Every summer, I find snakes in the chicken tractors. So far, the only ones I have seen are black rat snakes. They're fairly ubiquitous here, and since they're black. in the gloom at the back of the tractor, they're hard to see. They can be long--three feet or more.
Usually they're in the grass, and I see them when I move the tractor. This year, they're proving to be more assertive--and numerous. I have found two in a tractor on a couple of occasions. Sometimes they have even been in the nest box.
The one pictured above was in the Delawares' tractor. The egg at the top left is a ceramic one, but you can see that the snake has eaten a real egg--look for the bulge just behind the S-curve near the head. I don't know how intimidating this is to hens that are trying to lay, but it is also true that on days I find snakes, I find fewer eggs. The birds are either holding off or the snake is eating them. This is obviously sub-optimal from my perspective, although it works out well for the snake. An egg is an easier food source to exploit than rodents, who have to be caught and killed. I don't think they pose any danger to adult birds; although they might be able to kill one, the birds are far too large to eat.
However, the roosters have a role to play, and they aren't stepping up. I have never seen any indication that the roosters care about the snakes. Earlier this week, a snake slithered out of the tractor when I opened the door, and the rooster, who was nearby, glanced at it and kept walking--he stepped on the snake, in fact, not to try to restrain or attack it, but just because it was in his way. Another rooster looked at a snake that was slithering away in the grass, but made no aggressive moves.
Snake sightings seem to be grouped, where I'll find a snake (maybe the same one) in a tractor several times on consecutive days, then no snakes for weeks or months. It's a fairly low-level problem, but one without an obvious solution. A tractor will always have potential points of entry that a stationary coop might be able to be proofed against.