Dutch designer Klaas Kuiken made ceramic roofing tiles that are shaped like birdhouses. He writes:
Nowadays birds are not welcome under rooftiles anymore, so they lost a beautifull nest place. Placing a house on top of a rooftile, now created a nice place for a birdsnest. And also better to spot!
Link (scroll down) via DudeCraft
Size of the entrance also needs to be widened.
@ K!P >> Bird houses need to be cleaned out. Once the first set of birds have built their nest in the house, and have laid the eggs, hatched, raised the birds..etc.. They move out. A new pair of birds move in and build upon the previous nest. Repeat the process four times, and the bird house is no longer useful. If you do not clean it out, then you have a clay box full of bird nests.