Hello,
First, let me offer that if the birds are too tiny in your images when you shoot 35mm. film, they will still be too tiny when you shoot images with a DSLR. Yes, the image is "cropped" but it's also basically smaller. I see it as akin to taking a 35mm. negative and clipping away the edges. The bird in the center is still tiny, despite the cropping. Yes, the sensor size/number of megapixels complicates this comparison, but trust me, the 300mm. WILL be too short for your bird photography, so you need to consider something longer.
What you end up purchasing is going to depend, almost totally, on your telephoto budget. Given unlimited funds, something like a 500mm. f4 AF-S lens would be ideal, especially if coupled with high-quality teleconverters. But I'm guessing you don't feel like shelling out many thousands of dollars, at least to start.
Another very attractive possibility is an 80-400mm. VR zoom lens, either Nikon's or Sigma's. These are very convenient and they produce good images, and the vibration reduction mechanism helps you handhold pictures in the field. But these, too, are fairly expensive.
As Andy suggests, you could get a 1.4x teleconverter for your 300mm. lens and you'd have a decent 420mm. setup without spending a lot of additional money. Another possibility is to scour the used lens ads at ebay or elsewhere and get a high-quality 400mm. f5.6 tele. I have always liked the Tokina ATX SD AF tele, which is compact and which autofocuses surprisingly quickly. The problem is that you would have to send the lens back to the distributor for "rechipping" at a cost of $65. Ugh. The Sigma Macro APO 400mm. f5.6 lens is also good and it focuses closer, but it's larger and will cost a bit more.
A more "controversial" solution is to purchase a good quality 500mm. or 600mm. mirror lens. Most bird photographers hate these because they are slow and the produce doughnut-shaped out-of-focus highlights. But if you look for a good used say, Tamron 500mm. f8 mirror lens or a Sigma 600mm. f8 mirror lens you will have a very decent optic with lots of telephoto power for a surprisingly low cost. These lenses are very difficult to use much of the time because they are "slow." You need to use very fast film and/or have very strong sunlight to work with. However, the advantage you get with digital is that you can ramp up the ISO setting to 800, 1000, or even higher and you still can get very good results. I have used a 600mm. f8 mirror lens handheld in the field using ISO 1000 frequently and have ended up with quite satisfactory results. If your images end up a little "grainy" due to digital noise you can ameliorate this by using an inexpensive noise reduction program like Noise Ninja or Neat Image.
Other disadvantages of the mirror lens are that you must focus manually and you lose all automatic light metering. Yikes! However, the latter is not really as much of a problem as some people indicate. Because you can check your images and/or histograms immediately after exposure, you can easily use trial-and-error to figure out optimal exposure of your images in the field. What I have done is to shoot some preliminary images of objects that "simulate" the coloration/light level of my birds, and then get a range of appropriate exposures in my head as I approach my birds. This is not that hard. And the truth is, when you use autoexposure with a DSLR, you constantly are making exposure adjustments anyway, since images containing white birds have to be "underexposed" and images containing black birds have to be "overexposed." You end up doing a lot of mental calculation and adjustment in digital bird photography in any case, so setting manual exposure is just one more step along this path.
No, the mirror lens is NOT the equivalent of using a high-quality, fast non-mirror telephoto, but it's a very inexpensive way of getting started in bird photography and when coupled with your DSLR you can get good results.
Finally: one option I would avoid is a cheap long tele zoom such as the Tamron 200-400mm. or the Tokina 80-400mm. These lenses always compromise telephoto quality at the long end of their zoom lens, which is exactly where you will be using them the most. On the other hand, many users seem happy with the slightly more expensive Sigma 170-500mm. and 50-500mm. zooms, so these might be worth investigating.
Something like a 300mm f2.8 can be very useful (with the in-camera 1.5x and maybe a tc) if you intend doing more evocative in-flight shots of larger birds (wildfowl etc). For close up portrait style shots of smaller songbirds, get every mm of focal length that you can, you can get very close to the subject but even then, a bird like a Goldcrest will need a big lens + extension tubes (to reduce the lens minimum focus distance) and still maybe a teleconveter.
