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
