The Hawaiian hoary bat (Lasiurus cinereus semotus), known as the ʻōpeʻapeʻa in Hawaiian, is the only endemic terrestrial mammal currently found in Hawai’i. Fossil records suggest that the bats were once plentiful across the State, but they are now observed primarily on the Big Island, Kaua’i, and Maui. In 1970, the U.S. Fish and Wildlife Service (USFWS) listed the ʻōpeʻapeʻa as endangered under the Endangered Species Act due to increasing threats from habitat destruction, invasive predators, and increased insecticide use (USFWS 1998). However, because of their rarity and cryptic nature, the population status and distribution of the bats is still not well known.
Nine National Wildlife Refuges (NWRs) have been established across Hawai’i with the mission to protect threatened and endangered plants and wildlife and their habitat (Figure 1). Except for anecdotal observations, the ʻōpeʻapeʻa has been systematically documented only at Hakalau Forest NWR on the Big Island (Gorresen et al. 2013). Recognizing the lack of information concerning the bats, in January 2017 the USFWS’s Hawai’i and Pacific Islands Inventory and Monitoring Team initiated a 2-year acoustic monitoring project to record the presence or absence of the bats on NWRs by recording their distinctive echolocation calls. Results of the surveys will inform future management decisions to help conserve ʻōpeʻapeʻa and restore their habitat at Hawai’i’s NWRs.
Figure 1. National Wildlife Refuges in Hawai'i.
Gorresen, M.P., F.J. Bonaccorso, C.A. Pinzari, C.M. Todd, K. Montoya-Aiona, and K. Brinck. 2013. A Five-Year Study of Hawaiian Hoary Bat (Lasiurus cinereus semotus) Occupancy on the Island of Hawai‛i. Technical Report HCSU-041
U.S. Fish and Wildlife Service. 1998. Recovery Plan for the Hawaiian Hoary Bat. USFWS, Portland , OR, 50 pp.
Figure 2. Bat detector unit deployed at Kīlauea Point National Wildlife Refuge on Kaua'i.
Starting in January 2017, we deployed 22 stationary acoustic recorders across the nine Hawai’i NWRs to determine the presence or absence of echolocating bats. Deployment locations were selected non-randomly and based on the size of the refuge, accessibility, and availability of suitable foraging or roosting habitat, and near navigation corridors like streams, roads, trails, and power line right-of-ways.
Detector units consist of an SM4BAT full-spectrum acoustic recorder (Wildlife Acoustics, Inc.), equipped with a multidirectional ultrasonic microphone (SMM-U1), and powered by an external 12-volt battery charged by a 5-Watt solar panel. Each time a bat crosses a microphone’s path, the recorder saves a single audio (.wav) file with a time and date stamp. The units were programmed to record from one hour before sunset to one hour after sunrise.
We used the Kaleidoscope® software to discriminate bat calls from noise and categorized each recording as either a navigation call series or foraging activity (i.e. a "feeding buzz"). Because it is impossible to differentiate between multiple recordings of one bat from recordings of several bats, direct population density estimates are not feasible using acoustic surveys. However, we can calculate relative bat activity as the number of individual recordings made each night.
Figure 3. The distinctive "swoosh" of an individual Hawaiian hoary bat call visually represented in Kaleidoscope. This call was recorded at Huleia NWR at 2245h on 9/8/2017.
An indiviudal ʻōpeʻapeʻa echolocation call modulates from high to low frequency typically in a range of 25-40 kiloHertz (kHz) with a duration of only 10-20 milliseconds (ms) (Figure 3). These unique characterisitcs allow us to discriminate bat calls from those of insects, rodents, and other noise that might be recorded. An ʻōpeʻapeʻa's navigation call series typically includes 5-10 calls per second and are evenly spaced (Figure 4). They often begin at low magnitude, become louder as the bat nears the microphone, then fade as it continues past. Bat foraging activity is indicated by a rapid succession of calls (10-20 ms-1) often stopping abruptly when the bat captures its prey or cuts off the pursuit (Figure 5).
Figure 4. Hawaiian hoary bat navigation call recorded at Hakalau Forest NWR at 1944h on 11/8/2017.
Figure 5. Hawaiian hoary bat foraging calls ("Feeding Buzz") recorded at Hakalau Forest NWR at 0013h on 8/3/2017.
Table 1. Hawaiian hoary bat occupancy on National Wildlife Refuges
Figure 6. Average nightly bat activity at 5 Hawaiian National Wildlife Refuges during 2017
Figure 7. Average nightly Hawaiian hoary bat foraging calls (i.e. "feeding buzzes") at National Wildlife Refuges during 2017. Refuges without recorded foraging calls are not included.
• In 2017, we recorded 133,971 bat calls with 22 recorders deployed at 9 NWRs
• Bats were present every night at Hakalau Forest, and every night except one at Hanalei
• Bat occupancy at Kilauea Point and Huleia was more seasonal, but remained high most of the year
• Bats were occasionally present at Kakahaia with occupancy ranging from 0.10 to 0.43
• Bats were rarely detected at James Campbell (8 recordings) and Pearl Harbor (2 recordings)
• Bats were not detected at Oahu Forest during 2017, but a single call was recorded in Jan. 2018.
• Bat activity was highest at Hakalau Forest, ranging from 50 calls per night in April to 380 in October
• Bat activity was lower at the other refuges but showed distinct peaks during August and September
• Bat foraging activity was detected at Hakalau, Hanalei, Huleia, Kilauea Point and Kealia Pond
• Foraging activity showed noticeable seasonality, but was not consistent across refuges
These audio recordings correspond to the recordings displayed in Figures 4 and 5. The raw recordings are slowed down to 1/8 speed so that the bat calls can be heard by human ears.
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• This survey confirmed the presence of the ‘ōpe‘ape‘a at all 9 of Hawai'i's NWRs.
• NWRs on Kaua'i, Maui, and the Big Island are signficant havens for the ‘ōpe‘ape‘a.
• NWR managers should consider options for enhancing or restoring roosting and foraging habit for the ʻōpeʻapeʻa, where possible.
• To help expand our knowledge of the ʻōpeʻapeʻa, NWR managers and biologists should pursue collaborative research partnerships with univeristies, USGS, and others.
• Future surveys at the NWRs should concentrate on determing bat habitat preferences and evaluating habitat restoration effects on bat occupancy.