Читать книгу Nature Obscura - Kelly Brenner - Страница 11

They come from the north. Every night as the sky darkens and the sun touches the horizon, hundreds of them fly over our house in one long, continuous line, a river that may change course to the east or west of us but always within view. They are the commuters of the sky, unimpeded by traffic lights and not limited by the routes of the road. They fly together on the same plane, but once in a while an individual will dive or swoop over a tree or over nothing at all, seemingly just for fun. They fly through the rain, the wind, the fading sunset. Many nights I stop what I’m doing to watch them. The half-closed curtains are still gripped in my hands when my mind and eyes flutter back down to earth. I can’t seem to help following them. They enchant me.

I can’t say exactly when my appreciation of crows began. It’s like a love that grows gradually but, once realized, feels like it has always been there. Every time I watch a murder of crows, as a group is called, whether they’re foraging along the lake’s edge or gathering in trees, my fascination steadily increases. It wasn’t long after we moved into our house in south Seattle one September that I first noticed their astonishing numbers. Truly, they’re hard to miss as they fly south over the Rainier Valley every night in the hundreds, perhaps thousands. I counted crows one evening while I was participating in the Great Backyard Bird Count citizen-science project. When I stopped at the end of my hour, which had started long before the crows began their commute, I had counted nearly three hundred in just one five-minute span of that hour. At the time, where they were going was a mystery to me.

When the weather turns cold and the leaves turn red, they start flying in larger numbers. When the bare trees sprout flowers and new leaves, fewer crows fly by while they spend their nights at their nests. However, every night in the autumn and winter all the crows around Seattle head to one of several roosts, where they gather to sleep after a day of foraging around the city. The roost on the north end of Lake Washington is one of the best known and perhaps the largest, with 10,000 crows on Bothell’s University of Washington campus. I wanted to experience that many crows in one place, so I headed to Bothell one January night.

After getting to the campus, I followed a winding path downhill from the parking garage, passing the playing fields and heading toward the wetlands. At the bottom of the hill I stood with the densely packed, bare trees of the wetlands on my left and the playing fields to my right. High on the hill towering over it all sat the campus buildings. My breath hovered in the cold air, and my toes began to tingle from the chill. Eager for the experience—and the crows—I had arrived well before dusk, so I waited, and waited. For three-quarters of an hour I scanned the empty skies all around and worried that the crows had changed their minds and gone somewhere else for the night. I saw not a single one. Eventually, after my toes had gone fully numb, a slow trickle of birds started to arrive from the south; but instead of gathering around the wetlands as I had expected, they continued right on past. Again I worried that they weren’t coming as I watched crow after crow ignore the campus and continue on a path toward some unseen destination.

But then, an hour after I had arrived, the flow of crows shifted and they started to fly in. At first a small stream of black silhouettes flowed in from one direction, and then more and more from several directions. As I watched the sky, the small streams morphed into wide rivers, flowing in from many directions, and I stood at the center of the confluence. Soon the sky was covered with evenly spaced crows, black as India ink against a deep blue sky. Lower along the horizon the crows were outlined against pink illuminated clouds. Instead of heading straight for the trees in the wetlands, where I expected them to roost, they flew to the tall Douglas firs on top of the hill by the campus buildings, five hundred feet away from me. There they circled, mingled, and landed unseen in the thickly needled branches of the trees, cawing all the while. As the evergreens filled, some birds started lining up along the roofs of the campus buildings while others chose the bare branches of deciduous trees. More and more continued to arrive, the sky never empty, the noise unabating.

At some unknown signal, even while the latecomers were still arriving, the crows began swooping down the hill, over the sports fields, and over the path where I stood, into the wetland trees. It was noisy, chaotic, and felt as if the sky were falling—and yet, despite the ruckus, there was a sense of order to the process. The crows didn’t all move at once, but in one slow rolling wave. The first to fly downhill landed in the trees at the south end of the wetlands; then birds slowly began filling up the trees northward. The sky became black with crows as they continued flying down the hill and into the trees. I felt like the sun, a static point with everything moving around me. Standing beneath the crows, I turned in continuous circles—I watched the latecomers still arriving from the north, then turned to watch birds flying down from the top of the hill, then turned again to watch them land in the leafless trees of the wetlands.

The crows’ flying formation is very uniform, not at all like the frantic starling murmurations that create art in European skies, or the flight of Vaux’s swifts that gather in the nearby town of Monroe and swirl in a giant funnel into an old chimney to roost at night. The crows space out—not too close together, yet not too far apart—creating an orderly, steady formation of flying bodies. As the crows continued gathering in the wetlands, I walked down the short boardwalk into the middle of the trees. Where organized chaos reigned outside, inside felt like complete chaos. The crows shifted about, fluttering from one branch to another, only to jump up and move again. The noise was incredible, the caws of many thousands of birds like the feedback echo on an amplifier cranked up to eleven. It felt like the chaos might continue all night as some unseen force caused the birds to move from one branch to another, one tree to another, without stopping. Finally, about forty-five minutes after the crows started to arrive, they seemed to settle down, and the noise eased slightly. Then, as I walked out of the wetlands, an immediate silence fell. The show was over.

Perhaps the question related to crows scientists have studied most is why they roost together in such large numbers. The roost of 10,000 crows in Bothell is small compared to the reported two million observed roosting in parts of Oklahoma. There are crow roosts of all magnitudes around the country, often near and in cities. In North America, ravens also roost communally, although most frequently away from civilization. In Europe other corvid species—rooks, jackdaws, carrion crows, and hooded crows—roost communally, sometimes in large numbers. One roost of rooks and jackdaws in Norfolk, England, has been estimated to contain 80,000 individuals.

Birds usually form groups because there is safety in numbers. And that is indeed believed to be one reason they gather together at night to sleep: being in a large group reduces their chances of being attacked by a predator. At the wetlands in Bothell, the cottonwood, alder, and willow trees where the crows roost act as cages, enclosing the birds as they perch on branches near the middle of the tree. If a predator should land at the top of the tree, that motion would ripple down the branches, alerting the crows to the intruder’s presence.

But safety is only part of the reason for such large gatherings of corvids. The months when they roost communally also happen to be the colder months of the year: autumn and winter. Thousands of bodies packed together in trees helps with thermoregulation, keeping the birds warmer on cold nights.

Yet, perhaps even more importantly, roosts are social hubs, places where crows come together to exchange information, particularly about the location of food. Young crows are known to follow their parents to roosts, so the knowledge of roost locations is thus passed along. Once at the roosts, young crows learn from their elders where food can be found, and not necessarily only from their family group. Curiously, when family groups arrive at roosts, they tend to roost apart from each other.

Crow roosts also aid in the development and maintenance of a complex social hierarchy. Each night the crows jostle for prime position within the roost; the ruckus I heard in the wetland trees as they landed was exactly that. Higher branches—though not the highest—are prime roosting position because they offer protection from wind and rain. Juvenile crows, lower in the social order, are conscribed to the lowest branches—and as anyone who has spent time sitting or standing below birds in trees knows, that’s not an ideal place to be. Young crows are thought to settle in rings around older, experienced individuals as a way to gain favor, and as a result, potential access to a good food source the next day.

In addition to jostling for position in the social hierarchy, some crows may be looking for new mates, particularly those who have recently lost a partner. Crows tend to stay with their mate year after year—unless one dies or is killed—and a roost with thousands of birds is an ideal place to find a new partner.

There are benefits to roosting nightly in cities, as opposed to more rural locations. Roosting sites in cities can be up to 10 degrees warmer thanks to the urban heat island of roads and buildings absorbing solar radiation during the day to emit warmth at night. Cities often have a greater abundance of trees suitable for roosting, compared with surrounding landscapes, which may be intensively farmed or logged.

Surprisingly, cities are also free of many dangers that face crows in the countryside. Not common in most cities, great horned owls are mortal enemies of crows because they regularly kill and eat adult crows as well as nestlings. Some great horned owls are particularly ghoulish; if they have an abundance of food, they have been known to decapitate their prey and eat only the brains, leaving the rest of the body behind. A crow roost at night in the dark countryside can be easy pickings for this large owl, which has the benefit of night vision.

Crows, as diurnal animals, have poor night vision, and some scientists also believe that the lights of a city help keep crows safer by helping them see a predator coming before it catches them unaware.

In rural areas where shooting is allowed, humans are crows’ second most deadly enemy. Over most of our complex history together, some humans have spent a great deal of time and effort killing crows. As John Marzluff and Tony Angell point out in their excellent book In the Company of Crows and Ravens, crows that are used to being regularly hunted by humans are much more wary of humans. Those crows even recognize particular hunters, by their appearance, their weapons, and their cars or trucks. Country crows are much more wary of people, having endured a continued persecution that city crows generally escape.

Finally, crows have long collective memories, and it’s entirely possible that some crows may have always roosted and gathered in a specific locale long before humans moved in and that place became a city. The birds may have found that the more urban environment suited them just fine.

I often encounter murders of crows when I happen to be out before dusk, in places such as Genesee Park in south Seattle, which, I discovered, used to be a landfill, a popular foraging opportunity for many birds. Crows gather in these large groups all over the city, and for the uninitiated, the gatherings can be confused for roosts; but what the crows are doing is called staging. The closer to the roost that these gatherings occur, the larger they are, as more and more birds congregate before making the final push to their nighttime roost. Why crows do this is still a mystery. One thought is that these staging areas are crossroads of sorts, where the commuters from one aerial road meet up with others, and they offer a last chance to forage before continuing on to the roost.

Another big question that preoccupies scientists is, where do the crows at each roost come from? Somewhere in Seattle there’s an invisible divide, with the crows on the north side of that line going to Bothell, and those on the south side, for example, flying over our house in southeast Seattle to Renton, at the south end of Lake Washington. Some crows may travel to some of the smaller roosts around the city, but these roosts have not yet been well documented or observed.

Which individual birds go to which regional roosts is not yet entirely understood, but it’s likely somewhat flexible depending on where the crows are at the end of a day of foraging. It’s probable that resident pairs—those mated birds that have established territories—are more likely to go to the same roost each night. Younger birds without territory to defend may be more flexible in their choice of roost, depending on where their source of food is at that time or if they’re looking for information about a new source.

Crows abandon their roosts from time to time as well. Until recently, for many years crows roosted on Foster Island, right across Union Bay from the University of Washington campus in Seattle. In the morning they would land at a nearby parking lot by a place locally known as “The Fill” but officially called the Union Bay Natural Area. The nickname gives a hint to its past—it was once the local landfill, an attractive place for crows to forage. The landfill is long gone, but the crows remained roosting near there and gathering each morning by The Fill before the roost finally dissolved and the birds established a new one at the UW Bothell campus. It is unknown what led them to yet another UW campus several miles to the northeast—perhaps they have a preference for purple and gold.

Although—or perhaps because—I had experienced the full roost in Bothell, I wanted to find and experience the roost where my crows—the birds flying over our house—went each night. Based on blog posts and listservs, I had a general idea that they roosted somewhere near Renton, and I used that information as a starting point. So one January evening, my husband and daughter joined me and we headed to the south end of Lake Washington to find our crows.

We didn’t have any trouble locating the birds as we drove; we just picked up and followed one stream of crows until we found all the streams converging and landing in Renton. With my husband behind the wheel, I was free to keep my eyes on the sky. I could see the crows gathering in the middle of a large block of industrial and commercial buildings—they were tantalizingly close, but at that time of night, the driveways that could give us access were all gated and locked.

As I watched the crows arriving, we continued around the block to try and find a way in. Finally I saw an open driveway, and as we pulled in, I saw the crows in the middle of a giant parking lot surrounded by twos-tory buildings.

We drove through the lot—excitement building with each parking aisle we passed and as the numbers of crows grew and grew—until we came to the center of the chaos. I yelled to my husband to stop the car, but before the car fully stopped, I had already jumped out.

Thousands of crows covered every surface—trees, buildings, streetlights, curbs—even the parking lot’s tarmac was covered in crows. They perched in the branches of scrawny trees, silhouetted against the orange horizon and setting sun. They sauntered along the railroad tracks that divided the parking lot in half. More and more flew in from different directions. As I was trying to fathom the number of crows, I looked up, and my jaw dropped. A huge flock of starlings, a hundred feet across, had snuck up behind me and now flew directly overhead. The tight mass of small starlings flew higher, creating a cloud over the looser group of larger crows. The quickly fading deep blue of the sky filled with thousands of moving shadows—the starlings going one way and the crows going every way. The combined spectacle of starlings and crows was like the grand finale of a fireworks show.

I stood in awe watching the crows and starlings in constant movement. The murmurations of starlings flew out of sight, only to return larger in size, one moment a long, sinuous shape, the next a tight ball of black. Then the flock broke apart and there were two groups of starlings, then three. While the starlings were silent shadows, the crows’ caws echoed a thousand times over, creating a ruckus to rival Bothell’s massive roost.

Cars kept driving by me, and then I finally noticed small shuttle vans before realizing I was standing in the overflow parking lot for the nearby IKEA store. A security truck pulled up just across the train tracks from me, and the guard simply yelled out that the birds do this every night. Before I could ask any questions, he was gone, driving slowly through the rabble of crows.