Birds, for example, are predominantly nose breathers, aside from diving birds such as penguins, pelicans, or gannets. Generally, when an animal breathes through its mouth it is a sign of sickness, injury, or distress. Guinea pigs and rabbits will continue to breathe through their noses even under heavy exertion and will only breathe through their mouths if they have developed a breathing abnormality. The same goes for all farm animals, including the cow, sheep, donkey, goat, and horse.
