out with their professors, looking just to the right of the lecturer. Often, their gaze unconsciously inclines her (or me, as the case may be) to move more and more to her left to get back within her cla.s.s's line of sight.

Some research suggests that the very presence of signs, traffic lights, crosswalks, and raised curbs, all intended to make walking safer for the pedestrian in a car-filled city, actually make it less safe. In the Netherlands, the traffic engineer Hans Monderman came up with the idea of a "naked street," empty of all these safety accoutrements. His idea was that by forcing us to look at each other-walker to walker, walker to driver, driver to driver-we could use eye contact to negotiate our routes. A few cities are attempting to enact this planning idea; at a main intersection in one Dutch city, Drachten, through which pa.s.s tens of thousands of cars, bikes, and walkers a day, traffic moves slowly and perfectly smoothly.

When we arrived across the street, my gaze left others' faces and, as is often the case, it went to what was underneath our feet. I asked Kent how much he thought about what was underfoot.

"Designers think it's very important," he hedged. I knew a geologist who did, too.

"From your perspective . . ."