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Where do all the birds go? Every day at sunset, massive flocks of birds—hundreds and hundreds of them—fly south over the Potomac, to destinations unknown. Where are they going? Why do they go there? Please, I’d like to know the secrets of the bird commute.

The birds are likely in search of a place to spend the night. Grackles, starlings, blackbirds, and crows travel in large groups. Though they spread out during the day to eat and explore, they form enormous flocks at twilight and set off to find overnight shelter. This roosting behavior has not been well-studied, according to Eric Kershner, a wildlife biologist in the migratory bird management division of the U.S. Fish and Wildlife Service, so it’s hard to know exactly where the birds land every night. But in general, they look for a spot safe from predators and weather. “They may move up and down the Potomac, and they find a place where there’s lots of good fruits on the trees or whatever they’re looking for, and they use that area,” Kershner says. “They’ll roost nearby there so they don’t have to fly as far the next day, or if there’s no food, they just keep moving.” Just like many of D.C.’s human laborers, the birds head out of town to rest their heads and stay warm after long days of work. (And just like the daytime humans who soar in from Maryland and Virginia, they’re safe from a commuter tax.)