Its hard for the average Joe to become a full fledged botanist, and its helpful to have a book with illustrations to tell you what plant to eat and not. So its good to learn a few plants that are in many regions that are edible. There are few places in the world where there isn't edible vegetation, and you don't have to hunt or trap it.
Also just because an animal may be eating a plant it doesn't mean its safe for human consumption.
Below is the plant edibility test...Copy to a word document and print it out and put in you kit to have for future reference.