This morning I'm walking to the bus top and I notice I'm getting annoyed by something so I stop and I realize it's the noise. All the robins and the sparrows are making this very loud, repeating "beep" sound. I look up and there's a juvenile Cooper's Hawk (most likely? My raptor ID skills aren't great and also I didn't have any scope or anything on me). Eventually it flies away and a couple of robins follow him, beeping the whole time. Just heckling the poor guy. Screaming at everyone that there is, in fact, a hawk. And probably calling him names, I assume.
Bonus picture of two squirrels coming out of their squirrel holes as soon as the beeps die down, to check out what's going on but also like, not get involved.
For more on this bird behavior, see this post and excellent art by @Ballance
(actually click through, there's an article explaining the behavior)