Backyard and Beyond

I’ve been watching squirrels rush along mid-building parapets and window casements to get to this spot all winter.
Thought it was a nest, and voila, four youngsters! Parent on the left in these pictures. I gather that there’s plastic covering a small A/C unit here. The outer lining, open at the top? Don’t know what that’s about. There must be space between the window and the A/C unit, which is presumably where they nested. The young ones have been tearing around here, but only yesterday did they venture as far at the screen window, and underneath this whole set up.
Note the House Sparrows on both sides in the lower corners. There are also bird nests under this cage, on both sides. It was scolding sparrows that first alerted me to the rambunctious squirrels.