An "evolutionary arms race" can be seen between predator and prey species. The Lepidoptera have developed a number of strategies for defense and protection, including evolution of morphological characters and changes in ecological lifestyles and behaviors. These include aposematism, mimicry, camouflage, and development of threat patterns and displays. Only a few birds, such as the nightjars, hunt nocturnal lepidopterans. Their main predators are bats. Again, an "evolutionary race" exists, which has led to numerous evolutionary adaptations of moths to escape from their main predators, such as the ability to hear ultrasonic sounds, or even to emit sounds in some cases. Lepidopteran eggs are also preyed upon. Some caterpillars, such as the zebra swallowtail butterfly larvae, are cannibalistic.