As it happens, the common American crow does the same thing.
Around this time of year, all over the country, huge swarms of crows flock across the skies and mob the trees at city parks every afternoon and evening. Here in Portland, they seem to start around the northern end of the Eastern Promenade, then they flock over to Lincoln and Deering Oaks Parks before settling in for the night near the new Mercy Hospital buildings near the Fore River.
A few months ago I started working at Maine Audubon, and I asked the staff naturalist, Eric Hynes, what these crows were doing. He told me that, essentially, they're just socializing. While crows are fairly territorial in the summer, in the winter, when food is scarce and predators are more of a threat, they prefer the company of other crows. Thousands of other crows.
It's not all for fun: "They get together every afternoon and check each other out," said Eric. "They might say, 'hey, that guy looks fat and happy today, we'd better follow him and find out where he's getting his food.' Or, 'that one looks sick and scraggly, stay away from him.'" When it gets dark, they bed down together by the thousands to provide safety in numbers against predators.
Crows have also figured out that they're safer in cities, where their biggest nemesis, the Great Horned Owl, is less likely to venture. As a result, these wintertime social flocks tend to gravitate towards urban areas.
To see them for yourself, head out to your local city park this afternoon around 4 pm and walk towards the noise of a thousand crows cawing.