see http://medfmt.8k.com/mf/telephoto.html and
                      /third/mirror.html

basically, prices jump ALOT above default values (like 400mm f/5.6 could
cost $125 on up in a t-mount, while 400mm f/4 could cost ten times that,
not counting cost of filters and lens hood.  see mf/beat.html the lens
hood alone on nikon 600mm f/4 AF-S costs $900 MSRP for replacement - not
the lens, just the lens hood ;-)

most folks buy a 300mm f/2.8 and teleconverters ($800+ used 3rd party and
more for OEM models).

on the other hand, 500mm f/8 and 600mm f/8 T mount lenses can do very well
and cost under $100 used (I picked up 600mm f/8 with case for half that
;-). I prefer the glass lenses as noted in above URLs

I would start with the long lenses at affordable speeds, then rent the
faster ones for weekends once you know the birds habits and spots, and
decide if the extra speed is worth the high cost and weight etc. ;-)

regards bobm