This story was originally published on December 5, 2014.
During November and December, people in many U.S. cities may notice large flocks of American crows congregating in raucous urban roosts. Here’s the lowdown on why they’re here, and some insight on what they’re up to.
LISTEN: Crows Come Home to Roost
The large flocks of crows in our region now are primarily migrants from more northerly locations that are here to spend the winter. The number of roosting crows tends to build up steadily through November and December. These large winter roosts were historically in rural areas. But over time, as crows adapted to people, they moved their winter roosts into urban areas. Here they benefit from the warmth of the city. They are attracted to well-lit areas, which may enhance their ability to detect approaching predators. Their roosts can number in the hundreds of thousands of birds.
Roosts are most common in medium-sized cities with easy access to rural lands. The crows roost in the city during the night. Then at dawn, they scatter, often flying more than 10 miles to feed in fields and neighborhoods.
In mid-afternoon, the birds gather in pre-roosting groups. As the day progresses, flocks band together, forming larger and larger flocks. Just before dusk, the assembled group disperses. The flocks form lines across the skies. It brings back perhaps an idea of what large flocks of the now-extinct passenger pigeon might have looked like.
Crows recognize “bad” humans—and talk about them
Crows are known for their intelligence — just ask anyone who’s tried to get crows to move away from their roosting sites. In one city, workers wearing bright green vests shot off noisemakers to scare crows away. After a time or two, the birds recognized the green vests and flew away as they approached. But as soon as the vested individuals left, the birds flew right back.
Crow behavior was even more remarkable in another study. Researchers found crows could communicate to flock mates what human predators look like. The researchers wore masks with distinguishing caveman facial characteristics while they banded crows. When the masked people returned near the roost, the crows that had been banded — and the crows that had never been exposed to the researchers — noisily scolded the people wearing masks, but not those without masks.
WATCH: Yes, Crows Can Recognize You
Locals hang with the visitors
Crows call as they fly into the roost, perhaps encouraging their compatriots to join them. As dark settles, they crowd together. They roost in dense clusters in trees on urban streets and parks. These migrant crows — winter visitors to the area — are occasionally joined by some of the “townies.” These are the resident crows. They include crows that might have nested in local backyards over the summer.
Research has shown that the locals who join the roost tend to be the teenagers. They’re birds that may have helped mom and dad raise young but don’t have their own home territory. The older birds tend to stay at home and do not go down to the roost.
Crow roosts are considered “traditional,” meaning they occur in the same areas year after year. Younger birds learn roost locations from older members of the flock. In late February or early March, the roost breaks up as crows depart and migrate north for breeding.
So next time you see lines of crows darkening our skies at night, take a minute to marvel in the wonder of these smart, social birds.
Bird calls for this segment were provided by The Macaulay Library at Cornell Lab of Ornithology and recorded by Geoffrey A. Keller, Arnoud B. van den Berg, Michael J. Andersen and Wilbur L. Hershberger.
Top photo: Tim Spouge via Flickr