Our bird feeders are positioned where I can either shoot photos from a bench that's approximately 15 feet away, or from inside the house through a window at around 20 feet. The finches and hummingbirds tolerate me at the bench, but the spookier birds such as Grosbeaks and Oak Titmice require shooting through the window from inside the house (at least for now). I find the results vary dramatically depending upon the time of day and direction/quality of light. If the light is strong and coming from certain oblique angles, the images can look washed out, which I believe is a result of flare caused by the window glass. On the other hand, if the light is even slightly diffuse and is coming from a more acute angle, the window pretty much disappears. Of course, it helps if the window is clean, and it seems best to keep the lens as close to the window glass as possible.
The photo above of a female Black-Headed Grosbeak was taken through a window with good lighting conditions as described above (click the image to see the full res). I'd be curious to hear others' experiences shooting bird photos through glass.