A bit more than a flight simulator. This program simulates being a bird. You can choose to be a gull or a hawk or Canary. As a Canary, you have to flap your tiny wings, madly and look for bugs and worms. Flitter down and devour them to maintain your energy level. Drink from puddles. Try to hide
from the Birds of Prey.
As an Eagle, search out canaries, soar, calculate thermals. Perch on mountaintops and phone poles looking for prey in the air, and on the ground.
Some basic flight controls are the same as a standard flight simulator, but there, of course, are no engine controls. Instead, you have "flapping" controls. Bird flight dynamics would have to be simplified since an actual bird can change the shape of it's wings, continuously and nearly infinitely. They can also dynamically alter the delivery of force (flapping) as lift or forward thrust. Might work well with a 3D controller. One might have to create a new controller altogether.
Environments would have to be varied. Some areas of suburban housing with roofs and power lines to perch on merge into forest areas, and coastal zones and mountainous Eagle and Condor domains.
I think it would be cool, but would probably fail to be a good seller, after years of elaborate development.