Ruby-topaz HummingbirdChrysolampis mosquitusCredit: Luis Mazariegos
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Hummingbirds have the highest oxygen requirements of all vertebrates. To transport oxygen to their vital organs, they have the highest density of red corpuscles, about 6,590,000 per cubic millimetre for the Ruby-topaz Hummingbird (Chrysolampis mosquitus). Its respiratory system, composed of two symmetrical lungs for gas exchange and nine minuscule air sacs for ventilation, is adapted to use a large volume of oxygen. At rest, its breathing rate is about 300 breaths per minute, but it may rise to 500 breaths per minute during flight. In comparison, starlings and pigeons have a breathing rate of 30 per minute, and humans 14 to 18 per minute. The volume of air with each breath is between 0.14 and 0.19 cm3 for hummingbirds, 100% higher than mammals of the same size, such as the shrew.