Grass snakes are carnivorous reptiles that eat mostly small mammals, birds, and amphibians. Their diet consists of 85-90% rodents, with the rest being made up of lizards, frogs, toads, and the occasional bird or fish. When hunting for food, they will follow prey into their burrows or homes. Once they find their prey, they will strike quickly to subdue it before swallowing it whole.
While most of their diet is composed of small animals, grass snakes have been known to eat much larger prey on occasion. One individual was observed consuming a young deer weighing over 20 kg (45 lb)! This is an extreme example though, and such large meals are not typically part of a grass snake's diet.