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Acoustic evidence for Hoary Bat migration in the coast mountains of British Columbia.

A migratory tree bat, the Hoary Bat (Lasiurus cinereus) is a summer resident in British Columbia that migrates to a winter range in the United States or Mexico (Nagorsen and Brigham 1993; Cryan 2003; Cryan and others 2004). Information on its summer range and migration in the province is limited to occurrence records derived from museum specimens or opportunistic live captures by bat researchers. Consequently, this species' migratory routes and migration timing in British Columbia are largely unknown. For 5 mo in 2009, we monitored bat activity at a site proposed for a wind turbine on Grouse Mountain in the North Shore Mountains of the southern Coast Mountains in the District of North Vancouver. From acoustic recordings, we found evidence for a migratory passage of Hoary Bats. Herein, we review the seasonal activity patterns observed at the site and their implications for Hoary Bat migration.

We passively monitored bat activity on Grouse Mountain from 21 May-22 October 2009 with 2 ultrasonic acoustic detectors (AnaBat[TM], Model SD1, Titley Scientific, Columbia, Missouri) placed in trees. AnaBat[TM] High Mount cable microphones with waterproof coverings were attached above the units and positioned over plexi-glass reflectors set at a 45[degrees] angle. The acoustic detectors were protected in waterproof containers and powered by rechargeable 12-volt batteries with associated solar panel charging units. We programmed detectors to record from sunset to sunrise.

The detectors monitored 2 sampling stations below the summit of Grouse Mountain (Fig. 1). Both stations were in forest dominated by Mountain Hemlock (Tsuga mertensiana) with scattered Yellow Cedar (Chamaecyparis nootkatensis) and Amabilis Fir (Abies amabliis). Station 1 was on a Mountain Hemlock tree at 1200 m elevation (UTM: Zone 10, 494653E, 5470568N, NAD83) with the microphone positioned 12 m above ground. The microphone faced north and sampled airspace above the Thrasher Creek valley. Station 2 was located on top of a 6-m-high cliff face, 60 m southeast of the other station, in a Yellow Cedar snag at 1202 m elevation (UTM: Zone 10, 494577E, 5470503N, NAD83). The microphone was positioned 12 m above the ground facing east towards the Mosquito Creek valley and sampled airspace over a cleared ski run.

We downloaded data recorded on the digital storage cards of the detectors bimonthly and analyzed acoustic files with call analysis software (AnalookW[TM]; WinAnalook.htm#AnaLookW_Contents). A bat pass was defined as a file containing 2 or more echolocation calls. We identified files as those of a Hoary Bat if they had a sequence of calls with either fluctuating minimum frequencies or a sequence of calls uniform in frequency that had a narrow bandwidth, a maximum frequency <25 kHz, and a minimum or characteristic frequency below 20 kHz (O'Farrell and others 1999; 2000). We categorized passes as either commuting passes (calls with a regular pulse rate) or feeding passes (some calls with a rapid pulse rate, steep slope, and short duration).

We obtained weather data from the National Climate Data and Information Archive, Environment Canada (www.climate.weatheroffice. Nightly temperature ([degrees]C) and relative humidity (%) were calculated from data recorded at 15-min intervals from 20:00-06:00 h at the Cypress Bowl North weather station situated at 953 m elevation, 10 km west of Grouse Mountain. Nightly barometric pressure (kPa) and visibility (km) were calculated from hourly data recorded between 20:00 and 06:00 at the Vancouver International Airport station situated at 4 m elevation, 21 km south of Grouse Mountain. We calculated moon phase and illumination using moon phase software (Quick-Phase Pro[TM] ver. 3.3.5, CalculatorCat, Park City, Utah).

From 306 detector nights sampled, we recorded 3688 bat passes consisting of 1466 passes ([bar.x] passes/night = 0.5) at Station 1 and 2222 passes ([bar.x] passes/night = 7.1) at Station 2. We identified 1162 of the bat passes as those of Hoary Bats, with 69 recorded at Station 1 and 1093 recorded at Station 2. The 1st detection of a Hoary Bat was on 3 July. Hoary Bat activity in July and August was sporadic, with 0-9 passes/night (Fig. 2). However, on 11-12 September there was a marked spike in Hoary Bat activity, with 343 passes recorded (340 at Station 2, 3 at Station 1). Smaller (60-141 passes) activity peaks occurred on nights from 12 to 25 September. Activity declined sharply after 27 September, with the last detection occurring on 8 October. Of the 1162 passes of Hoary Bats recorded, most involved commuting calls; we identified only 15 (1.3%) sequences with feeding calls, all recorded from 11-21 September. We assessed temporal variation within a night by assigning the 343 Hoary Bat files recorded on 11-12 September into 30-min intervals (Fig. 3). The first detection occurred 3.5 h after sunset and the last detection was 1 h before sunrise. Most activity (82% of the 343 passes) was concentrated in a 2-h period from 02:00-04:00. The night of peak Hoary Bat activity had moon illumination 47% of full and was warmer with lower relative humidity and barometric pressure than the previous night when only 1 Hoary Bat pass was recorded (Table 1). Moon phases on September nights with high Hoary Bat activity extended from the third quarter (11-12 September) through a new moon (17-18 September) to the first quarter (25-26 September).

Although our acoustic monitoring began on 21 May, well before spring occurrences of the Hoary Bat in the Pacific Northwest according to Cryan (2003), we found no evidence for spring migration at Grouse Mountain. Spring records of Hoary Bats exist for the British Columbia interior (Cryan 2003), but there are none from coastal regions in that season. Valdez and Cryan (2009) and Johnson and others (2011) suggested that Hoary Bats fly at low heights during spring migration. If spring migration is confined to coastal lowlands or mountain valleys rather than mountain ridges, our failure to detect Hoary Bats from 21 May-2 July may be a sampling effect of having only 2 sampling stations, both located at approximately 1200 m elevation. Alternatively, this species may use a spring migration route east of the Coast Mountains. Cryan (2003) noted the lack of evidence for female Hoary Bats migrating along the Pacific coast in spring. The few Hoary Bat detections in July and August likely represent individuals from a local summer resident population in the North Shore Mountains. The pronounced spike in acoustic activity recorded in September, particularly the night of 11-12 September, is consistent with an influx of migrants. This represents the first evidence for an autumn migratory passage of Hoary Bats in British Columbia. The mid-September peak is concordant with the general timing of autumn migration for Lasiurus species reported by Cryan and Brown (2007). However, migration at Grouse Mountain was nearly 1 mo later than the autumn migration period observed by Baerwald (2008) for Hoary Bats in the foothills of the Rocky Mountains of southwestern Alberta, suggesting different migration patterns east and west of the Rocky Mountains.

Cryan and Brown (2007) demonstrated that during autumn migration Hoary Bats arrived on an offshore island during overcast nights with low wind speeds and barometric pressures associated with weather fronts. No data were available for nightly wind speed or % cloud cover at Grouse Mountain. The night of 11-12 September was clear and the available data we analyzed demonstrated significant changes in temperature, relative humidity, and barometric pressure from the previous night. To what extent the weather change triggered migration activity is unknown. It is noteworthy that the period of Hoary Bat migration at Grouse Mountain in September coincided with nights of darker phases of the moon.

The nightly timing of the September activity peak and paucity of feeding calls is consistent with Hoary Bats commuting over Grouse Mountain during migratory flight. However, we cannot rule out the possibility that in September Hoary Bats roosted in trees on Grouse Mountain during the day, using the area as a transient stop-over site (Cryan and Veilleux 2007). The origin of the migrants and their direction of flight are unknown. From an analysis of museum records, Cryan (2003) suggested that this species migrates southward along the Pacific coast in autumn. The North Shore Mountains, with their extensive ridge systems, deep valleys, and close proximity to the Pacific coastline (Fig. 1), would provide a migration route with distinctive landscape features for bats moving southward in autumn.

Key words: acoustic detections, AnaBat, British Columbia, Coast Mountains, Hoary Bat, Lasiurus cinereus, migration

Acknowledgements.--We thank Grouse Mountain Resorts Ltd for supporting this study. M Sarell installed the Anabat detectors and S Leech downloaded the Anabat files. P Jost prepared the map.


BAERWALD EF. 2008. Variation in the activity and fatality of migratory bats at wind energy facilities in southern Alberta: causes and consequences, [thesis]. Calgary, AB: University of Calgary.107 p.

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CRYAN PM, Brown AC. 2007. Migration of bats past a remote island offers clues toward the problem of bat fatalities at wind turbines. Biological Conservation 139:1-11.

CRYAN PM, VEILLEUX J. 2007. Migration and use of autumn, winter, and spring roosts by tree bats. In: Lacki M, Hayes J, Kurta A, editors. Bats in forests: Conservation and management. Baltimore, MD: The John Hopkins University Press, p 152-175.

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JOHNSON JS, WATROUS KS, GIUMARRO GJ, PETERSON TS, BOYDEN SA, Lacki MJ. 2011. Seasonal and geographic trends in acoustic detection of tree-roosting bats. Acta Chiropterologica 13:157-168.

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O'FARRELL MJ, MILLER BW, GANNON WL. 1999. Qualitative identification of free-flying bats using the Anabat detector. Journal of Mammalogy 80: 11-23.

O'FARRELL MJ, CORBEN C, GANNON WL. 2000. Geographic variation in the echolocation calls of the Hoary Bat (Lasiurus cinereus). Acta Chiropterologica 2:185-196.

VALDEZ EW, CRYAN PM. 2009. Food habits of the Hoary Bat (Lasiurus cinereus) during spring migration through New Mexico. The Southwestern Naturalist 54:195-200.

Mammalia Biological Consulting, 4268 Metchosin Road, Victoria, BC V9C 3Z Canada; mammalia@ [DWN]; Roberston Environmental Services Ltd., 1525-200th Street, Langley, BC V2Z 1W5 Canada; Current: EDI Environmental Dynamics Inc., Suite 640-1140 West Pender Street, Vancouver BC V6E 2R9 Canada [IR]; Grouse Mountain Resorts Ltd., 6400 Nancy Greene Way, North Vancouver, BC V7R 4K9 Canada [DM], Submitted 11 July 2013, accepted 7 November 2013. Corresponding Editor: Paul Cryan.

TABLE 1. Nightly weather data (means and standard deviations) for the
night of a migratory passage of Hoary Bats (11-12 September) and the
preceding night (10-11 September). P values are based on one-way
ANOVA's comparing means.


Weather data                  10-11 September       11-12 September

Temperature (1)              12.2 [+ or -] 0.34    17.1 [+ or -] 0.54
Relative humidity (%) (1)    70.1 [+ or -] 2.59    51.8 [+ or -] 1.92
Barometric Pressure         102.1 [+ or -] 0.08   101.1 [+ or -] 0.06
  (kPa) (2)
Visibility (km) (2)          31.3 [+ or -] 7.49    34.0 [+ or -] 5.37

Weather data                F-ratio     P

Temperature (1)             1893.32   <0.001
Relative humidity (%) (1)   1064.76   <0.001
Barometric Pressure         1009.00   <0.001
  (kPa) (2)
Visibility (km) (2)            0.77    0.392

(1) data from Cypress Bowl North station, 10 km west of Grouse

(2) data from Vancouver International Airport, 21 km south of Grouse
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Article Details
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Title Annotation:GENERAL NOTES
Author:Nagorsen, David W.; Robertson, Ian; Manky, Devin
Publication:Northwestern Naturalist: A Journal of Vertebrate Biology
Article Type:Report
Geographic Code:1CANA
Date:Mar 22, 2014
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