The woods behind the house are silent where they, until recently were alive with chatty and playful squirrels. I think it's up in the air. We are not seeing squirrels because, as one correspondent hinted, they have a better supply of food elsewhere. And that's up in the trees. But think about it.
Squirrels live in trees.
They come down to the ground only when necessary to get food. Right now, it's not necessary at all. It may look easy as you watch squirrels scoot about trunks, but climbing up and down trees is a lot more work than climbing across trees. Test it out. Stand underneath a good-sized oak or hickory, and see if you don't notice pieces of nutshells falling down through the leaves to the ground.