Nature Obscura. Kelly Brenner
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,