Overwintering Monarchs in Mexico are often eaten by black-headed grosbeaks. These birds are immune to the glycoside. Other birds, such as orioles and jays, have learned to eat only the thoracic muscles and the body parts inside the abdomen. These parts have less poison than the rest of the body. Some mice are also immune to large amounts of the poison. Over time, overwintering monarchs become less poisonous. This makes them more likely to be eaten by predators. In Mexico, about 14% of the overwintering Monarchs are eaten by birds and mice.