Apparently swans (and similar) birds have long necks to provide a constant amount of re-breathed air (eg a built-in inefficiency threshold).

Because their lungs are tied to their flight muscles, they would overbreath at cruising speed. So the long necks provide just the right amount of re-breathed CO2 to offset this.

(I hope I have this correct)

Some falcons have nostrils that are designed to slow down the air flow so that they can continue to breathe while diving at very high speeds. Even though they exercise hard to catch their prey they aren’t winded as a result. Unlike say cheetahs that are completely out of breath at the end of a chase and have to breathe for a bit before they can eat..

'pushing the envelope' is actually a term from the bird-racing nerds community for packing those things while staying aerodynamic