They’re so good at not looking in the windows as they walk by, not intruding. (I imagine their job depends on it.)
We still have utility workers who come to read the meters once a month.
Mine has to walk past the birdfeeder. 
It was a young guy this time, someone I hadn’t seen before, and as he walked studiously forward he couldn’t help but look up–and he cracked up.
Just a small slit in the bottom for the hanging chain to thread through; it’s been working perfectly for awhile now. The squirrels look down, they see the inside, and they know they can’t climb their way out of a paper bag.
(p.s. If you get a chance to see it: at 7:15 am California time, Glenn Stewart will be rappelling down to the nestbox area from the roof of San Jose City Hall and banding the three young peregrine falcons Tuesday, while their parents, who can dive at 241 mph, will be swooping at the guy’s hardhat-covered head. If you want to see it live, the cam feed is here.)