I've been through a range of lenses from 17mm to 600mm various zooms etc. etc. My "aircraft kit" is now an f/2.8 80-200 and f/2.8 300, (both used with 1.4 and 2x convertors for flexibility) a 35mm and sometimes 24mm. My experience with long zooms is poor - on the one hand you have a long telephoto which is perhaps 2 stops less bright, and of inferior optical quality to a normal tele, and at the other extreme, a very unweildy and slow 50 or 70mm lens.
In a typical spotting situation, I prefer to work with 2 camera bodies - one with the 80-200, the other with the 300, possibly plus convertor, depending on the situation. Of course this means carrying a fair bit of kit which can be a pain.
Colin K. Work, Pixstel