Use a window feeder to attract the birds. Fasten a twig, bark or moss etc on to it to make a more natural shot.
Like this .......... If you have a camera with a close up lens, put it near the glass. A remote shutter release, or a wireless release in essential. A flash may be required depending on the light available - you'll have to experiment
This is the kind of result you'll get .............. Not too impressive yet
A bit of cropping and image adjustment and you can get this .................