The Murdering Crows

This is the first post of a series on everyday animal behavior. I grew up in woodland, so deer, foxes, and owls were normal visitors to my backyard. And they were always up to something worth wondering about, living their own dramas. Now I live in Baltimore city—and the rats and roaches I meet here are really no less interesting than mighty black bears. Any city has plenty of wildlife to be found. This is my journal of the animals I meet in my day to day, the odd things they do, and my attempts—aided by whatever scientific litrature I can find—to figure out what the hell they’re up to. I’ll start with a story describing some weird incidence of animal behavior, and then I’ll move on to a discussion about what it might mean—for the animals, for us, and for the general experience of living a life on Earth.

Corvids have always written collective dot-dash sky-code. They are past scornfully enjoying the obliviousness of human observers. They no longer shape out rude slogans or rococo aerial slanders. No provocation ever provoked. There is a mournful air to their more recent, equally unread, messages.

China Miéville

I decided to go on an aimless walk today after a long and relaxing spell of procrastinating on my master’s thesis. After wandering through the low-slung dog park up the street (beautiful, but fragrant), I found myself following a narrow red-brick path toward the stately red-brick buildings of the Johns Hopkins main campus. I don’t belong at this school per se, and I always expect someone will detect me as an interloper and throw me off the hallowed academic grounds. This will never happen: I am a young white woman, clad in cut-up t-shirts, squint-eyed and pale from endlessly procrastinating on my own graduate work—albeit for a lab at a distant and much less impressive institution. I may not belong at Johns Hopkins, but everyone assumes that I do. My privileges solidly intact, I followed the red brick road.

I was plotting a new running route past the water fountains and dining halls when a stirring in the air caught my attention. I slowed down and looked up, to the forest lining the campus. The tops of the trees were letting loose a raucous noise, and as I came to a stop their feathered black bodies pelted through the air.

I have never seen so many crows, I thought. They were leaping and flying from tree to tree, tens and hundreds of them—the treetops bent with the weight of crows. They were raising such a croaking ruckus that people began poking their heads out of doorways to peer up at the sky. When one flew, others followed, and the air grew thick with crows.
I thought for a moment of the legendary flocks of passenger pigeons that blotted out the sun over pre-industrial America. I thought about how a flock of crows is rightly called a “murder,” and thought that this must be the largest murder I’d ever seen. I thought about crows as an omen, with their deep insistent voices and black ragged wings. I wondered what this murder might be gathering around, what might be coming towards me, slowly, through the darkness of those trees, heralded by a noisy entourage of hundreds of crows. As the hair stood up on the back of my neck, I silently and hopefully eliminated zombies as a possibility. The crows, I realized, were headed my way, flight by aimless flight.

They were travelling north, as a noisy group. They took off and bee-lined over my head with their wispy snaggle-toothed black wings, croaking all the while—pairs, tens, and then hundreds at once. They were alighting on the tallest aspects of their landscape, the stocky architecture of the Hopkins halls: on the ridges of peaked roofs, and along the Georgian flourishes of square-sided turrets high above the ground. They lined the rooftops like little black-feathered soldiers. I followed the vanguards, others flying behind me to catch up, until I was at the center of a quad, and all around me, hundreds of crows—gathered at the highest point of every building I could see.

For a moment, the birds were quiet. The stragglers were catching up. The evening blue was chasing the receding light of day at this point—a late summer sun about to dip below the horizon, and the sky was tinged with pink and buttery yellow. A soft cool breeze was blowing, the remnants of the afternoon rain. I studied the sky, and for a moment I forgot about the crows high up on their turrets.

But then the murder, all as one, took flight. The hundreds of crows lifted off all around me, as if by some tacit agreement, and wheeled together high above the quad against the yellow sky. A flock of crows is called a murder, and a flock of starlings is called a murmuration—the haunting mass of tiny birds that pulsate and turn as one body, reacting to each another in intimate space. The murder of crows now became a murmuration as hundreds of birds turned about one another, forming chimneys and swarms, and their rasping caws thundered from them all the while.

After this great wheeling and touching of wings had gone on for about a minute far above the ground, the crows resolved themselves into small groups and pairs. They straightened out northward, over the roof of the tallest broad building in the quad. The endless croaking went with them, and then they were gone as quickly as they’d appeared. The air felt empty and silent without them—but for a few lone swallows darting upwards on pointed wings, snatching up the last quick bugs of the day.

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The Secret Nighttime Lives of American Crows

I honestly have no idea whether this series will be as fun to read as it is to write (and you all will have to let me know. “Meg! This shit is totally boring to everyone but you!”)

Because I started with a really simple question: “Why did this big flock of crows gather at dusk?” But this sort of simple question is not simple to answer. It requires a basic knowledge of the daily life of the average American crow–and that sort of knowledge must be based on carefully collected data. In order to answer my question, someone else needs to have asked it before, and actually put in the work to answer it. And I’m reliably amazed to find that, in the field of animal behavior in 2016, many of the basic questions have indeed been answered.

In this mysterious case of the evening crows, I mainly turned to the report of one project in which scientists radio-tagged 21 crows in New Jersey and followed them over the course of a couple years (which, incidentally, were the first couple years of my life, 1990-1992). This paper—“Roosting behavior and group territoriality in American crows“—was published in The Auk in 1997, and gave me more information about the daily lives of crows than I ever hoped to find.

Of course, this study is old, was done in New Jersey (and I’m watching crows in Baltimore), and involves a perilously small sample size (n=21). But if there are crows in New Jersey living by this routine, it might apply—at least somewhat—to animals in my area. So what does the life of an American crow look like?

Lesson 1: Crows babysit for each other.

First off, I knew crows are social. I know from my own observations that they flock, travel in groups, have complex vocal communication, and form mobs to attack predators–all suggesting that crows have something like a strong sense of community. But I learned from this paper that they also participate in alloparenting—as in, members of a family group will actually care for each other’s babies. This is the sort of behavior we usually associate with, say, human beings or wolves—animals that have adopted exceptionally strong social systems, including altruism, to survive. Alloparenting is interesting because it’s both self-sacrificing and human-like, and it’s relatively unusual in the animal world.

Lesson 2: Crows live on one territory, with their families, for years on end.

The researchers found that, for the most part, crows stuck to specific territories with the same small interrelated groups (usually composed of around 5 individuals, who might all be part of a family). The researchers didn’t describe the average size of these territories—they just use the word “small”—but they did find that group members aggressively defend these spaces from other families, and that they stick to the same place for years on end. This, to me, is a lovely little revelation, because it means that the crows any of us might run into in the course of our daily lives may actually be our permanent neighbors. If you tend to see crows around your house or workplace, you may very well being seeing the same families over and over again. Watch them if you can. You might start to get to know them as individuals.

Lesson 3: Sometimes crows sleep at home, but they’ll also fly off to sleep in big neighborhood groups far away.

Now let’s put this information into the context of what I saw this week: Tons of crows flying together at dusk. The researchers found that much of the time, their crows spent their nights sleeping within their home territories (especially if they’re young). But some of the time—as much as half of their nights on average during the summer—their crows took off from their homes as dusk approached, and flew to a mass roosting site about 20 km away. And once there, crows did not sleep next to members of their family groups. It’s almost as if, during the summer, everyone in your hometown were to leave their houses around dusk to gather, meet with friends, gossip, and sleep in the town square. And, for the most part, the kids stay home until they’re old enough to take part in the ritual.

This, I suspect, is what I saw in Baltimore the other night: tens or hundreds of crows, from disparate family groups, meeting over the Hopkins campus and flying together towards a group roost site. Another paper, “Preroosting aggregations in the American crow,” corroborates the fact that crows in California will group up in areas near to group roosting sites but not quite at them, and fly the last legs of the trip in a big group before settling down for the night. Bingo: I suspect what I was watching the other night was a summertime preroosting aggregation of a few hundred crows local to the area around the Johns Hopkins main campus. Case basically closed.

But why?

This post is already getting too long, so I’ll keep this brief (especially knowing that while “what are they doing?” is a hard enough question to answer when it comes to animals, “why are they doing it?” is much harder still, and subject to a ton of inaccuracy). The researchers in that 1997 paper found that their crows stopped at high-yield forage sites both going to and coming back from the mass roosting site in the evening and morning after. This led them to believe that sleeping far away from home, especially at locations close to high-density food sites (in their case, a landfill), gave their crows the chance to hit the same far-from-home eating spot twice in the same 24-hour period with minimal traveling effort. Of course, group roosting may provide other benefits—like protection from predators (strength in numbers), or access to potential unrelated mating partners (new friends from far away). None of these are mutually exclusive.

Conclusion: While I was out for an evening run, I ran into a preroosting aggregation of Baltimore-native American crows that had gathered for the evening and were wandering, en masse, towards a group roosting site somewhere near the Johns Hopkins Homewood campus.

Until next time, folks!

Hoo boy, have some references.

1.  Caccamise, D.F., Reed, L. M., Romanowski, J., & Stouffer, P. C. (1997). Roosting behavior and group territoriality in American crows. The Auk, 114(4), 628-637. (Link to PDF full text)
2. Moore, J. E., & Switzer, P. V. (1998). Preroosting aggregations in the American crow, Corvus brachyrhyncos. Canadian Journal of Zoology, 76, 508-512. (Link to abstract)