Fallguy, there is a very wide range of telephoto lenses available for your wife's camera. I think for starters however you may want to establish your price range.
For smaller objects such as birds a 300 or 400mm lens would be a starting point. Of course if you can get closer to your subject you can get by with less(I use to set up a blind up in my back yard to get photos of birds at the feeder with a 300mm).
Most Canon lenses get pretty good reviews as does many of the Sigmas, Tokinas and Tamrons. There is a wide variety of lenses that will do the job and a wide range of prices as well. Zooms are versatile but single-focal length lenses(primes) can be less expensive, lighter and more compact.