Additional points to Watzman's post
1. It should be remembered that a 55mm lens provides a 1:1 object to image ratio. Thus a 550mm lens will have a tele effect of x10.
For example the Canon SX10 IS has an x-20 (28-560 mm) range which means it has a wide angle capability of x-2 and a net tele capability of x-10
2. An important point is "image stabilization" - the "IS" on your camera model. Using a tripod is not always practical - the birds are not always that cooperative.
3. It's not necessary to be over-wary about cropping. The resampling capability and other enhancing tools(e.g. in Photoshop) provide very satisfactory results.
I should recommend starting off with a good quality digicam with image stabilization.