I'm not sure how large the birds you intend to shoot are, so I hesitate to recommend focal lengths, but for air-to-air stuff IS will be very helpful, aircraft not being the most stable of shooting platforms. If the focal range is right, the 70-200/2.8L IS (perhaps with a 1.4x teleconverter) should be high on your list of choices. If you need something longer, Canon makes, 300, 400, 500 and 600mm L lenses with IS. Choose the lens that will get you the framing you want within your budget, maybe a 400/4 L IS.