Predators are often another organism's prey, and likewise prey are often predators. Though blue jays prey on insects, they may in turn be prey for cats and snakes, and snakes may be the prey of hawks. One way of classifying predators is by trophic level. Carnivores that feed on heterotrophs are secondary consumers; their predators are tertiary consumers, and so forth. Because only a fraction of energy is passed on to the next level, this hierarchy of predation must end somewhere, and very seldom goes higher than five or six levels. For example, a lion, an apex predator (at the top of its food chain, if parasites are not considered) that preys upon large herbivores such as wildebeest, which in turn eat grasses, is only a secondary consumer. Other apex predators include the sperm whale, Komodo dragon, tiger, and most eagles and owls. Many predators eat from multiple levels of the food chain. A carnivore may eat both secondary and tertiary consumers, and its prey may itself be difficult to classify for similar reasons.