I know this post is old, but seeing as someone has brought it back up I really need to dispell the information here.
Garter snakes are completely and utterly harmless to your chickens. Their diet consist of frogs, toads, earthworms, slugs, fish, and sometimes (rarely) pinkies and mice.
They have no interest in your chickens, you, or your pets. Their bites are not painful at all. Some people can be mildly allergic (like me) but the worst you'll get is a small rash.
Do not kill them and just let them go on their way please.