I once saw a large beautiful birdhouse covered in small seashells ... an acquaintance's husband made it for decorative purposes and it sat on a shelf inside their home.
I think a birdhouse covered in small river pebbles would be really pretty ... I might have to try it! I doubt if birds would use it here in our climate because the pebbles would probably really heat up when we have 90ยบ+ during the summer and the poor baby birds would fry.
It sure would make a decorative garden accent though so I'm putting it on my list of things to do!