At any given time in our yard, we have at least 20 birds, up to the same number of squirrels, 15 or so chipmunks, and a few rabbits. Our many feeders and birdbaths are mostly under big trees and next to large bushes, although many of the birds and other things feed more out in the open. Needless to say they're very noisy, and it's a gorgeous cacophony!
Sometimes, all of a sudden, it will get eerily quiet out there, and when we look out, almost everyone will have disappeared into the trees or bushes.
Within a few minutes, a hawk comes through. It doesn't stop, but there have been times it will swoop down and grab something vulnerable, usually a chipmunk, or baby bunny, that hasn't taken cover in time.
A little while after the hawk is gone, everybody comes out again and resumes their regular routine.
I'm curious as to how they know the hawk is in the vicinity. Do some of them see it and somehow communicate that to the rest? Do those who see it hide and others follow suit by hiding even if they don't know why? Does it emit a sound that we can't hear? Is it a smell? In the yard, we've only ever seen one hawk at a time, so it doesn't seem like they feel surrounded.
If it was just a few birds or animals it would be easier to understand, but I'm referring to a variety of sizes, species, speech patterns, nesting behaviors and feeding behaviors.