Hi
I think that all you need is reach.
I am assuming you will be shooting in good light and not needing rapid auto focus.
With the D90 you can pretty much crop in half and still have a useable pic if not printing huge....so...find the longest lens you can afford and use a tripod if necessary.
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If you could afford it maybe something like a 300 2.8 manual focus lens (still pricey though) and a couple of extenders...1.4x, 1.7x and 2x maybe even a 3x one though IQ would be degraded it should still be "ok".
Maybe get something like a 300mm f4.5 and a 2x converter to match.
Lesser but maybe still ok would be a 500mm mirror lens ....you can buy "1000mm " kits on ebay...mostly a 500mm mirror lens and 2x converter...will not be the greatest but might be what you want.
Maybe have a look at the likes of ebay or pawn shops or Photographic trader magazine($4.75 at most newsagents)....get a cheap mirror lens second hand and have a play....it will get expensive if you want auto focus, speed and IQ and reach....take away the ones you do not NEED first (from what you have posted that would be auto focus first to go reach the last)....your compromises will be for lens speed and IQ.... though generally the two go up and down together sometimes they do not...
How much you are prepared to pay will determine those things.
You can get some 500mm auto focus slow super zooms as stated and they will be good IF 500mm is enough.....I suspect it will be enough SOMETIMES but others will still not be long enough for you.
IF it is not long enough a lot of the time , then a 300 2.8 manual focus or slower if needed budget wise 300mm prime with extenders would be the way to go. (Tamron adaptall 300 2.8 can be had for under $1000 AU ...well under if lucky and often comes with 1.4 and 2x matching converters for 420mm f4 and 600 5.6). It is a lens better suited to cameras that are stabilized..........I use mine on my Pentax K-x but not on my Nikon d50 but for YOUR uses it would be fine (and hold its value if not suited to what you want BECAUSE it works so well on Pentax stabilized so you could get your money back...maybe even make a profit).
Good luck
neil
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