My wife just bought herself a Canon EOS 400 XTi. Now she is interested in getting a telephoto lense for it. I was wondering if anyone out there has one of these camera's and what kind of lense they feel works best. She wants to take pictures of the birds in our back yard.

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.

Thanks for the help Jeff.

If birds are her main subject I'd look for a used Canon 400m f5.6.  Very sharp lens!

Jerry