The Postscript / Carrie Classon NOBODY WAS USING the old wren house. My grandfather built it. Grandpa started building birdhouses when he retired from milking cows and his second oldest son took over. That son, my mother’s brother, is now 87 and retired 20 years ago. It’s a pretty old birdhouse. “My dad never built fancy birdhouses,” my mother explained. Grandpa put on a tarpaper roof and, if…