I use a 400 mm lens, which is usually considered the minimum acceptable focal length for serious bird photography. With a 400 mm lens, you must be about 10' away from a bird the size of a cardinal to get a frame-filling image. You are very, very rarely 10' away from a wild bird, and when you are, it's usually not for very long. Even for larger birds such as herons and egrets, you still have to be quite close to the bird to get a frame-filling image. Really devoted bird photographers typically opt for 600-800 mm lenses, often used with teleconverters, and still complain about not having enough focal length most of the time.
Bigger lenses are heavier and much more expensive, however.
Mirror lenses seem like an ideal solution because a long focal length can be folded into a small, light package. If you can't get a frame-filling image, carefully consider the composition. The natural tendency for me, and most photographers, when faced with an opportunity to photograph a desirable bird that might fly at any moment, is to place the bird dead center in the frame and fire away. Resist the tendency. It's difficult to think about composition when you're excited and rushing to get that first shot of a particular species, but generally shots where the bird is relatively small in the frame and placed dead center are visually boring.