Adult vampire bats eat only blood, while young vampire bats feed on their mothers' milk until they are three months old before then switching to a blood diet. The common vampire bat mainly eats blood from large domestic mammals such as horses and cattle, but it occasionally feeds on human blood. However, the hairy-legged vampire bat and white-winged vampire bat both prefer the blood of birds.
Vampire bats emerge at night from caves, old buildings, hollow trees and other dark roosts to hunt. When they locate prey, they land and move toward it on all fours. To locate points on their victims where blood is flowing just under the surface, they use thermoreceptors on their noses to detect heat on the skin of their prey through infrared radiation. Crawling up onto their victims, they use their sharp incisor teeth to make a cut. The saliva of vampire bats has anticoagulants that aid blood flow as they feed.
In 20 minutes, vampire bats can consume half their body weight in blood. Although they digest it quickly and begin to urinate soon after feeding, they are extremely heavy when they attempt flight. Vampire bats are the only bats able to take off from the ground. They crouch down and throw themselves up to 3 feet into the air while flapping their wings. After feeding, vampire bats return to their shelters to roost and finish digesting their food.