Ok, I am going to relate this story because it happened by accident about 12 years ago. I should have known better, but I wasn't thinking as I sometimes don't.
I had been feeding birds in my backyard for some time, when all of a sudden, several large rats started showing up and eating the bird seed that had made it's way to the ground. Knowing that where there are rats, there are going to be snakes, I quickly devised a plan. Went to Walmart and bought some D-Con rat pellets and sprinkled them all along the edge of the house where the rats were coming into the backyard.
Well, about 3 days pass, and I am coming home one afternoon, and I catch 3 rabbits sunning themselves in the middle of the street right in front of my house. Prior to that day, we had a lot of rabbits in the neighborhood. After that day, it was like they all moved away.
Moral of the story is, don't put any rat poison on the ground, unless all you want in your backyard are songbirds.