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 lung

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)