Telephoto lenses can be an especially useful tool with bird photography. A
telephoto lens zooms in your cameras lens and can allow you to get much closer
to your subject. A photograph taken with a telephoto lens can look like your
standing beside the bird when it is really fifty feet in the air.
Telephoto lenses are usually only sold for higher end cameras and are sold in a variety of different zoom lengths. In many cases a photographer will have several lenses available when they go out on a photography shoot. They can then change the lens on their camera to accommodate their location, and the photograph they would like to take.
When you go to purchase a telephoto lens for your camera it might be a good idea to take the camera itself with you. In many stores you may be able to attach the lenses you are looking at to your cameras body and get a feel for how they will be on your camera as well as what the zoom will look like. Telephoto lenses can be heavy especially ones with higher zoom numbers so it can be a good idea to get a feel for what the lens will actually feel like on your camera (and if it will inhibit your photography) before you buy.
Telephoto lenses can also be extremely expensive. Depending on the type of camera you have, you may be looking at spending as much o a camera lens as you did on your entire initial camera purchase. However, the end result or quality bird photographs will be worth it.