Not sure when he got into my garage, but I suspect it was sometime over the winter because there were several weeks, maybe a month, I didn't go into the garage. Of course, it's also possible the neighbor's cat caught it and dropped it there, but I've seen the results of the cat's handiwork and it's more a yardful of feathers than a little corpse.
Two years ago when I was visiting Noah Fentz's haunt (The Creepy House Next Door) one of his props was a set of corpse hands reaching out between bars in a basement window. One of the hands was upturned and sitting within it was the corpse of a bird. He explained that the carcass was actually several years old and had been sitting there ever since a worker had picked it up and placed it there. He figured there was something magical about it since it never decayed.
While my wife thought I should take my robin and incorporate it into a prop, I opted for discretion and disposed of it instead.