There was a whole murder of crows in the cornfield across the highway from our house a few weeks ago. They were doubtless all gathered for migration. It was a LOT of very noisy crows.
Edit: And if you see a really huge crow, it might not be a crow; it might be a raven! Not that there aren't also just really big crows, too, but anyway, just the ramblings of a bird nut. Carry on!
A murder of crows is scary. Not just because of the name. When I was in college, there were a few months one year when the crows (American crows...entirely black and about the size of a pigeon) found something particularly tasty in all but one tree on campus. So they roosted in the trees. Going to class became dodging bird crap and preparing to run in case one or two took offense to people passing too near. People were often chased.
The trees were black with them. And there was constant rustling in the leaves. They didn't caw as much as one would expect (and sometimes sounded a bit like squirrels, for some reason), but when they did it usually meant someone was going to get chased soon.
That was also the year I saw a hawk-or-falcon-like (I was too freaked out and late for class to really study it so I could figure out what it was) bird feeding on a chipmunk on campus. It's the first and only time I've ever seen a bird like that so close and actually feeding. BUT, I knew it wasn't anything I would know on sight as it was way too small for any of the birds of prey I was used to seeing.
Since then, I've understood a certain Alfred Hitchcock movie a little better. Though I would have been more frightened if all the crows were blue jays. They're pretty, and all, but I had some words with a couple when I was a kid. They took over a robin nest in a tree in our backyard one year and that pretty much kept other birds (and a couple of my friends) away.