Originally Posted by
GrumpyDiver
Christina - it really depends on your shooting needs and style. If you are going to rack out a zoom and shoot at (or near) the long end of the zoom range; then I can certainly see going for a long prime lens. On the other hand, if you do shoot at the intermediate distances, you will find yourself limited by the lens.
I assume you are looking at the AF-S Nikkor 300mm f/4D IF-ED as it is a less expensive lens than some of the alternatives. Caution is warranted here; it is not a stabilized lens, so keep that in mind before moving off in that direction. You will have to shoot at a fairly high shutter speed to compensate for the lack of VR. The lens will work out as the equiivilent of a 450mm on a full-frame, so you are certainly looking at a lens that will be useful for bird shots. Being a f/4 lens, you could even look at adding a 1.4x or possibly 2x teleconverter on it to extend the range and still be able to use autofocus.
You could look could certainly look at some of the other manufacturers; but understand that you get what you pay for. My personal experience with Sigma is not particularly good. My wife owns the 150-500mm and it has had to go back for repairs twice. I have looked at Tamron lenses a few times, but frankly was not particularly impressed with their build quality either.