I never notice birds flying with their beaks ajar - not even when they're zipping around all over the place, or flying against the wind, or traveling for miles at a time (like those jerkwad Canada geese when they fly south for the winter). And yet if I'm out walking and the sidewalk starts to slant uphill even a little bit, POW, I gotta start breathing through my mouth.
What's up with that?
Edit: The Boy and I have decided that the velocity of flying must push air right up a bird's nostrils. Then the air flows right through the bird and out its ass - the whole wing-flapping thing is to help pump the air out like a bellows. SCIENCE!