I built some bird boxes from offcuts of wood from making a veranda for my daughter's playhouse. I needed a few more, so I bought some 150 mm x 25 mm four meter lengths of framing timber (untreated) from the local woodyard which were perfect, except I should have cut the roofs slightly longer to give a bit more overhang. The thicker wood gives better thermal stability. The jury is out on whether pressure treatment is bad for birds. On the gable I have tried to make a "sparrow terrace" for tree sparrows, although I am not sure anyone has moved in. Starlings are using the martin box at the top of the gable!