Thick, solid, untreated wood — no zinc, no tar, no dark coat. The mass holds a steady temperature through a July afternoon instead of spiking to the lethal range that cooks pups in thin-walled boxes. A nursery should never get hotter than the bat can survive. Most do by noon.

A bat never enters from the front. It drops in through the open bottom and wedges up into the tight interior grooves — a crack, not a room — until its back and its belly both touch wood. That pressure is what tells it it is safe.

A bat lands low and climbs up into the bottom, gripping its way in. The rough, bark-on wood and the grooved interior give it that hold from the first contact. Smooth, sanded wood is a death trap — the young slide and the colony moves on.

Fallen black locust and white oak — among the most rot-resistant woods in the Eastern forest — let the house survive years of weather with no stain or sealant at all. Nothing to off-gas, nothing the bat has to breathe in a sealed cavity all day.

Multiple crevices give a colony room to spread by temperature — females and pups in the warm pockets, others where it is cooler. It is built to be a working summer nursery, because a population only recovers one brood at a time.

Every house is cut, bored, and grooved by Earl himself with hand tools — most of a day of work in each one. No crew, no machine stamping them out, and no second batch once the barn wall is empty.