Sorry, just my two cents worth:
I must admit as much as I enjoy birds and birdhouses and the fact that the Purple Martin eat a ton of insects...I would not want to cause harm to a beneficial snake - the snakes keep down the rat/mouse population.
Wish it were possible to find a way to exclude the snake from the bird house without killing the snakes -
maybe put up a sign "Private Propery, No Trespassing"...
