Wild bunnies are herbivores and eat all kinds of vegetable matter, including leaves, roots, plant stalks, and even flowers, nuts, and seeds. Unlike similar woodland herbivores, such as squirrels or rats, bunnies do not store their food or bury it for later eating. Bunnies must forage daily for their food, eating whatever they find.
This means their diet in the wild has a great deal of variety, as they must be opportunistic feeders.
However, a large portion of a wild bunny's diet is grass, as it is readily available in most fields and woodlands. The fibrous grass keeps a bunny's digestive system in working order.
To get the most nutrition from the grass, bunnies will ingest their own waste after eating grass so the food will get a second pass through their digestive system. Specialized bacteria will use the re-eaten grass to produce nutrients via fermentation. This changes a food poor in nutritional value aside from a few trace vitamins and fiber into a nutritionally rich food that gives the bunny all the vitamins it needs to live.
Because they have adapted to being dependent on a diet high in fibrous food like grass, wild bunnies do not eat much in the way of sugar or carbohydrates. Although the popular image is of a bunny munching mostly on carrots, carrots are a root vegetable packed with sugar and starch. Eating such a sugar-heavy diet would make any rabbit, wild or domestic, ill.