if it is a metal door, as most exterior doors are, you could try some of the stick on magnets that you find at craft stores (provided the "bones" arent too heavy). As an alternative, you could try the sticky back velcro. This may leave some residue upon removal, but goo gone or goof off should take care of that. Another possibility is the 3M stick on hooks (if you can properly hide them. you could tie some fishing line to the back of the bone and hang the string from the hook as if hanging a painting / picture with a cable hanger on the back.