1) Why does a bird require energy to get "up there"?
To get up there a bird has to overcome the pressure with its own density. Because it has big wings, it can spread them our against the air pressure underneath and push lots of it out of the way which forces it up against the air above. It does this by closing its wings and straightening it's beak to look like a missile so it can overcome the pressure and friction from above. The second that friction slows it, it's wings come back out like an oar on a rowing boat where the wing twists to slice through the above air pressure, then twists again to make it's volume larger for the next push, as so on.
The smaller winged birds do the same only extremely fast.
1) Why would it require less energy to go back. Or, to make you less nit-picky, why would it require less energy to fly from a perch on a cliff to the base of the cliff.
The opposite of what I said about flying upwards. The bird can parachute down by holding its wings out and spreading its feathers, then it can use the force of the air against it by gliding down slowly by the same twisting motion using the friction under its own density with the pressure above it to play one against the other to create a lift then fall and lift then fall, or it can use any wind against it to create a lift and hover. It depends on many variations.
If it wanted to simply get to the bottom quick, it just slices it's wings through the air resistance below it by folding them back until it wants to land, then opens them right up to prachute down to the bottom.
A sky diver with a parachute will do the same thing in a fashion, except a sky diver is much denser. If he/she wants to go faster downwards, the fold their arms down their sides and point their heads down to create less resistance against their mass.
2) No air was displaced below the bird or the crap. The bird flew from 15,000 FT to 5000 FT. The only air that was displaced was air above and behind the bird.
The bird and the crap displaced the air. Once you pick up any object and put it into the atmosphere, that object displaces air. If you use energy to put something into the air, the air will push it back, always, because you made it unequal, you put an action into force which requires a reaction. That reaction is against the object and you, until you release that object, then the reaction is against that object, always.