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Occupancy of Buildings by Bats

Characterization of Buildings Used by Bats

Despite prevalent use of anthropogenic structures by bats, and the associated implications for public health, management, and bat conservation, very little quantitative information exists about urban roost characteristics and their selection by bats.

During the summers of 2001 to 2004 we conducted fieldwork in Fort Collins to address questions of roost selection by the big brown bat (Eptesicus fuscus). The city has experienced its greatest growth in the past half century, with its population increasing by 30% in the last decade. Similar growth in new buildings has occurred, with the number of new housing permits issued annually doubling in the past decade. A total of 142 roosts were located using radiotelemetry or by citizen calls in response to a newspaper article and flyers. To determine characteristics of roost selectivity by bats, variables for known maternity roosts and randomly selected buildings were compared at microhabitat and landscape scales using logistic regression; an information theoretic approach was used to determine which variables were most important.

Forty-four and 100 buildings were considered in the microhabitat and landscape scale analyses, respectively. At the microhabitat scale maternity roosts had exit points with larger areas that were higher from the ground and had warmer average temperatures than randomly selected buildings. At the landscape scale distances to similarly categorized roosts were smaller, and urbanization variables such as lower building density, higher street density, and lower traffic count density were most important. Results for variables important to urban-roosting big brown bats were often similar to studies that characterized maternity roosts found in tree snags and rock crevices. In addition, changes in the landscape, not only in the form of anthropogenic structures, but also in water availability and vegetation structure such as riparian forests, may have led to population increases and range expansions of the big brown bat.

Maternity roosts in Fort Collins

Random Surveys of Buildings for Use by Bats

We surveyed the exterior of 406 randomly selected buildings throughout Fort Collins for evidence of use by bats on 27 days during the summer of 2004. These included 327 single-family homes, 33 businesses, 18 apartment buildings, 18 duplexes or town homes, 6 outbuildings, 2 schools, and 2 churches. Although we designed our sampling to account for detectability, rates of verification at emergence were low and the number of positive cases was too sparse to allow estimation of a detectability function.

The number of buildings in which one or more independent observers judged occupancy by a maternity colony was 12 out of the 406 surveyed, with both independent observers agreeing on 10 of the 12 positive cases. However, verification based on exit counts was low. (One of 10 was occupied at the time of sampling. Two others of the 12 positive cases selected as random addresses had been previously known to harbor bats from radiotracking earlier in the study; 1 of these 2 was not checked and 1 had bats at the time of sampling). One plausible assumption is that the presence of bat guano or other sign detected by observers indicates use by a maternity colony of big brown bats at some prior point in the summer of 2004, or that these buildings may have included temporary roosts not occupied on a daily basis (assumes detectability of use, rather than occupancy, is close to 1). Given this assumption, the completely random survey suggests that about 2.9 % (12 of 406) of the buildings in Fort Collins may be used by big brown bat maternity groups at some point within a summer, but only 0.5-0.7 % may be occupied on a given day (2-3 of 406).

Guano pileCrystalized urine

Evidence of minor use by bats was obtained for 20 additional buildings. These were cases where the likely use of the building was as a night roost (droppings below porch overhangs without openings for internal access for day-roosting) or possibly occasional use by solitary bats during the day. These signs were more subtle and positive reports by both observers occurred in only 4 of the 20 cases. Assuming no false positives, a minimum of an additional 4.9 % of the buildings (20 of 406) in Fort Collins showed evidence of minor use by bats. Only one of these was observed for counting bats at emergence. However, this building proved to harbor a maternity colony of about 30 bats, showing that false negatives can occur in the surveys for maternity colonies. Interestingly, the 12 buildings with likely use by maternity colonies and the 20 classified as showing only evidence of minor use were generally located in areas of Fort Collins where our past radiotracking and efforts to obtain local knowledge also yielded a concentration of maternity colonies in buildings (Fig. 1).

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