Although horror movies might lead you to associate bats exclusively with caves, bridges - especially the concrete kind common in Oregon - also make good habitats for bats. The underside is dark and quiet, just the way daytime sleepers like it. And the concrete mass stores heat, so bats can regulate their mammalian body temperature by climbing up when it's cool and down when it's warm.
Though no bat species in Oregon are yet listed as threatened or endangered, bat populations are declining almost everywhere. The bridge program recognized that rebuilding bridges across the state gave us the chance to help bat populations recover.
Before any construction began, biologists went to the bridges at night with flashlights to identify which species were living underneath each bridge. The bridge program then worked with engineers to design bridges with bats in mind. Whenever possible, engineers incorporated one of three types of habitats into the design: crevice, cave-like and "Oregon wedge."
Crevice habitats, ideal for mouse-eared bats, were built into the concrete structure to provide a gap that runs along the span of the bridge. Cave-like habitats accommodate brown bats; they are boxes built into the bridge to provide a cavernous space. The "Oregon wedge" design is a simple plywood box affixed to the outer bridge beams that can house hundreds of bats of different species at a time.
In total, 88 bridges were in compliance with environmental performance standards for bat habitation. The program is a proven success: The guano (bat waste) that was found beneath bridge crevices after construction shows that bats are using the bridges as homes.