Mammals and birds have four-chambered hearts. However, birds have much larger hearts in proportion to their size than mammals do, and their hearts pump more blood per minute than mammals of the same size.
The heart is a muscular organ that pumps blood through the circulatory system of an organism. Blood provides bodies with oxygen and nutrients while also ridding the body of metabolic wastes. Four-chambered hearts consist of the upper left and right atria and the lower left and right ventricles. The left atrium receives de-oxygenated blood from the body and pumps it into the left ventricle, which pumps the blood to the lungs; the blood is oxygenated and then enters the right atrium. The right atrium pumps blood into the right ventricle, which pumps blood through the body.
This system creates two loops from the heart: one to the lungs and one throughout the body. Because of the force required to pump blood through these loops, ventricles have thicker walls than atria. This is especially true in birds. Reptiles and amphibians often have three-chambered hearts, which are also organized into two loops. Fish have two-chambered hearts, and worms and other very simple organisms have single-chambered hearts. The hearts of fish as well as worms and similar organisms only pump blood in one loop.