Why Do Crows Flock in Large Numbers?

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Head outside about an hour or two before night in the fall and winter and you may be greeted by a large and noisy bunch of crows gathering together. You won’t see this behavior happening in the summer, so why does it happen during the cooler seasons?

Crow is the common name for a wide variety of species under the genus Corvus. Known collectively as corvids, this group of medium to large-sized birds includes crows, ravens, jays, and magpies.

Crows can be found on all continents except for Antarctica. The American crow is widespread across the United States and across parts of Canada and Mexico, giving residents both urban and rural areas access to this particular wintertime roosting behavior.

During the late spring and summer, crows hang out in small family groups. Usually between 2 to 8 in size.

Crows sit in a nest under a bridge near Polychrome Pass, Denali National Park.  Photo:  NPS Photo / Emily Mesner, public domain.
Crows sit in a nest under a bridge near Polychrome Pass, Denali National Park. Photo: NPS Photo / Emily Mesner, public domain.

Why Do Crows Gather in Large Groups?

As temperatures get cooler, crows form large groups that number in the hundreds and even thousands. Despite this common phenomenon, not much has been studied as to why crows gather in such large numbers.

But experts in corvids have some hypotheses.

Some researchers suggest that large groups of crows provide a number of benefits. Roosting in large numbers provides heat, safety, and better access to food sources during the winter months.

The gathering of large numbers of crows to sleep together at night is called a “roost”. Crows typically roost in trees that are sturdy enough to hold large numbers of these birds.

Sleeping close to other warm bodies makes it easier to fend off the winter cold.

Sleeping together in large numbers is believed to provide collective protection against predators.  Great horned owls, red-tailed hawks, peregrine falcons, and eagles are all predators of crows. Roosting together sets up a “safety in numbers” that discourages attacks by these raptors.

Gathering in large numbers helps crow to find sources of food by using the collective wisdom of the group,

Where in the Trees Do Crows Roost?

There are optimal spots within trees to roost. Roosting too far to the top makes the crow vulnerable to be picked off by a predator.

Crows that sleep on the lower branches of the tree often are covered with bird droppings released by their neighbors higher up in the trees.

Research hypothesize that crows have a pecking order that determines which crows get the choice spots and which crows end up white with feces in the morning.

Crows are social animals. Some researchers believe that roosting is also a form of socialization. Crows gather in large numbers to communicate food sources and to establish breeding partners for the spring.

How Do Crows Roost?

Crows don’t immediately find a suitable tree for sleeping. Observers have reported that about two hours before nightfall, crows start gathering close to their roosting site.

As the light of the day beings to weaken, crows start to gather near their roosting spots.  Photo: Caitlin Dempsey
As the light of the day begins to weaken, crows start to gather near their roosting spots. Photo: Caitlin Dempsey

This period is a noisy time, with plenty of crows cawing as they jostle each other. Playing, fighting, and foraging for food are all activities that occur during this period.

As night begins to descend, the crows will start to move from their spots to the actual roosting spot or spots for the night. Crows will find their sleeping spot on branches on the tree.

With larger roosts, crows may be spread out over many trees in a clustered area.

While most roosts number in the hundreds or even thousands, one survey done in 1971 estimated a roost a Fort Cobb, Oklahoma to be two million (Gerald Iams, 1972, State of Oklahoma Upland Game Inventory W-82-R-10).

Roost have been observed to happen in both rural areas and in cities.

Crows Don’t Always Roost in Trees

The crows at the University of Washington, Bothell actually roost in a nearby restoration area in northern Seattle. John Marzluff, a professor of Wildlife at the university explains that the crows use the rustling sound of the willows to warn against predators at night.

The experience of seeing large numbers of crows gathering to roost is quite spectacular. This video showing the gathering of crows at nightfall at the University of Washington, Bothell. The hundreds, and perhaps thousands, of crows collect on rooftops and in nearby trees before their nightly roost.

References

Frequently asked questions about crows. (n.d.). Cornell Lab of Ornithology—Birds, Cornell Lab of Ornithology : Birds, Cornell Lab of Ornithology. https://www.birds.cornell.edu/crows/crowfaq.htm

Lindholm, J. (2015, November 25). Something to crow about as flocks gather by the thousands. Vermont Public Radio. https://www.vpr.org/post/something-crow-about-flocks-gather-thousands#stream/0

Where do Seattle-area crows go at night? (2018, October 24). KUOW – KUOW Public Radio. https://kuow.org/stories/where-do-seattle-area-crows-go-night/

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