The wrens that set up housekeeping outside my office window each spring owe their little house to the bird’s-mouth bit. These bits — there are three different ones in the set — enable you to mill one edge of boards and assemble them into hexagonal, octagonal and other-agonal forms.
If you’re looking for a good way to construct multi-sided boxes, here it is. You don’t have to calculate bevel angles. You just decide how many sides your construction will have, chuck the appropriate bit in your table-mounted router, and start routing. When you’re done, the routed bird’s-mouth is self-aligning.
I first tried the bits several years ago, making multi-sided tubes. I made an eight-sided form from thin strips of Spanish cedar, then transformed it into a kind of cigar humidor by adding rabbeted disks for a bottom and top.
The six-sided form made of Western red cedar strips began my wren’s house, simply by mounting it to a board hung on a nail in a deck post. Maybe there’s something extra in the bird’s mouth joinery. Every year, Mamma and Poppa Wren show up, push the old nest out onto the ground, then recycle the twigs and strings and other debris into a new nest.