Don't forget to look area the area you live for a nearby wildlife refuge. Many times they will have blinds set up for bird watchers, and you can use it too! Also, to get an idea of what the bird would look like on a print/slide consider using a pair of field glasses to see what magnification you might need. If you like the way something looks with 8x field glasses, then you will need something around a 400mm lens, 10x - 500mm. Oh, and don't forget that you will want the fastest lens you can afford - unless your eyes are better than mine..I can't seem to see much of anything anymore.
Check out Elliot Porters bird photos, he did some of them using 4x5 with flash, triggers and blinds. Some of them are amazing.