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Monitoring Bats of the Lower Colorado River

Background: bats1

As the Lower Colorado River’s (LCR) riparian habitathas beendegraded due to humandevelopment, Arizona Game and Fish workingin conjunction with the U.S.Bureauof Reclamation on the Multi Species Conservation Program which looks to mitigate and reduce the effects of dam operations on wildlife at the LCR. Our part in the plan is to study four species of bats: the Western red bat (Lasiurus blossevillii), Western yellow bat (Lasiurus xanthinus), California leaf-nosed bat (Macrotus californicus), and the Townsend’s big-eared bat (Corynorhinus townsendii pallescens). The purpose of our study is to determining their specific habitat requirements, so that we can then create/restore habitat areas for these species. Among the conservation actions proposed are measures to create 765 acres of western red and western yellow bat habitat.



As seen in the map, our area of study extends from Davis Dam to Laguna Dam near Yuma Arizona. We have divided the study area into 3 reaches. In each reach we are surveying four distinct vegetation types: marsh, mesquite, salt cedar, and cottonwood/willow.




I In order to survey the bats in the study area, LCR Reaches 3, 4, 5 and small area of Reach 6 (see map fig.) will be sampled each of the four seasons using Anabats, acoustic devices that record bat echolocating calls. This is a monitoring method allows researchers to be less invasive. Yet detection and identification of individual species can at times be difficult, therefore mist-netting will be used to confirm bat distribution. To provide information on bat migratory movement along the river we have placed 4 permanent bat detector stations (picture to the left), which allows us to evaluate variability of bat activity as well as migration patterns. This permanent stations are equipped with temperature, wind speed and humidity sensors that can aid researchers to find a correlation between such environmental variables and bat activity

Important finding patterns: 

Thus far we have found positive correlations between high temperatures and bat activity, meaning that in warm nights we can expect many bats flying in the skies. Detection rates for Thownsend’s big ear bat were highest in mesquite habitats. The California leaf nosed bat does not appear to have a clear habitat selection; The Western yellow bat detection rates were very low and appeared to avoid cotton wood habitats, and selecting for mesquite. The Western red bat detection rates were low on most habitat types except in salt-cedar habitats. This is unusual because the red bat is a tree roosting species, but we speculate that the red bat is roosting in cotton-wood and mesquite habitats and foraging in salt cedar. The four bat species appear to be present in the four vegetation types. We recorded much night-to-night variability in bat activity at our permanent stations during late winter and spring, which may have corresponded to migration pulses or to the influence of temperamental weather patterns.  The latter explanation is supported by the fact that call minutes were highly correlated to mean evening temperatures during this time at most stations.  Declines in activity but increases in consistency during April and May were perhaps a result of less influence from migrants and more consistent weather.  Overall, the number of call minutes from the 4 focal species comprised only about 2% of the total sample from the permanent stations, an indication of their relative scarcity along the LCR.


The primary purpose of this study is to determine the distribution and habitat use of the bats mentioned within the study area during all seasoned of the year; and to evaluate their migration patterns. This data can then be used to make recommendations as to the management activities in the area, as far as to in which areas to concentrate restoration efforts.


For more information contact:

Michael Ingraldi, Ph.D.
Arizona Game & Fish Department
5000 W. Carefree Highway, Phoenix, AZ 85086


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