Some instinctive behaviors depend on maturational processes to appear. For instance, we commonly refer to birds "learning" to fly, because they cannot fly at first, but can a week or two later. However, young birds have been experimentally reared in devices that prevent them from moving their wings until they reached the age at which their cohorts were flying. These birds flew immediately and normally when released, showing that their improvement resulted from neuromuscular maturation and not true learning.