Depends on your budget.
You could look at the 2nd hand market, the 1D MkIIn is still out there and going for reasonable money, still beats most of the customer offerings (canon 7D, 50D, 40D, Nikon D300, D90 for focus speed) to the finishing line.
But glass is usually the biggest hole. A canon 100-400mm f4.5-5.6 and say a 40D would set you up, but were in the £1500-2000 range. It's really the glass you attached to the camera that's makes significant difference and you can spend alot of money. Canon fall down at that stage for a fast long zoom after the 100-400mm. Next step is the primes. A 2nd hand 300mm f2.8 for £2500 - 3000, significantly more new.
A nikon D90 is an alternative, but the nikon 80-400mm (is £$%"), so an alternative lens would be needed. The sigma 100-300mm f4 would meet that requirement, or nikon 300mm f4, otherwise your into the mega $$ faster primes and the nikon 200-400mm f4 zoom.
I don't rate sigma's new budget zooms (120-400, 150-400 etc) and the sigma 120-300mm f2.8 isn't for aviation, more for motorsport.
Although your budget controls what you buy