Why Do Crows Gather in Huge Flocks? Winter Roosts Explained

What is a crow roost?

A roost is a place where birds sleep. During the nesting season, American crows sleep on their own territories, but in fall many begin to travel each evening to a shared roost in a stand of tall trees. Numbers build through autumn and usually peak in midwinter, then the roost breaks up in late winter and early spring as pairs settle down to breed.

Each afternoon, small groups of crows leave their feeding areas and fly toward the roost, often along the same routes every day. They first gather at staging areas such as fields, parking lots or rooftops, where they call, feed and preen. As light fades, the whole crowd moves into the final roost, settling with a great deal of noise. At dawn they leave again in streams.

Safety in numbers

The main nighttime enemy of crows is the great horned owl, which can snatch a sleeping crow from its perch. Crows cannot see well in the dark, so sleeping in a large group helps in several ways. With thousands of birds present, the chance that any one crow is taken is small, and many eyes and ears make it more likely that someone notices an owl and raises the alarm.

Crows hate owls. By day, they mob any owl they find, gathering from all around to dive and caw until it moves on. This hostility may partly be payback for nighttime raids on their roosts and nests.

Sharing information about food

In 1973, biologists Peter Ward and Amotz Zahavi suggested that communal roosts work as “information centers”: birds that had a poor day can follow successful neighbors to good feeding spots the next morning. The idea is hard to test, but it has support from studies of common ravens in Maine, where juvenile ravens that found a carcass recruited other birds from the roost to it, and the group flew out together at dawn.

For crows, whose food is often patchy, such as a newly plowed field or a fresh landfill load, learning where others are going could be a real advantage.

Why do crows roost in towns?

Over the last few decades, many crow roosts have moved from rural woods into towns and cities. Researchers suggest several reasons:

• Warmth. Cities are often a few degrees warmer than the countryside at night, an effect called the urban heat island, which saves energy on cold winter nights.
• Light. Street and parking-lot lights may help crows spot an approaching owl.
• Fewer predators. Great horned owls are less common in busy urban centers.
• No shooting. In the past, crows were shot or even blown up at rural roosts. In towns they are largely left alone.

Food is a bonus too: lawns, sports fields, trash bins and landfills provide easy meals near the roost.

Famous crow roosts

The roost on the restored wetland at the University of Washington Bothell campus, near Seattle, is one of the best studied. Up to about 16,000 crows commute there from around the region, and the university holds an annual crow-watch evening each November. Other well-known roosts have formed in cities in New York, Pennsylvania and the Midwest, some of them holding tens of thousands of birds.

Historic roosts could be even larger. In the early 20th century, some rural winter roosts in the central United States were estimated to hold hundreds of thousands of crows. Campaigns to destroy them, including one in Oklahoma from 1934 to 1945 in which explosives killed about 3.8 million crows, had little effect on overall crow numbers and were abandoned.

Do other birds roost like this?

Communal roosting is common in birds. In Europe, rooks and jackdaws share large winter roosts, often joined by carrion crows. European starlings form huge roosts too, and their swirling flights at dusk are famous; see Why do starlings murmurate? Other species, from swallows to vultures, roost in groups for similar reasons: safety, warmth and information.

Living with a crow roost

A roost of thousands of crows is a remarkable sight, but it also brings noise and droppings on cars and sidewalks below. Crows are protected in the United States, and towns that want a roost to move must work with wildlife agencies rather than harm the birds. Experience shows that roosts are very hard to shift for long; when disturbed, they often simply settle a short distance away.

For many people, the evening streams of crows across the sky are one of the great wildlife spectacles of winter. They are also a reminder of how social crows are, as explained in What is a group of crows called?