Hi Ellen,
I would also recommend trying a stovepipe baffle to help deter snakes as well as coons, cats, and other climbing predators. I’ve included a link I found with instructions and the dimensions of the baffle. Or check with Wild Birds Unlimited and see if they carry one. Like Gin said the baffle must wobble on the pole to help keep predators from climbing around or over it. Here is the link:
Hope “your” bluebirds return soon. Keep us posted.
Dana
Lancaster, PA