For example, cats smell their urine and start digging in cat litter to bury their waste. They rarely check the results of their efforts to see if they have met their goal; they simply respond in a preprogrammed way to a set stimulus, in this case the smell of their urine or feces. Grooming behavior in multiple animals provides additional examples of fixed action patterns. Birds preen themselves, cats lick themselves, and dogs shake themselves when wet. Additional examples include gnawing behavior in pigs and horses whinnying and shaking their heads.