I have several guiding principals that I try and follow when I'm building birdhouses. The first is to find inspiration from the simplest places and find objects that tell a story or have personal meaning. An old cabinet drawer, a favorite fallen apple tree or a broken camera that grandad had.
Second is to recycle or reuse whenever possible. Reused material has history and personality that just can't be found in new material.
Third is to challenge myself to build one style or look and move on to another look. There are advantages to doing the same process over and over, but for me the world is full of ideas to explore and new things to learn.
Lastly I look for surprises. The birdhouses I like the most start out different than they end up, evolving into something else and becoming much more interesting. I start with a general plan but once started the house takes on a personality of its own and I simply follow that course. I don't believe we should have to walk in a straight line.