Apparently I didn't put out the birdseed on time yesterday morning. When I opened the back door onto the screened porch, there was a squirrel on the porch!
I'd noticed a few days ago that part of one screen had a few holes in it near the bottom. Now I know why. Apparently those holes were the size of a squirrel's nose. Yesterday they expanded to the size of a squirrel's body -- complete with a nose that can smell birdseed!
The squirrel dashed around the perimeter of the porch a few times and then scooted back outside the way he/she came in.
Now the birdseed is inside the house and the hole is blocked with duct tape.
No harm done, and both of us have a story to tell!