Please do not be discouraged if a 500 mm lens isn't in your budget right away.
Some options which could bring you considerable enjoyment and good photos include use of your existing lens (in the field or from a blind) and use of a lens plus a teleconverter (which could result in some loss of image quality).
Perhaps you'd be happy with results from a long mirror lens, potentially at lower cost than a non-mirror lens of equal length. I hope you can try a mirror lens under various lighting conditions before buying that specific lens.
Please consider use of a tripod, monopod, support of a beanbag on a car's window frame, and/or other means of providing stability when using a long lens. (I have had some acceptable results with a handheld lens up to 300 mm.)
Some resources for bird photographers are
http://www.birdsasart.com/
Arthur Morris/BIRDS AS ART http://birdphotographers.net
Arthur Morris, prominent photographer of birds, has a book on DVD and an earlier book in print. Although he uses Canon equipment, much of what he writes is useful to photographers using other equipment.
Best wishes!