Active flapping flight needs a lot of energy to maintain. This in turn necessitates an efficient and effective circulating system. Birds have evolved such a system and it is very similar to a mammal's. Bird blood is similar to ours in that it contains both red cell (erythrocytes) and white blood cells called leucocytes. The red blood cells are iron-based proteins like ours and do the work of moving oxygen around the system and taking the waste carbon dioxide away from the muscles and other organs. However, unlike ours, a birds red blood cells are nucleated, i.e. they have a nucleus where our red corpuscles have no nucleus.

Flight muscles need a lot of oxygen on a regular basis and to get it the blood must be kept moving rapidly around the system. To achieve this birds have, like mammals, evolved a four chambered heart (reptiles have only a three chambered heart). Two of these chambers are basically receiving vessels called atria, into them the blood flows at the end of its journey around the body, or to and from the lungs. The other two chambers, called ventricles, are the pumping power houses that send the blood off on its endless journey again. Thus the blood travels in a figure of an 8 as in mammals.

The oxygenated blood (red) is pumped out to the various parts of the body by the left ventricle, where after giving up its life fuelling oxygen and collecting the carbon dioxide, it returns, as deoxygenated blood (blue) to the right atrium through three large veins called the caval veins - (left caval, right caval and post caval). From here it is shunted to the right ventricle which pumps it out to the bird's lungs via the pulmonary arch where the carbon dioxide is dumped to be exhaled (breathed out) and a new load of oxygen picked up. This newly reoxygenated (red) blood returns to the left atrium of the heart via four large pulmonary veins. (We mammals only have two pulmonary veins). From here it is shunted to the left ventricle so that the cycle can start all over again. The possession of four pulmonary veins, along with the fact that a bird's heart is generally larger and more muscular per pound (or kilogram) or body weight than ours, explains why a bird's circulatory system is more efficient than ours. The left ventricle in a bird's heart is by far the largest chamber and has to work exceptionally hard in small birds which have hovering flight such as humming birds.

Some resting birds' heartbeat rates(in beats per minute)

It is a general rule in nature that smaller animals have larger hearts in proportion to their body size and faster heart rates. The relative size of a bird's heart is also affected by its lifestyle - Tinamous are flightless birds and therefore do not need such athletic hearts. Also, birds with primarily gliding flight will need less capable hearts than those that practice active flight - particularly hovering. Like us, a bird's heart rate rapidly increases when it is involved in exercise and the heart rates of small birds can easily rise above 1000 beats per minute during flight.

Heart weight as a percentage of total body weight in man and several bird species:

Most information on this page was contributed by EarthLife.