The issue that I have with a ball head in that situation is if you loosen it too much, it will move in 3 axis; elevation (birds, up/down tilt), azimuth (rotating around the vertical axis) and roll (left/right tilt).
IMHO, the only axis that is of value, is elevation.
Azimuth, you can get simply by rotating the monopod on the ground.
Roll to me is the worse, as that unlevels the horizon, and is harder to move the camera than elevation.
IF you can control the head movement, it will work.
I just attach the foot of the lens directly to the monopod, simpler. KISS
Birds in the air, totally different story.
This depends on how far they are from you, and how high they fly. As the elevation angle change could make the use of the monopod more of a hassle and useless. More usable when the elevation angle change is small.