Typically, people that are interested in attracting song birds into their yard will also want to purchase bird feeders and baths.  Aside from these items, you may also want to sell accessories that will help individuals use homemade birdhouses for indoor purposes.  This can include an installable roof hatch to facilitate conversion into a pen holder.  You can even sell  imitation animals and insects for decorative purposes.  You may even want to sell design plans so that people can enjoy making birdhouses on their own.