> The mammalian lung has reciprocating ventilation with large terminal air spaces (alveoli) while the avian lung has a flow-through system with small air capillaries. As a result the environment of the pulmonary capillaries is very different between the mammals and birds.[1]

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2681264/#:~:tex....

Water is a much more efficient heat exchanger than air.