Big Free-tailed Bat (Nyctinomops macrotis)

Protection Status Notes
The big free‑tailed bat was proposed as a federal candidate species in 1994. This species is currently on the BLM's special status species list for Utah and Colorado. It is considered a Species of Special Concern by the states of California and Utah. In Texas, it is not considered Endangered or Threatened, but little is known about population trends.
Description
Nyctinomops macrotis can be distinguished from other molossids (= free‑tailed bats) based on size. With an adult forearm of 58‑64 mm it is larger than T. brasiliensis or N. femorosacca, and smaller than Eumops perotis. Also, it has vertical grooves or wrinkles on the upper lip, which are lacking in Eumops.
Life History
This species is a seasonal migrant, and a powerful flyer. The species forms maternity colonies, and females bear one young in late spring or early summer. Lactating females have been taken in July, August and September, and volant juveniles recorded on 8 and 27 August. Maternity roosts have been documented in rock crevices, with evidence of long term use. It appears that the return to the roost site by this bat involves ritualized behavior, including a general reconnaissance of the site and several landing trials before entry. Owls are the only documented predators of this species. N. macrotis has an audible echolocation call, which is characterized as loud and with a frequency range of 17‑30 kHz. Surveys based on echolocation calls for this species may be possible, as captures appear to be uncommon (outside of Big Bend National Park, where the most animals in North America have been documented). Easterla, however, reports that the populations at the Park fluctuate greatly from year to year. Little is known about the species population dynamics and ecology.

Big free-tailed bats roost mainly in crevices and rocks in cliff situations, although there is some documentation of roosts in buildings, caves, and tree cavities.

N. macrotis forages almost entirely on large moths, but some data exist to document occasional foraging on other insects, including grasshoppers, beetles, crickets, leafhoppers and flying ants.
Distribution
N. macrotis ranges from most of South America northward to include Mexico, Arizona, New Mexico, southern and western Texas, southern California and southeastern Nevada, southern Utah, and north to central Colorado. The species is migratory, and there are some extralimital records from British Columbia, Iowa, Kansas, and South Carolina. The known elevational range is from near sea level to about 8,500 ft. N. macrotis appears to be mainly an inhabitant of rugged, rocky habitats in arid landscapes. It has been found in a variety of plant associations, including desert shrub, woodlands, and evergreen forests. It appears to be associated with lowlands, but has been documented at around 8,000 ft in New Mexico.
Threats and Reasons for Decline
No known threats to the species have been identified to date. However, some of the general threats to bats could apply to N. macrotis. These could include impacts to foraging areas from grazing, riparian management, the use of pesticides, and in some places disturbance to the roost site (e.g. blasting of cliffs or water impoundments).
Ongoing Recovery
Although big free-tailed bats are locally abundant, they are often absent from seemingly appropriate habitat. Since these fast flyers are rarely netted over any but the largest and most obstacle-free ponds, it is quite possible that they are limited by suitable drinking sites, which are known to have decreased in number during historic times.
Other
Information is needed on N. macrotis regarding roosting ecology, seasonal movement patterns, and breeding colony distribution. Current evidence suggests that the species breeds farther north than previously thought, including southern Utah and Colorado. Vocalization recordings are needed to train researchers and managers that may attempt to survey for the species based on audible calls. Reference calls need to be established and geographically verified, and made available at a depository, and/or establish a site where recordings can be sent for verification. It will be important for bat biologists to be able to distinguish between the different audible bats in the southwest.
For more information
  • Additional details can be found in the online version of The Mammals of Texas for the Big Free-tailed Bat

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