A good example of the associative learning called operant conditioning in the wild is the interaction between monarch butterflies and certain birds. For example, as part of their life cycle, female monarch butterflies lay their eggs exclusively on milkweed plants. After a few days the eggs hatch and a yellow-, black-, and white-striped caterpillar emerges from each egg. These caterpillars are totally dependent on milkweed plants. Although the plants contain toxic substances (cardenolides) that are poisonous to other animals, the toxin is harmless to the monarch. Blue jays spend much of their day searching for food and will often eat insects such as adult monarch butterflies to supplement their otherwise-vegetarian diet. However, if the food tastes bad, the blue jays will vomit up the food and will then learn individually to avoid the food in the future. Thus, wild monarch butterflies with high levels of cardenolide concentrations are less susceptible to natural predation by birds—in other words, a good example of operant conditioning in the wild.