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Why do birds have large hearts compared with most mammals?

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A zebra finch (Taeniopygia guttata) with a bright orange beak, gray head, orange cheek patch, and black-and-white striped tail perches on a branch. — Bird A brightly colored zebra finch (*Taeniopygia guttata*) perched on a branch.

Birds possess relatively large hearts compared with most mammals to support their high metabolic demands, particularly those associated with flight. While a typical mammal's heart accounts for 0.24 to 0.79 percent of its body weight, a bird's heart is significantly larger, ranging from 0.2 to over 2.4 percent of its body weight.12

This anatomical adaptation is essential because birds maintain a high metabolic rate and have the highest oxygen consumption at rest of all vertebrates, a requirement that increases substantially during flight. To facilitate this, the avian circulatory system features a complete separation between pulmonary and systemic circulation, ensuring efficient oxygen delivery to tissues. This efficiency is further supported by a specialized respiratory system that utilizes unidirectional airflow and air sacs to maximize gas exchange beyond the capabilities of the mammalian lung.12