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Determining Foraging and Roosting Areas for Mastiff Bats (Eumops spp.) Using Radio Telemetry

Organ Pipe Cactus NM
Brian Barns
Underwood’s mastiff bat (Eumops underwoodi) is a large, little-understood tropical species, reaching its northern distribution limit in extreme Southern Arizona. One of the few locales where it occurs is Quitobaquito Pond in Organ Pipe Cactus National Monument (OPCNM). Located in southwestern Arizona, Organ Pipe Cactus NM is comprised of approximately 330,689 acres of Sonoran Desert plains, bajadas, and rugged mountains. OPCNM is on the international border with Mexico and its “sister” park, El Pinacate Reserva de la Biosfera, is nearby to the southwest. Quitobaquito is located right on the U.S. - Mexico border next to a busy highway, and is subject to various threats. Further, the border area nearby is undergoing changes due to increasing human population, tourism, increasing industrialization, and changes in land uses. Quitobaquito and the border area are clearly of importance to this bat for foraging, roosting, and accessing water in an arid landscape. To expand our knowledge of this bat’s life history and to identify potential management issues, we sought to determine foraging and roosting areas using radiotelemetry. Eumops underwoodi were found to forage widely across and along the international border area. Foraging habitat ranged from rugged wilderness topography to agricultural and semi-urban areas. Unexpectedly, they were found to be roosting in woodpecker cavities in saguaro cactus (Carnegia gigantea). This is the first documentation of this species roosting in cactus cavities, and a rare documentation of any bat species doing so.

We captured, outfitted with radiotransmitters, and tracked four E. underwoodi. We captured two E. underwoodi in 2001 and another two in 2002. Of the four E. underwoodi that were instrumented, three yielded valuable data. These three individuals, Eumops #218, #239, and #284, were each tracked almost nightly for up to two weeks. For all three bats, we were
able to determine foraging areas, travel routes, and roost sites, with a high degree of precision. A program called "Locate II" helped triangulate bearings from various telemetry stations. It is a DOS-based program which replaces manually drawing bearings on maps and guesstimating where animals are located. It was published commercially by Pacer Computer Software, but released into the public domain in June of 2000. ArcView 3.x with ANIMAL MOVEMENT ANALYSIS EXTENSION created by USGS - BRD, Alaska Biological Science Center was used to make maps and analyze home ranges and movement paths.

The saguaro-cavity roost sites used by E. underwoodi appear to be, on the surface, a nearly unlimited resource. These cavities are excavated by Gila woodpeckers (Melanerpes uropygialis), and northern flickers (Colaptes auratus, including the “gilded flicker” form). Many other bird species in the study area use these cavities subsequently for nesting, and are known as “secondary cavity nesters” because they are unable to excavate cavities themselves. Saguaros selected for roosting were all fully mature, large individuals with several arms and multiple cavities. While the roost cavity was often high on the plant, on several occasions the bats selected lower cavities, sometimes on an arm. All three bats changed cavities from night to night, but seemed to cycle between a fairly small number of cavities. These favored roosts were typically within 1 km (1.6 mi) of one another. Each bat was confirmed day-roosting in only two individual saguaros each. Roost saguaros differed only slightly in setting. Eumops #239 used saguaros in rolling, undeveloped bajada topography south of the Rio Sonoyta. Eumops #284 roosted in a small area of undeveloped, slightly undulating topography surrounded by agricultural fields, and also used a saguaro within a few hundred meters of Mexico Highway 2 as a temporary night roost. Eumops #218 day-roosted in several saguaros also located within a few hundred meters of Mexico Highway 2.

The three Eumops bats displayed comparable home ranges and foraging areas. All three roosted in the western part of the Rio Sonoyta valley, approximately 1.5 to 4.2 km (1-2.6 mi) south or southeast of Quitobaquito, near the northeastern corner of El Pinacate Biosphere Reserve. All three ranged eastward and southward during probable foraging movements. In movements that we assumed to constitute foraging, we found E. underwoodi to be foraging in a range of habitats, including undeveloped wilderness, Sonoran desertscrub, mesquite-tamarisk riparian woodland, agricultural areas, rural development, topography ranging from flat to very steep, and the semi-urban small city of Sonoyta. All three engaged in movements generally along the axis of the Rio Sonoyta valley, but ranged north and south onto adjacent bajadas and mountain slopes. All three incorporated the city of Sonoyta, Sonora, in their home ranges, approximately 21-24 km (13-15 miles) from roosting areas. The time spent by our radioed E. underwoodi in or near Sonoyta - usually 1- 3 hours per visit - strongly suggests the bats foraged for arthropods attracted to artificial lights or other anthropogenic sources, and/or was visiting water sources in or near Sonoyta. The ability of E. underwoodi to range up to 24 km (15 mi) or more on foraging bouts from its roost site demonstrates that roost sites do not need to be available in close proximity to foraging areas.

The differences among the three Eumops in total cumulative home ranges seem likely to have been correlated with the amount of movement data collected. The bat tracked the least number of nights (Eumops #284) had the smallest cumulative home range, while the bat tracked the longest (Eumops #239) accumulated enough data points to define a much larger home range. Each bat exhibited night-to-night variation in the location and areal extent of its movements. Eumops #239 was radiotracked over the most nights, and likely represents the minimal total home range for the species in this area over the longer term. The species demonstrated an ability to cover large areas in a single evening. The largest single-night home range recorded was 284.6 square kilometers (109.89 sq. miles), for Eumops #239 on June 14, 2002. All three bats commonly ranged over approximately 100 sq. km (37 sq. mi.) on a typical night. It is difficult to estimate total linear distances flown per night, because of the meandering routes often used by individual bats. However, the bats on most nights traveled 20-30 km (32-48 mi) at the very least, and often more in the range of 50-100 km (80-160 mi) as a minimum estimate. We observed extensive back-and-forth movements within a given night’s home range, e.g. back and forth along mountain ridges or back and forth across Sonoyta. Such movements suggest that bats may fly hundreds of kilometers per night – and often in just several hours.

The three Eumops bats we examined spent the majority of their flight time over relatively flat terrain in the Rio Sonoyta valley and adjacent bajadas. However, as in a light-tagging project undertaken in 2000, we also observed radioed Eumops flying around and along topographic features ranging from small hills (e.g. Quitobaquito Hills) to steep, high, rocky ridges (e.g. Puerto Blanco Mountains). This use was best illustrated by Eumops #239, which was radiotracked the longest. Other bats (e.g. #284) may also have visited these topographical features, but this was not detected or confirmed due to limited or inconclusive radiotelemetry data. Observers on ridgetops commonly heard the distinct vocalizations of non-radioed Eumops overhead, sometimes seen and heard in groups of six or more, indicating that these bats commonly forage along ridgelines, hilltops, and other raised topographic feature. Our observations suggested Eumops may favor windward slopes, as if the bats used favorable wind currents for flight energetics, and/or that upslope winds created predictable concentrations of nocturnal aerial invertebrate prey.

The results of this project have been published in the NPS report called "Determining Foraging and Roosting Areas for Underwood’s Mastiff Bat (Eumops underwoodi) Using Radiotelemetry, At Organ Pipe Cactus National Monument, Arizona." A poster is on display in the visitor center and has been presented at several conferences. It has also been translated into Spanish for the Pinacate Biosphere Reserve. This was a cooperative effort with investigators from the Pinacate Biosphere Reserve in Mexico, with which the monument shares a common boundary. More information is available by contacting the park. Project contributors are Tim Tibbitts, Ami Pate, Yar Petryszyn, Scott Sweet, and Brian Barns.

This project was funded largely by a grant from the Desert Southwest Cooperative Ecosystems Studies Unit at The University of Arizona, Tucson (DS-CESU Cooperative Agreement CA-1248-00-002 Acct 306910).
April 08, 2004