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At other times eggs have no meaning to them. The switching on and off of these programs often involves complex inborn releasers and timers. In birds, preparations for spring migration, as well as the development of sexual dimorphisms, territorial defense, and courtship behavior, are all triggered by the lengthening period of daylight. This alters hormone levels in the blood, thereby triggering each of these dramatic but essential changes in behavior. In general, however, no good explanation exists for the way in which motivation is continually modulated over short periods in an animal's life. A cat will stalk small animals or toys even though it is well supplied with food; deprived of all stimuli, its threshold will drop sufficiently so that thoroughly bored cats will stalk, chase, capture, and disembowel entirely imaginary targets. This unaccountable release of what appears to be pent-up motivation is known as vacuum activity-a behavior that will occur even in the absence of a proper stimulus. One simple mechanism by which animals alter their levels of responsiveness is known as habituation.

Habituation is essentially a central behavioral boredom; repeated presentation of the same stimulus causes the normal response to wane. A chemical present on the tentacles of its archenemy, the starfish, triggers a sea slug's frantic escape behavior. After several encounters in rapid succession, however, the threshold for the escape response begins to rise and the sea slug refuses to flee the overworked threat. Simple muscle fatigue is not involved, and stimulation of some other form-a flash of light, for instance-instantly restores the normal threshold . Hence, nervous systems are prewired to "learn" to ignore the normal background levels of stimuli and to focus instead on changes from the accustomed level. The fourth contribution ethology has made to the study of animal behavior is the concept of programmed learning. Ethologists have shown that many animals are wired to learn particular things in specific ways at preordained times in their lives. One famous example of programmed learning is imprinting. The young of certain species-ducks, for example-must be able to follow their parents almost from birth. Each young animal, even if it is preprogrammed to recognize its own species, must quickly learn to distinguish its own particular parents from all other adults. Evolution has accomplished this essential bit of memorization in ducks by wiring ducklings to follow the first moving object they see that produces the species-specific exodus call.