I took Fountain's Birdhouse, and added two suction cup holes to the back.

Now you can attach it to your window. :-)

Took all of five minutes using the existing suction cup hole Thing ( http://www.thingiverse.com/thing:207 ) and Fountain's original design, and most of that was making sure the scales were correct.

Further refinements might include adding a little tab sticking out the back (from the floor piece) to keep the house level once the suction cups are attached to a vertical surface.

Cut it out, assemble it as per Fountain's original instructions, and slip the nubs of two standard bathroom fixture suction cups into the holes in the back plate.

With a lick and a promise, it should stick to your (clean) window and hold a full bird family in all security.