Assessing Tricolored Bat Acoustic Monitoring for Regulatory Purposes in Relation to Reproductive Period, Cover Type, and Presence of Eastern Red Bats

Abstract

The U.S. Fish and Wildlife Service (USFWS) allows use of acoustical surveys and automated identification software to determine the presence of the proposed endangered tricolored bat (Perimyotis subflavus). Analytical software is required to assess the probability of species absence on a sitenight basis using a maximum likelihood estimator (MLE) that accounts for interspecific misclassification rates. The current standard for occupancy determination is a returned MLE P-value ≤0.05 at the nightly level irrespective of the number of files identified as tricolored bats. For this species, MLE P-values can vary based on presence and proportion of other bat species with similar echolocation characteristics such as the eastern red bats (Lasiurus borealis). Large numbers of eastern red bat echolocation passes may lead to a swamping effect, causing false-negative tricolored bat determinations. Therefore, we investigated the relationship between MLE values returned by Kaleidoscope Pro automated software and nightly tricolored bat counts, along with count proportion relative to eastern red bat detections, for various summer reproductive stages (e.g., pregnancy, lactation, and volancy) and vegetation cover types. We also examined the effectiveness of using the 14-site-night level of effort (LOE) set by USFWS for the endangered northern long-eared bat (Myotis septentrionalis) for determining how many cumulative nights of tricolored bat presence would occur under that LOE standard. Automated identification software identified tricolored bat echolocation pass presence at the file level. However, software returned nightly MLE values that were not significant when tricolored bat passes were infrequent (<20 passes) and proportionally low relative to eastern red bat passes, suggesting an acoustic swamping effect. The probability of a significant MLE for tricolored bats was greatest during the late summer when juvenile bats were volant and actively foraging on the landscape. Cumulative nights of tricolored bat presence based on significant MLE values were greatest during the late summer and in riparian cover types.