lucubrando

Depending on the level of intricacy associated with your birdhouse plans, a few simple lines may do.  On the other hand, if you want decorative birdhouses that will take on the shape of a historic building, or even model a particular item, you may need some drafting tools.  Typically, you can purchase fairly inexpensive drafting software for this purpose.  That said, if you prefer the old fashioned way of doing things, a ruler, protractor, and some triangles may be all that you need.