They can move around the whole yard by jumping from tree limb to tree limb... but they do get brave and run across the yard. If Gidget sees them and I'm out there, she'll look at me to see if I will scold her to leave it alone. She does chase them, and she's gotten one before.... but the squirrels can usually race up the nearest tree - six feet or so, and turn around and taunt them :~D
I bet that's fun to watch! It must drive Gidget crazy that she can't get to them!