Hi Brian,
You are finding out the hard way, the problems of stepping into DSLR ownership. I have a D80, and believe me, it is a fantastic camera, and regardless of what anyone else might say, it will deliver exceptional photo's.
As you rightly say though, a 135mm lens isn't going to get you near enough for most off-the-cuff bird photos. I have the Nikon 70-300 lens which again is a fantastic lens, if used within it's limits, but even that isn't too good for birds. (Luckily, I'm more into wild flowers & fungi) so I don't need extreme focal lengths.
Due to the present economic climate, (and some blatant profiteering if you ask me
), the cost of the Sigma 50-500 has practically doubled in the last two months, and nowadays you would be lucky to get one for less than £1000.00. Realistically, if you don't have a big budget, you will be hard pressed to get anything longer than 300mm.
This very problem, is one of the reasons why I have always advised people to go for a bridge camera, unless they want and can either afford a full DSLR kit, or unless they are prepared to compromise on what they can shoot.
A top end bridge camera costs about £350.00, and will give you most of what a DSLR can do (again - with the proviso of using it within it's limitations), with the advantage of having a lens offering everything from super macro, up to 400mm zoom on many models, and the equivalent of over 600mm zoom on the new Nikon P90 bridge. (I must admit, unless I'm on a dedicated photography outing, I still often go out just with my Fuji S9600, and leave the D80 kit at home).
Don't be too disheartened though, If you use your D80 plus whatever lens you can afford to get, within their limits, you will be very impressed.
Best regards
Mike.