There was a large nest near the top of a very tall poplar tree. Apparently, the crows raising their young in that nest took exception to our presence in their territory. From the amount of noise they made, a person might have thought there was an entire rookery nearby, but it was just this single nest and two very protective parents.
I did get a pretty good feel for why the lookout post way up on the mast of a tall ship is called a crow's nest, though. It's a pretty apt name.