How many chambers does a bird's heart have?

A bird's heart has four chambers, two atria and two ventricles, the same plan as a mammal's. That layout keeps oxygenated and deoxygenated blood completely separate, with one circuit sending blood to the lungs and another sending it to the body. The separation matters because it allows high pressure in the body circuit without damaging the delicate lung vessels, and it means the tissues receive blood that is fully loaded with oxygen rather than a mixture. For an animal that flies, which is one of the most energetically expensive things any vertebrate does, that efficiency is essential, and birds pair it with an unusually effective one-way lung and air-sac system. Birds and mammals arrived at the four-chambered heart independently, from reptilian ancestors with three chambers, which is a striking case of the same solution being reached twice under the same pressure to sustain a high metabolic rate.

Crocodilians are the exception among reptiles, having a fully divided four-chambered heart of their own.