Every spring, Mexican long-nosed bats traverse a lengthy migratory path north from Mexico into the southwestern U.S., following the sweet nectar of their favorite blooming plants like breadcrumbs. They return along the same route in the fall.

Scientists have long suspected that Mexican long-nosed bats migrate through southeastern Arizona, but without capturing and measuring the night-flying creatures, proof has been elusive. Researchers say they now have a way to tell the endangered species apart from other bats by analyzing saliva the nocturnal mammals leave behind when sipping nectar from plants and residential hummingbird feeders.