15,000 crows

I had imagined it like a beckoning flood.  A small sputter of water followed with increasing force until a great river finally makes its way.  Rather than water though, the flood I was trying to envision was the ascent of 12-15,000 crows to their nightly roost in Bothell, Washington.

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Witnessing it in person, I found that my water analogy was not entirely accurate.  Rather than being a steady stream with a predictable course, their arrival ebbed and flowed, sometimes leaving the sky lonely with only its fading grey light while other times exploding into seemingly endless black clouds.  They arrived from all cardinal directions, colliding into a mass that could be deafening at close range.  Although the movement of the flock as a whole was more restrained, individually they showed off with spontaneous dives and barrel-rolls.  Soon the light receded completely, and all I could sense was the cacophony of so many crows settling into the willow trees they would call their beds for the evening.

Time lapse of Bothell crow roost I took with my GoPro in December of 2016.  Music by Andy McKeen.

Since that first experience, I have visited the Bothell roost many times, each as awe inspiring as the time before.  This behavior isn’t unique to my region, however.  Cities and rural areas all over the world call themselves home to the upwards of thousands, even hundreds of thousands of crows that may seek their refuge when darkness falls.  Even in the greater Seattle area, Bothell is only one of two roughly equally sized roosts.  This kind of mass sleepover, known as communal roosting, isn’t unique to crows, but it certainly captures our attention in ways most other birds don’t.  So what exactly are the characteristics and functions of roosts?

For all species of corvid, roosts are places where anywhere from a small handful to hundreds of thousands of individuals may converge to spend the night together.  Though roosting occurs year round, it peaks in winter, when territorial pairs are free from the eggs or nestlings that demand all-night attention.  They may occur in wildlands, but more typically occur in cities, where sequestration of heat is higher than in surrounding areas.  Here in Bothell, the roost converges in a wetland outside of the University of Washington’s Bothell campus, but in other areas they may take over the rafters of abandoned buildings or trees dotted within a business district.

Historically Danville, IL hosted North America’s largest roost, a whopping 325,000 birds but I do not know if they remain the contemporary record holder.  The midwest is particularly primed to host such large numbers because many thousands of crows head there during winter from their too cold territories in Canada and because appropriate roosting locations are few and far between.

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Prior, or just after roosting crows attend “staging” or “pre/post-roost” areas where they gather in the trees or on the ground by the hundreds or thousands.  Since these staging areas often occur on asphalt or turf where there’s little food or water, their function continues to elude scientists though social or anti-predator implications seem likely.1 A new UW research study is attempting to parse why crows are so vocal during the staging period and what they might be trying to communicate.  Perhaps their findings will shed some much needed light on these events. 

Corvids get different things out of roost itself depending on the species or possibly even the region they live.  For example, for ravens roosts act, in part, as mobile information centers.2  A raven knowledgeable of a food bonanza such as a moose carcass will display to other ravens at first light, and recruit others to the food.  Rather than being a sign of food altruism, this kind of recruitment is often the only way a lone raven can gain access to a large carcass.  Finding and gaining access to an animal carcass is challenging both because its arrival is unpredictable but also because it’s intensely guarded by the pair whose territory happened to claim the animal’s life.  Overpowering a pair takes a small army, so by recruiting other birds, rather than giving up food in the name of helping others, the lone raven actually gains access to a resource it would have otherwise been boxed out of.

American crows on the other hand do not have this need because urban waste and invertebrate filled yards are so easy to come by.  For crows, roosts act in large part as predator protection.  The odds of successfully fleeing an incoming owl are much better when there are thousands of you, rather than just you and your mate.  They may serve other purposes as well though including socialization, mate finding, and thermoregulation.  Lastly, while there isn’t strong evidence of information sharing among crows it would be arrogant to claim we know it doesn’t occur.

How roosts are organized remains largely mysterious.  For example some evidence shows that ravens that come from the same food bonanzas also sleep near each other in a roost,2 whereas other work done on crows suggested that group cohesion is low at roosts.3  Still, other research suggests that while group cohesion from the territory is low,  it’s high leaving the staging area.  So perhaps there is deep rhyme and reason for who they sleep with, it just hasn’t been captured by the questions we’ve so far asked.  One thing is for certain though; the one place you don’t want to be is low in the trees with others above you.  There would be no escaping the white shower raining down throughout the night.

Even the people who share the UW’s campus are sensitive to this reality.  In perfect synchrony with the incoming cloud of birds, the umbrellas bloom like moonflowers.  Here in Seattle, people seem willing to take such measures to coexist with the birds (though I’m sure there are many who only do so only by rule of law).  In other areas though the cultural attitude or resulting damage makes such cohabitation difficult, even deadly.  In the most extreme case, 328,000 crows were killed in 1940 when the city of Rockford, IL elected to dispose of a local roost with dynamite.4  Today, crows are protected under the migratory bird treaty act and cities are usually required to take more creative, non-lethal approaches including noise and light deterrents.

City living doesn’t always lend itself to witnessing the kind of mass animal movements we fawn over when they appear in Planet Earth footage, but that doesn’t mean they are devoid of such spectacles.  The mass micro-migration of thousands of crows is an awe inspiring event,  grand in both scale and the mysteries it contains.  Any corvid or birdwatcher would be remiss to ignore such an opportunity and I encourage everyone to get outside, head to your roost, and watch the magic unfold.

Literature cited

  1. Moore JE, and Switzer PV. (1998).  Preroost aggregations in the American crow, corvus brachyrhyncos.  Canadian Journal or Zoology.  76: 508-512.
  2. Wright J, Stone RE, and Brown N. (2003).  Communal roosts as structured information centers in the raven, Corvus corax. Animal Ecology 72: 1003-1014.  DOI: 10.1046/j.1365-2656.2003.00771.x
  3. Donald F. Caccamise, Lisa M. Reed, Jerzy Romanowski and Philip C. Stouffer
    (1997). Roosting Behavior and Group Territoriality in American Crows. The Auk 114: 628-637
  4. Marzluff, J.M. and Angel, T. 2005. In the company of crows and ravens.  Yale University Press

8 Comments

Filed under Birding, Crow behavior, crow conflicts, Crow curiosities, Crow life history, Crows and humans

8 responses to “15,000 crows

  1. brightdark

    I can’t hazard a guess but we must have 5-10K crows roosting in Rochester,MN. Some evenings the air is just of them flying around, sitting on rooftops, etc before they settle in for the night. Many in the downtown trees but others in the trees around the lake or cemetery.

    Crows are always fun to watch. For some odd reason they like my spouse. Sometimes they will hope along behind her talking. When she shoos them away they think its a game.

  2. Judith Blades

    wowww I can’t imagine the sound

  3. Kirk Muter

    I would love to see this phenomenon first hand, but I’ll bet they make a huge racquet! In my neighbourhood, there is a small gang of five or six ravens that hang out together, acting like a group of teenagers – showing off, and hassling the local squirrels. They seem to be out there most afternoons, up to no good! I can watch them for hours…
    Anyways, just want to let you know that I look forward to your new blog entries, and find them endlessly facilitating and informative. Thank you for making them available!
    Warmest Regards,
    Kirk Muter
    Kitchener, Ontario

  4. Andy Pym

    Once again, a superb post!

  5. FYI: In the first paragraph, “assent” should be “ascent”. Otherwise, a wonderful article. Thanks.

  6. Ivy Castleton

    I rescued a crow about two weeks old last spring. In the Henderson, NV area & entire Las Vegas valley, we had very harsh winds during April & May of 2017. A large number of baby birds & entire nests were blown out of trees. This crow was one of the many.
    She (I’m not sure if crows can be sexed by sight or if it has to be done either surgically or by observing the bird among other crows.) was found by the property owner where the tree stood or by a neighbor. This person didn’t know to leave the baby bird alone for the parents to help & took it to a local veterinarian. The vets in that clinic deal only with pet cats & dogs. They had no idea how to help the crow, so ‘she’ was given to me since I have rescued a number of birds, especially babies. I’ve never worked with a crow before & have several questions to which I need answers.
    The crow I named “Ribbitt” after the call it made when begging for food when very young, seems to be as intelligent as crows are known to be. Ribbitt does seem to have a few symptoms of a possible mild head injury from the fall from the tree. There were other minor injuries which have healed very well. I wanted to prepare her (not certain, as I said) to be released in October where there is a fairly large family group to help her mature & later select a mate. I wasn’t able to release her & now it’s too cold until spring.
    Basically, I need to get in touch with someone who has experience in crow rescue, but not someone who will try to take Ribbitt away from me because they know it all & I’m still learning how to work with these wonderful corvids.
    Thank you!

  7. Beth davies

    Fantastic post. Last winter my apartment building was the epicenter of a roost. It was surreal. I live in west Berkeley, and have been watching them for a long time. In the span of about a week, I would go out to my porch and watch them fly overhead by groups of fifty or more, totally silent at dusk., then just about a half hour before dawn, they would be in the five trees around my house, all sharing their opinion at once. Ground as white as snow, and none to be seen when the sun came up. The experience was wonderful, I am glad it happens elsewhere too. Not just my imagination.
    Thanks!

    Beth Davies

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