Many omnivores change their eating habits during their life cycle. They are sometimes called "life-history omnivores", because they are only omnivores if their whole life is considered.
Some species, such as grazing waterfowl like geese, are known to eat mainly animals at one stage of their lives, but plants at another. Also, many insects, such as the beatle family Meloidae, eat animal tissue when they are larvae, but eat plant matter after they mature.
All of these animals are omnivores, but have different feeding behaviors and favorite foods. Being omnivores gives these animals more food security in stressful times. It also makes it easier for them to live in less consistent environments; (those that change along with the season, for example).
Omnivore Facts for Kids. Homework Help - Kiddle Encyclopedia.