Use of Bridges by Bats (Mammalia: Chiroptera) in the Rio Grande Valley, New Mexico

Abstract

During a 2-year study, we documented eight species of bats using bridges in the lower Rio Grande Valley of New Mexico. Of 17 bridges surveyed, 15 (88%) contained day-roosting bats and ≥8 bridges (47%) contained maternity colonies with one or more species, including the Arizona myotis (Myotis occultus), Yuma myotis (Myotis yumanensis), and Brazilian free-tailed bat (Tadarida brasiliensis). The pallid bat (Antrozous pallidus), big brown bat (Eptesicus fuscus), silver-haired bat (Lasionycteris noctivagans), California myotis (Myotis californicus), and fringed myotis (Myotis thysanodes) roosted infrequently in bridges. Use of bridges by day-roosting bats differed seasonally, with most individuals occupying bridges April–October. During November–March, relatively few individuals of only three species (L. noctivagans, M. yumanensis, and T. brasiliensis) occupied bridges. Individuals roosted at many sites in bridges, but 99.9% roosted in narrow cracks and crevices. A total of 36,629 day-roosting bats was counted, with 99.8% observed in bridges constructed from timbers and 0.2% observed in I-beam bridges constructed from concrete, metal, or both. Paucity of bats in I-beam bridges reflected lack of narrow spaces. Our study demonstrates that some bridges represent important roosting sites for bats in southern New Mexico. Bridges throughout the southwestern United States likely serve similar life-history functions for bats and should be managed as a resource, especially those occupied by maternity colonies or large numbers of individuals.