It's been a while since I did any avian anatomy, but I believe the intestines function by peristalsis. There is no rectum to store the accumulated waste, no bladder for nitrogenous wastes from the kidneys. Both of these waste systems and the reproductive function are served by a common body opening, but I don't think it relies on gravity. Birds seem to be eliminating waste all the time because they don't store it in a structure like the rectum found in terrestrial animals since the extra weight would inhibit flying. I think the actual elimination is the result of peristalsis, and gravity takes over after it leaves the bird, often above the operator of a tractor, which is why some form of head cover, commonly a cap, is given away with a new tractor sale.
