This seems like a great idea in theory. I decided to see if it would work!
Hook & Eye
I got a wooden bird house for $4 from Hobby Lobby. (LOVE that store!) Don't forget you can use a coupon to get that even cheaper! Also picked up my suction cups from there as well.
Next, VERY CAREFULLY, cut the back off the house. You could use a box cutter, but I used a knife. Once I got the knife in between the wood, it popped off fairly easy.
Keep the back to use later as a template! This next step you could really use anything that has clear hard plastic. Use the plastic from your spinach container lid, or from those impossible packing of toys or other items you can't seem to open easily. I decided to use my laminator! I just ran a pouch through the machine to get the hard plastic.
Take the back of the birdhouse you popped off and trace an outline. Then cut it out!
Use hot glue to place the plastic on the birdhouse. Don't go too thick just to keep it safe for the birds. Also add a hook and eye to the top. The wood is soft so it was easy to screw in.
Birds are attracted to light bright colors so I just painted the house white to keep it simple.
Add twine through the hook and eye.
Use your suction cup and hook birdhouse onto window.
Then just wait!