Here's the rig being used for taking pictures with a spotting scope.

The long reach of a spotting scope can get you close to the birds. However there are some limitations, chromatic abbrevation of the scope, vibrations and it takes about 20 seconds to put the camera on and get a picture taken. But under the right circumstances quite good results can be delivered. Flight pictures and active birds are difficult though.

Another drawback is that the manufacturers of these cameras doesn't support RAW mode. That would have improved the end result quite a bit.

Today there are scopes with builtin camera available (from Leica).