I'm no expert on snake bites- I've kept my distance from the rattlers we have here, but that looks like one to my eyes. Snakes eat eggs whole, and I'm guessing a rat would have broken it in the coop and eaten the insides, so if there's no evidence of eggshell I would guess snake. Have you...
Copperhead snakes generally have a 1'' span between fangs but that's an average so a small one could definitely be a possibility. They do eat eggs as well.