That makes sense. CPUs and GPUs convert most of their energy use into heat. May as well do something slightly useful with that.
No idea what the numbers are now, but last winter, I think my 3080Ti cost ~$2/day to run and generated ~$1/day in crypto.
From my limited understanding, you should get rid of the post since the birds you want don’t actually use it. Also make sure the hole is the right size for the birds you want to nest there. Lastly, you should break up house sparrow nests if it looks like they are building one.