70-300 mm on a crop body is enough, though you might want 400 on full frame. You should look into a waterproof option too, and be careful about weight - in and out of GPS has a very low weight limit, because aircraft carry their fuel for a round trip journey and cargo is at a premium (everything comes in and out of what was originally an old Army Air Force base).
You don't say if you're Nikon or Canon, but on the Nikon side of the fence, the new 80-400 mm is a much better lens optically than the 70-300 (which is also very good), so you may want to consider it for other reasons.