There are no native snakes in north america that can eat mature chickens but most chickens will find snakes to be good eats. Most of the stuff you hear about snakes bothering animals is based on old wives tales. In Fla there are some imported species that are very dangerous, even to humans and the venomous ones in various parts of the country but most of them won't bite animals they can't eat. I'm thinking a snake in a chicken coop is in more danger than the chickens :-)
Snakes are egg eaters and can be a nuisance to egg-seller businesses. As for being a danger to adult chickens - I would guess it depends on how lethargic sleeping hens are to ignore a possibly poisonous or constrictor snake to catch the chicken off-guard. My Blue Wheaten Ameraucana is a very wary skittish alert hen and just a creak of the egg collection door at night and she's standing bright-eyed and alert. The other hens might dope-illy blink but don't move a muscle when roosting at night - that's when we find the hens easiest to pick up for health maintenance checks. Now, during the day, I'd love to watch a flock of hens attack a snake that dares enter their domain!!!! We get a kick out of watching them chase stray cats out of the yard!