Bat use of roosting boxes installed under flat-bottom bridges in western Oregon

Bats are known to use certain types of bridges as roosts, particularly cast -in-place and I-beam style bridges. Recent concern about the importance of bridges for bats precipitated studies to address their use of bridges as da y and night roosts. We conducted a study to determine whether boxes install ed beneath flat-bottom bridges would provide opportunities for day and nigh t roosting by bats. We evaluated roosting by bats within boxes installed un der 15 flat-bottom bridges spanning streams in the western Oregon Cascades. We monitored use of boxes by roosting bats during 15 weekly surveys conduc ted each year between 1 lune and 19 September 1997 and 1998. We used guano collected from traps located under roost boxes to indicate day and night ro ost use by bats. Observations of bats during surveys confirmed that boxes w ere used as day roosts. We documented use by bats in 13 of 15 (87%) boxes d uring the study; 10 boxes were used within a year after installment. The nu mber of times guano was collected from traps under boxes used by bats range d from one to 16 (3 to 53% of 30 surveys). We observed one to 8 bats day-ro osting in boxes on 14 separate occasions at 5 boxes. These data confirm tha t boxes installed under flat-bottom bridges provide roosting opportunities for some species of bats. We contend that designs for flat-bottom bridges c ould incorporate opportunities for roosting by bats without the aid of supp lemental structures.