I have a tray on the ground underneath the bird feeder to catch all the discarded bird seed. Usually, the birds will eat some of it but the squirrel will eat the most. However, the last few weeks they've only been nibbling at it, even the squirrel. So I decided to put it all back into the bird feeder, which filled it up. And the next day it's like 75% empty!
My wife acquired a "squirrel-proof" bird feeder and set it up in a tree in our yard. Turns out that the local squirrels could climb out the branch and grab seeds while upside down, hanging from their hind legs or even their tails. Acrobatic little beggars!
To be fair, she did purchase some "squirrel chow" and place some out in the yard for them. One day she left the sack of feed too close to the screened interior of the porch, and the little varmints bit through the screen to get to it. A couple of them got inside the porch and ran wild, had to be chased out. our young son noted that it looked like Chip 'n' Dale ransacking Donald Duck's house in the cartoons.