Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 90 Vol. 25, Special Issue 9 Bats of the Boston Harbor Islands Joshua B. Johnson1,2,* and J. Edward Gates1 Abstract - Insectivorous bats are an integral part of ecosystems because they consume significant quantities of nocturnal insects. White-nose syndrome is decimating populations of susceptible bat species in North America; thus, inventorying and monitoring bats are critical first steps in managing these important populations. We inventoried bat species at Boston Harbor Islands National Recreation Area (BOHA), MA, from 2010 to 2011. Using a combination of capture and acoustic methods, we documented 6 bat species, including Eptesicus fuscus (Big Brown Bat), Lasiurus borealis (Eastern Red Bat), Lasiurus cinereus (Hoary Bat), Lasionycteris noctivagans (Silver-haired Bat), Myotis lucifugus (Little Brown Bat), and Myotis septentrionalis (Northern Long-eared Bat). Bats occurred at all inventoried islands, although most activity of Northern Long-eared Bat, a species Federally listed as threatened, was documented at a mainland site in Worlds End, near Ice Pond. Although the full extent of bat use on the islands remains unclear, we provide evidence of bats roosting and foraging on the islands. During long-term acoustic monitoring at Thompson and Lovells Islands, we assessed the effects of weather and season on bat activity; the latter analysis provided evidence of bats migrating through the area in spring and autumn. Introduction Much research has focused on the conservation and management of bats because of their beneficial roles in ecosystems; for example, bats consume a wide variety of insects, including harmful forest and crop pests (Boyles et al. 2011, Griffith and Gates 1985). However, some populations have been detrimentally affected by a variety of factors, including hibernacula and maternity-roost disturbances, landscape degradation, wind-energy development, and white-nose syndrome (WNS) (Fenton 2003, Frick et al. 2010, Hein and Schirmacher 2016, Kunz et al. 2007, O’Shea et al. 2003, Pierson 1998). The latter 2 factors have caused the deaths of millions of bats (Frick et al. 2010, Hein and Schirmacher 2016). Wind-energy developments primarily affect migrating tree bats, i.e., Lasionycteris noctivagans (LeConte) (Silver-haired Bat), Lasiurus borealis (Müller) (Eastern Red Bat), and Lasiurus cinereus (Beauvois) (Hoary Bat) (Hein and Schirmacher 2016). White-nose syndrome, for which the psychrophilic fungus Pseudogymnoascus destructans (Blehert and Gargas) Minnis and Lindner has been identified as the causative agent (Lorch et al. 2011), mostly affects populations of cave-dwelling bat species, including Eptesicus fuscus (Beauvois) (Big Brown Bat), Myotis leibii (Audubon and Bachman) (Eastern Small-footed Bat), Myotis lucifugus (LeConte) (Little Brown Bat), Myotis septentrionalis (Trouessart) (Northern Long-eared Bat), 1University of Maryland Center for Environmental Science, Appalachian Laboratory, 301 Braddock Road, Frostburg, MD 21532. 2Current address - Pennsylvania Game Commission, 2001 Elmerton Avenue, Harrisburg, PA 17110. *Corresponding author - joshujohns@pa.gov. Manuscript Editor: Joseph Johnson Research at the Boston Harbor Islands NRA 2019 Northeastern Naturalist 25(Special Issue 9):90–109 Northeastern Naturalist 91 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 Myotis sodalis (Miller and Allen) (Indiana Bat), and Perimyotis subflavus (Cuvier) (Tri-colored Bat) (Frick et al. 2010). This syndrome has affected bat species’ distribution, community composition, and relative abundance across landscapes (Ford et al. 2011, Francl et al. 2012). Many bat species of special concern, threatened, or endangered status occur on federal lands, including those administered by the National Park Service (NPS) (Johnson and Gates 2007, Johnson et al. 2005, Rodhouse et al. 2016). To implement informed management decisions regarding bat conservation on NPS lands, which can serve as contiguous tracts of protected land in otherwise fragmented landscapes, collecting baseline inventory data is an important and necessary initial undertaking (Johnson et al. 2008). Although a general assessment of bat-species composition for a given area can be determined by examining geographic ranges and historic records, verifying the presence and relative abundance of species through an inventory process facilitates more informed and accurate management decisions. Simultaneous use of different survey methods, e.g., capture and acoustic recordings, can provide a more complete assessment of species composition, and long-term acoustic monitoring may allow examination of possible weather and seasonal influences on activity as well as timing of migration in an area (Hayes 1997, Johnson et al. 2011a, Murray et al. 1999). The NPS Inventory and Monitoring Program “assists park managers in developing a broad-based understanding of the status and trends of park resources as a basis for making decisions and working with other agencies and the public for the long-term protection of park ecosystems” (Fancy et al. 2009:161). As a part of this program, we conducted an inventory of bat species in Boston Harbor Islands National Recreation Area (BOHA), MA, following discovery of WNS in the region (MDFW 2017, Turner et al. 2011). Although bats are unique among terrestrial mammals in their ability to move among islands, prior to our inventory, only anecdotal observations of bats flying over BOHA were available, and none were identified to species. Regional bat-community species composition and abundance vary seasonally because many bat species are migratory (Cryan 2003, Davis and Hitchcock 1965, Griffin 1945). Seasonal trends in bat activity generally follow a pattern—activity increases closer to summer in concert with increases in temperature, humidity, and precipitation, and gradually decreases to a minimum in winter as bats either enter hibernation or migrate south (Cryan 2003, Hayes 1997). In addition to these seasonal patterns in activity rates, seasonal patterns in species presence and absence also occur, with some species occurring in an area only during their spring and autumn migration. Thus, bat use of BOHA is likely to change throughout the year. The objectives of our inventory were to: (1) provide baseline information on bat species and their relative abundance at BOHA, (2) identify seasonal and nightly activity patterns in long-term acoustic monitoring data, and (3) detect possible migration events. Based on occurrence in the northeastern US, including the State of Massachusetts, we hypothesized that Big Brown Bat, Little Brown Bat, Eastern Red Bat, Silver-haired Bat, Hoary Bat, Northern Long-eared Bat, and Tri-colored Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 92 Vol. 25, Special Issue 9 Bat would occur at BOHA (Kurta 1995, MDFW 2017, Merritt 1987, Whitaker and Hamilton 1998). We did not envisage finding Eastern Small-footed Bat due to lack of roosting habitat, i.e., rock outcroppings on the surveyed islands; nor, did we think that we would detect Indiana Bat, which has not been reported in Massachusetts since 1939 (MDFW 2017). We hypothesized that long-term acoustic monitoring would detect seasonal patterns in bat activity, i.e., higher activity levels May–September, with spikes in activity associated with possible migration events, as previously documented on the Atlantic Coast (Hatch et al. 2013, Johnson et al. 2011a, Miller 1897, Sjollema et al. 2014). Further, we hypothesized that bat activity would be positively correlated with temperature and relative humidity and negatively correlated with wind speed, rainfall, and barometric pressure (Erickson and West 2002, Lacki 1984, Paige 1995, Parsons et al. 2003, Reynolds 2006, Turbill 2008) Field-site Description Boston Harbor Islands National Recreation Area is located in the Seaboard Lowland section of the New England physiographic province and consists of 34 islands and peninsulas in Boston Harbor, MA (Fig. 1). Human population in the Boston Metropolitan Statistical Area was ~4.5 million in 2010. Long-term mean summer (June–August) temperature in Boston was 21.9 °C, and mean annual precipitation was 108 cm. Based on our preliminary observations (Gates and Johnson 2009), information provided by NPS staff, land area, presence of trees, and accessibility, we selected 4 islands and 1 peninsula for our inventories. Oriented from the northwest to the southeast, they were Thompson Island, Peddocks Island, Lovells Island, Grape Island, and Worlds End peninsula (Fig. 1). The islands and peninsula varied in area (upland and intertidal) from 41 ha to 111 ha and in maximum elevation from 21 m to 43 m (Table 1). Thompson Island was the only island in our inventory that had fresh water (less than 0.1-ha patches of wetlands); however, standing water existed only during spring months, and vegetation growing in the wetlands likely prevented bats from using them as freshwater sources. There was a ~0.2-ha freshwater pond (Ice Pond) at Worlds End peninsula that was a potential water source for bats. Methods We employed mist nets and a combination of active and passive acoustic-monitoring to inventory the bat community at BOHA. Mist netting We conducted mist-net surveys in July and August 2010. To capture bats, we used 50-denier, 2-ply, 38-mm–mesh mist-nets (Avinet, Dryden, NY) measuring 2.6 m high and 6 m, 9 m, or 12 m long. We erected mist nets over hiking trails and access roads. The number of mist-net locations depended on size of islands, presence of trees, and logistical constraints. We used 1-tier and 3-tier mist-net arrangements. A 3-tier mist-net arrangement consisted of 3 mist-nets stacked vertically (7.8 m total height) and suspended between a rope and pulley sy stem on two 10-m telescoping poles. We typically deployed mist nets for ~5 h following sunset. No Northeastern Naturalist 93 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 bat surveys were conducted during periods of heavy rain, high wind (≥20 kph), or cold temperatures (less than 10 °C). We determined the species of each captured bat (Menzel et al. 2002), and recorded the weight, forearm length, sex, age, and reproductive condition of all specimens before release. We measured weight (g) and forearm Figure 1. Map of BOHA showing Thompson, Peddocks, Grape, and Lovells Islands and Worlds End peninsula (Image source: Tiner et al. 2003). Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 94 Vol. 25, Special Issue 9 length (nearest 0.5 mm) with a spring-scale (Pesola AG, Baar, Switzerland) and bird-wing rule (Avinet, Dryden, NY), respectively. We determined age, i.e., adult or juvenile, by examining the level of epiphyseal–diaphyseal fusion in finger bones (Kunz and Anthony 1982). To determine the reproductive condition of female bats, we palpated the abdomen and inspected the mammary glands (Racey 1988). We marked captured bats on the forearm with a non-toxic marker (Newell Brands, IL) to identify any recaptures. Bat-capture and handling protocols were approved by the Institutional Animal Care and Use Committee of the University of Maryland Center for Environmental Science and followed the guidelines of the American Society of Mammalogists (Gannon et al. 2007). We followed US Fish and Wildlife Service (USFWS) disinfection protocols to decontaminate mist-netting equipment and bat-measuring and handling equipment to avoid potential spread of WNS between captured bats (USFWS 2009). Active acoustic-monitoring We conducted active acoustic-monitoring in July and August 2010. We used Anabat II (Titley Electronics, Ballina, Australia) broadband, frequency-division bat detectors linked to compact flash-storage zero-crossing analysis interface modules (ZCAIM) to actively monitor for bat activity in various cover types, including Ice Pond, forests, and fields. The number of sample locations depended on island size and variety of cover types, i.e., larger islands and those with a greater variety of cover types had more sample locations. We conducted one 20-min survey at each sampling location between sunset and 0100 h (Johnson et al. 2002). We remained stationary at sampling locations while actively scanning with the bat detector for bat activity. We examined active acoustic-monitoring data and capture data for bat Table 1. Descriptions of islands and a peninsula surveyed for bats at Boston Harbor Islands National Recreation Area, MA, 2010–2011. Distance (km) to Island or Area Maximum and name of # of peninsula name (ha) elevation (m) nearest mainland buildings Vegetation description Thompson 69 24 0.50 to Squantum 14 Quercus robur L. (English Oak), Acer platanoides L. (Norway Maple), open lawns, saltmarshes, freshwater wetlands Peddocks 85 24 0.39 to Hull 26 English Oak, Norway Maple, Rhus typhina L. (Staghorn Sumac) Lovells 48 24 2.20 to Hull 0 Staghorn Sumac, Betula populifolia Marshall (Gray Birch) Grape 41 21 0.47 to Webb Park 0 Staghorn Sumac, Gray Birch, Populus tremuloides Michx. (Quaking Aspen) Worlds End 111 43 – 0 Oak and maple trees, fields, brackish wetlands, freshwater pond Northeastern Naturalist 95 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 activity within 20 min of civil dusk. Bat activity occurring within this arbitrary timeframe may indicate that bats were roosting on the islands. Long-term passive acoustic-monitoring From April 2010 to April 2011, we conducted long-term passive acoustic-monitoring to investigate the possibility of bat migration at BOHA. We established 2 bat-monitoring stations (BMS)—1 each on Thompson and Lovells Islands, ~7 km apart (Fig. 1). Each BMS consisted of an SD 1 Anabat bat detector (Titley Scientific, Columbia, MO) contained in a weatherproof enclosure attached to a guyed 10-m pole. To the top of the pole, we attached a polyvinyl chloride tubing assembly that protected an Anabat microphone. We positioned the microphone straight down toward a plexiglass deflector, which was oriented at a 45° angle relative to the microphone. The deflector redirected echolocation passes up to the microphone, which was attached to the SD 1 Anabat detector via a 10-m audio cable. Both BMS were >10 m from the nearest tree lines. Power for each BMS was maintained using an external 12-V, 24-Ah battery recharged by a 50-W photovoltaic panel. The BMS monitored bat activity from 1700 to 0700 hr nightly throughout the year (15 April 2010–15 April 2011). Bat echolocation-pass data were remotely uploaded daily by each BMS to http://getmylog.com; we downloaded the data for analysis. Echolocation identification We used Analook 4.8p computer software to visually inspect bat passes and assign identifications for both actively and passively collected acoustic data (Corben 2001). We used frequency and shape characteristics to manually identify echolocation passes (Britzke et al. 2002, Fenton and Bell 1981, Murray et al. 2001, O’Farrell et al. 1999). Echolocation passes were identified by comparing our recordings to a library consisting of echolocation passes collected from hand-released bats marked with chemiluminescent tags (US Geological Survey, Virginia Cooperative Fish and Wildlife Research Unit, Blacksburg, VA, unpubl. data). We attempted identification only of those echolocation passes containing ≥3 pulses (Johnson et al. 2002). We characterized a bat pass as ≥1 pulse emitted by an individual bat within a call file. We also noted feeding passes (rapid emission of echolocation pul ses, distinct from search-phase pulses), which are evidence of bats actively foraging. Modelling migration activity To analyze long-term bat activity recorded at our 2 BMS for evidence of possible bat migration, we used time-series analysis to account for effects of weather and to model nightly and seasonal bat passes. To examine effects of weather on bat activity, we obtained data from a remote automated weather station located at Logan International Airport, Boston, MA (NOAA 2011). Data included nightly rain accumulations (cm; Rainacc), number of nightly hours rainfall accumulated (Rainhr), mean nightly air temperature (°C; Temphr), daily maximum air temperature (°C; Tempmax), daily minimum air temperature (°C; Tempmin), mean nightly wind speed (m/s; Windmean), mean nightly relative humidity (%; RelHum), and mean nightly barometric pressure (kPa; Baro). We averaged hourly data during non-daylight Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 96 Vol. 25, Special Issue 9 hours to obtain nightly means. To achieve statistical normality, we used natural-log transformations for the number of hours that rainfall accumulated within a night (Rainhr) and square-root transformations for nightly rain accumulations (Rainacc) and wind speed (Windmean) (Zar 1984). Prior to analyzing effects of weather on bat activity, we tested explanatory variables for collinearity to reduce model over-fitting. We computed Pearson’s product-moment correlation coefficients for all pairs of variables and censored 1 member of any pair having a correlation >0.60 (Grewal et al. 2004). Collinearity was significant for 4 variable pairs, including Rainhr and Rainacc (r = 0.81), Temphr and Tempmax (r = 0.96), Temphr and Tempmin (r = 0.99), and Tempmax and Tempmin (r = 0.96). We retained Rainhr because the periodicity of rain may have more of an effect on bat activity than precipitation totals. We retained Temphr because it represented mean temperatures recorded throughout the night when bats are active rather than maximum and minimum temperatures recorded during a 24-h period. Seasonal trends may follow a sine or cosine function (Montgomery et al. 2008); thus, we incorporated a sinusoidal trigonometric function into our time-series analysis to account for seasonal trends in bat passes and weather patterns. Number of bat passes may be serially autocorrelated, i.e., bat passes are not independent on successive nights (Hayes 1997, Milne et al. 2005). As with seasonal trends, autocorrelation also must be accounted for in time-series analyses. We used an autoregressive (AR) modeling approach to estimate autocorrelation structure of bat passes (RDCT 2008). We iteratively incorporated AR (p) structures, where p = lag, to estimate appropriate AR order, i.e., the number of nights elapsed between independent nights of data. We used Akaike’s information criterion for small sample sizes (AICc) to rank models. Candidate models separated by ≤3 AICc were considered competing models. We used Akaike weights, wAICc, to select the most parsimonious model in the candidate set. After selecting the best approximating model, we incorporated the estimated correlation coefficient (Φ) in the final models to account for nightly serial autocorrelation of bat passes. We used generalized least squares to develop a predictive model of bat activity and account for seasonal trends and effects of weather. Models followed the general form of: natural-log(Passesj) = β0 + β1 natural-log(Rainhr) + β2 (Temphr) + β3 sqrt(Wind mean) + β4 (RelHum) + β5 (Baro) + β6 sin(2πj / T) + β7 cos (2πj / T) + εj, where Passesj was the number of bat passes recorded on night j; Rainhr was the number of hours of precipitation on night j; Temphr was the mean air temperature on night j; Windmean was the mean nightly wind speed on night j; RelHum was the mean nightly relative humidity on night j; Baro was the mean nightly barometric pressure on night j; β0-7 were coefficients estimated by regression; sin and cos terms described seasonality, where j was night and T was total number of nights per year; and εj was an error term. For each BMS, we developed models for: (1) all passes, including bat passes unidentifiable to species; (2) passes from tree bats, including Eastern Red Bat, Hoary Bat, and Silver-haired Bat; and (3) passes from cave bats, including Big Brown Bat, Little Brown Bat, and Northern Long-eared Bat. Northeastern Naturalist 97 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 We examined model residuals (predicted–observed data) that exceeded 2 standard deviations (SD) from the mean. Nightly bat-pass totals that exceed predicted values by >2 SD may be evidence of migration events, particularly if these events occurred during spring or autumn and occurred on the same nights at the 2 BMS (Johnson et al. 2011a, Montgomery et al. 2008). Results Through a combination of mist netting, active acoustic-monitoring, and passive long-term acoustic monitoring, we documented 6 bat species at BOHA. These included Eastern Red Bat, Big Brown Bat, Hoary Bat, Little Brown Bat, Silver- Haired Bat, and Northern Long-eared Bat. Mist netting From 26 July 2010 through 5 August 2010, we conducted mist-net surveys at 7 locations for 9 nights and captured 49 bats representing 4 species (Table 2). We captured bats at all locations, with the exception of 1 location on Grape Island. We captured reproductively active female Big Brown Bats, Eastern Red Bats, and Northern Long-eared Bats. At Thompson Island, we captured 3 Eastern Red Bats within <6 min of civil dusk; all other captures on islands were >20 min after civil dusk. We did not recapture any bats. Active acoustic-monitoring We conducted active acoustic-monitoring at 30 locations over 5 nights (26–30 July 2010) (Table 3), recording activity of identifiable bat species at 23 (76.7%) locations. There was no activity recorded at 3 locations, and bat passes were not identifiable to species at 4 locations. During active acoustic-monitoring, we recorded echolocation passes of 5 identifiable species—Big Brown Bat, Eastern Red Table 2. Bats captured using mist nets at Boston Harbor Islands National Recreation Area, MA, July– August 2010. EPFU = Eptesicus fuscus, LABO = Lasiurus borealis, MYLU = Myotis lucifugus, and MYSE = Myotis septentrionalis. Locations Nights Total Adult Juvenile Adult Juvenile Field site sampled sampled Species capturesA male male female female Thompson 3 3 EPFU 1 1 0 0 0 LABO 6 2 1 2 0 Peddocks 1 2 EPFU 11 6 0 2 2 LABO 2 2 0 0 0 MYSE 1 0 0 1 0 Grape 2 2 LABO 1 0 0 1 0 EPFU 1 0 0 0 0 Worlds End 1 2 EPFU 12 1 3 7 1 MYLU 1 0 0 0 1 MYSEB 13 4 0 5 2 ADiscrepancies in total number of bats captured and sum of males and females are because bats escaped before sex or age could be determined. BIncludes 1 male for which age was not determined. Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 98 Vol. 25, Special Issue 9 Bat, Hoary Bat, Little Brown Bat, and Northern Long-eared Bat. We recorded feeding activity from Big Brown Bats and Eastern Red Bats (Table 3). Twenty minutes before civil dusk, we recorded 2 Big Brown Bat echolocation passes at Thompson Island, 7 Eastern Red Bat echolocation passes at Grape Island, and 6 Eastern Red Bat echolocation passes at Peddocks Island. Long-term passive acoustic-monitoring The 2 BMS recorded 4320 bat passes from 6 identifiable species, including Eastern Red Bat, Big Brown Bat, Hoary Bat, Little Brown Bat, Silver-Haired Bat, and Northern Long-eared Bat (Table 4, Fig. 2). We identified 0.6% of bat passes as Myotis spp. due to poor pulse-quality. We were unable to identify 32.1% of bat passes to species because they consisted of less than 3 echolocation pulses or were of poor quality. Bat-monitoring stations at both islands functioned during all 365 nights of the sample period. Although we documented the same species at both Thompson and Lovells Islands, 1.73 times as many passes were recorded at Thompson Island. Bat activity began to increase at the beginning of May, peaked in July and August, and then declined through October (Fig. 2). Relatively few passes were recorded from November to April. We observed these trends at both Table 3. Bat-echolocation passes recorded using Anabat II bat detectors actively monitoring at Boston Harbor Islands National Recreation Area, MA, July 2010. NOID = unidentifiable bat pass, EPFU = Eptesicus fuscus, EPFUFP = Eptesicus fuscus feeding pass, LABO = Lasiurus borealis, LABOFP = Lasiurus borealis feeding pass, LACI = Lasiurus cinereus, MYLU = Myotis lucifugus, MYSE = Myotis septentrionalis, and MYSP = unidentifiable Myotis spp. Locations Total Species Field site sampled passes NOID EPFU EPFUFP LABO LABOFP LACI MYLU MYSE MYSP Thompson 9 53 10 20 0 8 0 15 0 0 0 Peddocks 7 14 2 0 0 10 0 0 0 0 2 Grape 7 108 9 84 5 12 0 0 1 0 2 Worlds End 7 239 21 90 0 59 2 2 12 16 39 All 30 414 42 194 5 89 2 17 13 16 43 Table 4. Bat-echolocation passes recorded using Anabat SD 1 bat detectors set for continuous monitoring (n = 365 nights) at Boston Harbor Islands National Recreation Area, MA, April 2010–April 2011. NOID = Unidentifiable bat pass, EPFU = Eptesicus fuscus, EPFUFP = Eptesicus fuscus feeding pass, LABO = Lasiurus borealis, LABOFP = Lasiurus borealis feeding pass, LANO = Lasionycteris noctivagans, LACI = Lasiurus cinereus, LACIFP = Lasiurus cinereus feeding pass, MYLU = Myotis lucifugus, MYSE = Myotis septentrionalis, and MYSP = unidentifiable Myotis spp. Species Total EPFU LABO LACI Field site passes NOID EPFU FP LABO FP LANO LACI FP MYLU MYSE MYSP Thompson 2738 820 845 11 951 26 1 75 1 34 1 11 Lovells 1582 565 488 2 468 14 1 20 0 25 1 14 All 4320 1385 1333 13 1419 40 2 95 1 59 2 25 Northeastern Naturalist 99 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 islands for tree bats and cave bats. Eastern Red Bats were recorded mid-April– October. Hoary Bats were recorded from May–early September. We recorded 2 passes from Silver-haired Bats: 1 at Thompson Island in April, and 1 at Lovells Island in late-August. Big Brown Bats were recorded mostly from May–October, but a few passes were recorded in November and mid-February. We recorded Little Brown Bats mostly from mid-July–early November. We recorded 2 passes from Northern Long-eared Bat; 1 at Thompson Island in early-June and 1 at Lovells Island in early-August. Unidentifiable Myotis spp. were recorded mostly from late July–October. Figure 2. Echolocation passes of (a) Big Brown Bat, (b) Eastern Red Bat, (c) Silver-haired Bat, (d) Hoary Bat, (e) Little Brown Bat, and (f) Northern Long-eared Bat recorded at Anabat bat-monitoring stations on Thompson and Lovells Islands, Boston Harbor Islands National Recreation Area, April 2010–April 2011. Note different scales on y-axes. Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 100 Vol. 25, Special Issue 9 Modelling migration activity The best candidate AR function for the time-series models differed between the BMS and species groups according to wAICc values. There were 3 or more competing models for each island and species group (Table 5). For each island and species group, we chose the model with the highest wAICc value: AR(12), AR(12), and AR(3) for all bats, tree bats, and cave bats, respectively, at Thompson Island; and AR(8), AR(11), and AR(8) for all bats, tree bats, and cave bats, respectively, at Lovells Island. After accounting for seasonal trends and autocorrelation, mean nightly temperature, mean nightly wind speed, and relative humidity significantly affected total bat activity, but only at Thompson Island (Table 6). Bat activity was positively related to mean nightly temperature, and negatively related to mean Table 5. Autoregressive (AR) model selection using Akaike information criteria (AICc) difference with correction for small sample sizes (ΔAICc), and model weight (wAICc) for determining temporal independence of bat-echolocation passes recorded at 2 bat monitoring stations at Boston Harbor Islands National Recreation Area, April 2010–April 2011. Only candidate models less than 3 ΔAICc are included. p = order of AR model Bat monitoring station Bat groupA AR(p) model AICc ΔAICc wAICc Thompson Island All 12 707.50 0.00 0.367 11 707.58 0.08 0.352 13 709.14 1.65 0.161 Tree bats 12 570.98 0.00 0.297 13 571.30 0.32 0.253 11 571.53 0.55 0.225 14 573.09 2.11 0.104 Cave bats 3 739.10 0.00 0.394 7 740.61 1.51 0.185 4 740.91 1.81 0.159 Lovells Island All 8 713.90 0.00 0.329 7 714.99 1.09 0.191 9 715.86 1.95 0.124 2 716.35 2.44 0.097 13 716.43 2.53 0.093 Tree bats 11 536.19 0.00 0.195 9 536.38 0.19 0.177 8 536.41 0.22 0.174 12 536.90 0.71 0.137 7 537.05 0.86 0.127 13 537.83 1.64 0.086 10 538.26 2.07 0.069 Cave bats 8 599.97 0.00 0.547 9 601.64 1.67 0.237 11 602.93 2.96 0.125 AAll = all bat echolocation passes, including unidentifiable passes: tree bats = echolocation passes from Lasiurus borealis, L. cinereus, and Lasionycteris noctivagans; cave bats = echolocation passes from Eptesicus fuscus, Myotis lucifugus M. septentrionalis, and unidentifiable Myotis spp. Northeastern Naturalist 101 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 Table 6. Effects of weather conditions on bat activity recorded at Boston Harbor Islands National Recreation Area, April 2010–April 2011. Bat monitoring station Bat groupA VariableB Coefficient SE P Thompson Island All Intercept 7.520 5.519 0.174 Temphr 0.012 0.006 0.035 Rainhr -0.046 0.190 0.810 Baro -0.062 0.053 0.240 Windmean -0.212 0.075 0.005 RelHum -0.005 0.003 0.049 Sine -0.700 0.198 less than 0.001 Cosine -1.129 0.234 less than 0.001 Tree bats Intercept 8.989 4.450 0.044 Temphr 0.005 0.004 0.320 Rainhr 0.005 0.160 0.974 Baro -0.075 0.042 0.077 Windmean -0.230 0.063 less than 0.001 RelHum -0.006 0.002 0.004 Sine -0.563 0.175 0.001 Cosine -0.817 0.205 less than 0.001 Cave bats Intercept 4.784 5.613 0.395 Temphr 0.011 0.006 0.066 Rainhr -0.009 0.221 0.968 Baro -0.045 0.053 0.406 Windmean -0.052 0.084 0.539 RelHum -0.003 0.003 0.333 Sine -0.288 0.102 0.005 Cosine -0.476 0.150 0.002 Lovells Island All Intercept 4.373 5.607 0.436 Temphr 0.005 0.006 0.427 Rainhr -0.160 0.199 0.421 Baro -0.036 0.053 0.505 Windmean -0.036 0.076 0.636 RelHum -0.002 0.003 0.436 Sine -0.654 0.193 less than 0.001 Cosine -0.911 0.227 less than 0.001 Tree bats Intercept 8.392 4.281 0.051 Temphr 0.001 0.004 0.800 Rainhr -0.160 0.156 0.306 Baro -0.078 0.041 0.057 Windmean -0.027 0.061 0.659 RelHum -0.001 0.002 0.532 Sine -0.456 0.169 0.007 Cosine -0.484 0.198 0.015 Cave bats Intercept 4.948 4.657 0.289 Temphr 0.005 0.005 0.343 Rainhr 0.004 0.174 0.982 Baro -0.044 0.044 0.323 Windmean -0.076 0.065 0.241 RelHum -0.003 0.002 0.191 Sine -0.251 0.127 0.049 Cosine -0.497 0.159 0.002 AAll = all bat echolocation passes, including unidentifiable passes: tree bats = echolocation passes from Lasiurus borealis, L. cinereus, and Lasionycteris noctivagans; cave bats = echolocation passes from Eptesicus fuscus, Myotis lucifugus M. septentrionalis, and unidentifiable Myotis spp. BTemphr = mean nightly air temperature; Rainhr = number of hours of precipitation; Windmean = mean nightly wind speed; RelHum = mean nightly relative humidity; Baro = mean nightly barometric pressure. Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 102 Vol. 25, Special Issue 9 nightly wind speed and relative humidity. Tree-bat activity at Thompson Island was negatively affected by mean nightly wind speed and relative humidity (Table 6). No weather variables significantly affected cave-bat activity, nor did they significantly affect overall bat activity at Lovells Island (Table 6). For Thompson Island, fits of final models for all bat activity (adjusted r2 = 0.708), tree-bat activity (adjusted r2 = 0.612), and cave-bat activity (adjusted r2 = 0.411) were generally better than final models for Lovells Island for all bat activity (adjusted r2 = 0.558), tree-bat activity (adjusted r2 = 0.409), and cave-bat activity (adjusted r2 = 0.369). Final models used to predict seasonal trends in bat activity varied by night, location, and among species groups. Residuals that exceeded 2 SD (n = 19) in models for total bat activity occurred at both islands during documented bat-migration periods, i.e., April–May and late-July–October, and on the same nights at both islands of 1 May, 24 and 27 July, and 4 and 10 August (Fig. 3). At both islands, residuals that exceeded 2 SD (n = 22) in models for tree-bat activity occurred during autumn migration periods, i.e., late-July–October, and on the same nights of 27 July, 18 and 19 August, and 16 September. Residuals that exceeded 2 SD (n = 31) in models for cave-bat activity occurred at both islands May–September, and on the same nights of 1 May; 6, 24, and 27 July; and 4 and 10 August (Fig. 3). From November through April, no residuals exceeded 2 SD at either island or for any species group. Discussion It is important to note that our inventory took place after WNS was confirmed in Massachusetts in winter 2007–2008 (MDFW 2017). Nevertheless, we detected 6 of 9 bat species that potentially could occur at BOHA. At the time of our inventory, all 6 taxa were considered globally secure or apparently secure (Harvey et al. 1999). Since then, the Northern Long-eared Bat has been listed as a threatened species by the US Fish and Wildlife Service and endangered under the Massachusetts Endangered Species Act. This forest-interior–dwelling species occurred throughout Massachusetts; however, because of its susceptibility to WNS, its population has declined 90–100% (Blehert et al. 2009, Caceres and Barclay 2000, MDFW 2017). Coastal areas in the Northeast, including Martha’s Vineyard, Cape Cod, Nantucket, and Long Island, however, continue to support small breeding populations (BiodiversityWorks 2016). The state-endangered Little Brown Bat also was once widespread throughout Massachusetts; its populations have been reduced by 90–100% by WNS (MDFW 2017). It is likely that, prior to WNS, we would have observed higher numbers of Northern Long-eared Bats and Little Brown Bats and in more locations than we did in our inventory. As we hypothesized, we did not capture or record Eastern Small-footed Bat or Indiana Bat; both are endangered in Massachusetts. Unique habitat, e.g., extensive rock outcrops, required by Eastern Small-footed Bat does not exist at BOHA (Johnson and Gates 2008); furthermore, within Massachusetts it has only been found in 2 counties in the western part of the state. Although habitat conditions may exist in BOHA that are suitable for Indiana Bat (Silvis et al. 2016), this rare species has not been documented in Massachusetts since 1939 (USFWS 2007). We also did Northeastern Naturalist 103 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 Figure 3. Standard deviations between actual and predicted nightly echolocation passes from (a) all bat species, (b) tree-bat species, and (c) cave-bat species recorded at Anabat bat-monitoring stations on Thompson (triangles) and Lovells (circles) Islands, Boston Harbor Islands National Recreation Area, April 2010–April 2011. Note different scales on y-axes. Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 104 Vol. 25, Special Issue 9 not document the Tri-colored Bat. Tri-colored Bats had a widespread distribution in Massachusetts, but populations in the Northeast have suffered losses averaging 90% from WNS (MDFW 2017). Tri-colored Bats may be locally extirpated or exist in such low abundance that they were difficult to detect. High bat-species diversity at Boston Harbor Islands National Recreation Area is likely due to the mosaic of forests and open areas found at BOHA (Ford et al. 2005, Krebs 1999). The location with highest bat activity (both active acoustic echolocation-call activity and number of captures) was at Ice Pond at Worlds End peninsula. This freshwater source likely is the most accessible and dependable at BOHA, attracting bats from within and outside the unit. Furthermore, mature intact forests at Worlds End may provide a refuge in an otherwise heavily developed area, similar to findings in Rock Creek Park located in Washington, DC (Johnson et al. 2008). Captures of Northern Long-eared Bats at Worlds End and Peddocks Island indicate BOHA is providing habitat in an otherwise urbanized setting for this species, though the extent to which bats are using islands at BOHA warrants further research. Our inventory included only a portion of BOHA, but other islands and peninsulas may provide roosting and foraging habitats for bats as well. For example, bats have been observed flying over Bumpkin Island, which we did not survey (M. Albert, NPS, BOHA, MA, pers. comm.). Our active acoustic-monitoring and capture data suggest Eastern Red Bats and Big Brown Bats may be roosting on BOHA islands. Interestingly, the islands we inventoried did not have available freshwater sources. Bats roosting on the islands must either fly to freshwater sources on the mainland, e.g., Ice Pond at Worlds End, or perhaps partly maintain water balance by intake of seawater, as documented in marine Myotis vivesi Menegaux (Fish-eating Bat) in Mexico (Carpenter 1968). Distances between the islands we inventoried and the mainland are within known home-range distances of Big Brown Bats (Menzel et al. 2001) and Eastern Red Bats (Walters et al. 2007) in urban settings. Further research could elucidate the home ranges and habitat use by bats that may be roosting on the islands. Long-term acoustic monitoring answered many questions regarding bat species occurrence and activity patterns at BOHA. For example, we documented Silverhaired Bats only through use of BMS in our inventory, though we caution that echolocation-call characteristics of Silver-haired Bats and Big Brown Bats are similar. We gained a broader knowledge of the seasonality of bat use at BOHA; information that would not have been obtained through our capture and active acoustic-monitoring efforts. Indeed, seasonality of bat activity was more consistent than weather variables in terms of predicting bat activity. It is unclear why effects of weather variables on bat activity at BOHA were inconsistent, and insignificant in most cases, between islands and bat groups. Perhaps overall differences in bat activity, and consequently model robustness, between Thompson Island and Lovells Island could be a result of more roosting habitat on the former. Regardless, when bat activity was significantly associated with weather variables, the associations were consistent with past research, with the exception of relative humidity, which we found was negatively associated with bat activity (Erickson and West 2002, Lacki 1984, Paige 1995, Parsons et al. 2003). Northeastern Naturalist 105 J.B. Johnson and J.E. Gates 2019 Vol. 25, Special Issue 9 It appears that bats may be migrating through BOHA, mostly in late July and early August, known migration periods for tree bats and perhaps cave bats (Cryan 2003, Davis and Hitchcock 1965, Griffin 1945). Annual patterns of tree-bat activity were largely in agreement with museum records for the area, indicating that these bat species arrive or migrate through the area in May and stay through October (Cryan 2003). It has been hypothesized that bats follow linear landscape features when migrating (Cryan and Brown 2007, Kunz et al. 2007). Furthermore, Eastern Red Bats have been observed flying near the Atlantic Coast during migration periods (Hatch et al. 2013, Peterson et al. 2014, Sjollema et al. 2014). Consequently, the geographic position of BOHA may be in migratory flight paths. It is difficult to say if bats are moving directly between the Deer Island and Hull peninsulas, using Lovells Island as a stopover site, or if they are using any other islands as they cross over Boston Harbor. It is assumed that tree bats travel in a north-south direction during autumn, but cave bats may migrate in any direction (Davis and Hitchcock 1965, Griffin 1945). In a banding study in New England, Little Brown Bats typically traveled in a southeast–northwest direction. One bat’s banding and recovery sites were on opposite sides of Boston Harbor, indicating possible migration through the area (Davis and Hitchcock 1965). Our results indicated that bats may migrate through BOHA, but the total number of echolocation passes recorded suggest the number of migrants passing through BOHA may be relatively small compared to other areas of the US (Arnett et al. 2008; Johnson et al. 2011a, 2011b; Sjollema et al. 2014). Though bats may not always echolocate during migration, the number of echolocation passes recorded and number of fatalities at wind-energy facilities have been shown to be correlated (Johnson et al. 2011b). At minimum, our results add to and corroborate the findings of prior research focused on bats migrating along the coast of Massachusetts (Cryan 2003, Miller 1897). Indeed, Eastern Red Bats, Hoary Bats, and Silver-haired Bats have been documented along the Atlantic coast since the 1800s, according to museum records, as well as other observations and studies (Cryan 2003, Hatch et al. 2013, Miller 1897, Peterson et al. 2014). During our study, Silver-haired Bat activity was much lower than for Eastern Red Bats and Hoary Bats, a difference that has remained consistent for decades (Cryan 2003, Johnson et al. 2011a, 2011b, Miller 1897). Similar to historical reports of tree bats in Massachusetts, our findings also showed increased activity levels of tree-bat species during autumn, a season known for migration of these species. However, more definitive research is warranted to confirm migration of tree bats through BOHA. Acknowledgments We appreciate the hard work of C. Daggett and J. Torzewski during the summer field season. We are grateful for the help of numerous National Park Service (NPS) staff, particularly M. Albert, S. Colwell, D. Hayes, C. Martin, B. Masson, S. Walasewicz, G. Waters, and volunteers who assisted us at all the NPS units that we inventoried. We thank A. Kozlowski for developing the MS Access database. We appreciate J. Scully and B. Dowd of Outward Bound, Thompson Island, for providing housing and assistance. University Northeastern Naturalist J.B. Johnson and J.E. Gates 2019 106 Vol. 25, Special Issue 9 of Massachusetts, Boston, graciously provided transportation at Boston Harbor. We thank several reviewers for their comments on this manuscript. Funds were provided by the Maryland Department of Natural Resources, Power Plant Research Program, to help with bat-monitoring on Thompson and Lovells Islands. The NPS provided funding for this inventory. This article is Scientific Contribution No. 5494 of the University of Maryland Center for Environmental Science, Appalachian Laboratory. Literature Cited Arnett, E.B., W.K. Brown, W.P. Erickson, J.K. Fiedler, B.L. Hamilton, T.H. Henry, A. Jain, G.D. Johnson, J. Kerns, R.R. Koford, C.P. Nicholson, T.J. O’Connell, M.D. Piorkowski, and R.D. Tankersley Jr. 2008. Patterns of bat fatalities at wind-energy facilities in North America. Journal of Wildlife Management 72:61–78. 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