JOllrl/ol oj'Mf.Jmmalogy. 90(6):1330--1340, 2009

CAUSES OF BAT FATALITIES AT WIND TURBINES: HYPOTHESES AND PREDICTIONS

P.,\L.'L M. CRYA:\'" A'<D ROBERT M R. BARCLAY

I.!niled Swte.\ (j('%gic<d S'UJ1.'('Y, Fort Cu/lim Science Cenrer, Fol'! Col/InS, CO 80526. USA (PMC) UniH·/.iity oleu/gary. Deparrmem o{Biologlcai SUei/LeI. Calgarl', AUJetta T1N IN-I, Canada (RMRBJ

Thousands of indu~trial-scaJe wind turhine\ are being built across the world each year to meet the gmwing demand for sustainable energy. Bats or certain species arc dying at wind turbines in unprecedented numbers. Species of bats consi';tendy affected by tnrhines tend 10 be those that rely 011 tree~ as roosts and most migrate Illng distance~. Allbough considerable progress has been made in recen!. years 1O .....·ard better underqanding the problem. the causes of bat fatalilies a[ turbines remain unclear. In this synthesi ... we review hypothe,ized causes of bal fal.alilie'> al turhines. Hypotheses of cause fall mto 2 general calegories-proximaLe and ldlimate. Proximal(, cau,c, explain the direct mcan~ by which bat, die at turbines and Include collision with towers ilnd rol<lling biade." and barolrauma. Uhimille callses explain Why hal., come dose to turbines and include 3 general type:>: random collisions. Cl)incidental collisions. and collision" that result from atlraction of bars to turbines. The random coli ision hypothesis POSilS that interactions hetween bats and turbines are random events and that ralalities arc representative of [he baLS prescnt at a site. Coincidental hypotheses posit lhal certain aspeClS or bat distribution or behavior put them ar risk of collision and includc aggregation during migration and seasonal increascs in night activity associated with feeding or mating. A surprising number of allraction hypotheses suggest that bats might be attracted to turbines out of curiosity. misperception, or as potential feeding, roosting, flocking. and m<lting opporlnnities. Jdentifying, prioritizing, and iesting hypothesized causes of bal collisions with wind turbine'> are vital step.. toward developing practical solutions to the problem.

Key word... i11HilctioJl, Oim, ('all~e", colliwlD. migration. mitlg~ltioll. mortality, wind energy

lndw;trial wind cnergy production is increa'>ing throughout to increase dramatically. and turbines will continue to be built the world to mcet growing demand for electricity generated in large numbers in windy a[Cas around thc world. witholJl carbon erni~sioo (United States Energy Information Bat fatalilics at wind turbines were an unanticipated and Administration. www.eia.doe.gov, accessed 15 M<l) 20U9}. unprccedclllcd phenomenon. Beginning in thc btl' 1990s, Advances in wind turbine [echnology and the cost effective­ biologists begi.ln finding hat carea~se~ beneath turbines while nc~s of ~vind energy have led to a tremendous increase in the studying bird mortality at wind energy ~i[es (.John~on d aL number or wi.nd lurbines deployed lhroughOLlt Ihe world 2003). Thi.,; trend continued, and during 20m high-profile during Ihe pim decade. As of \·1ay 2009, global induslrial­ evenrs involving hats and wind rurbines in the Appalachian ;;call:' developmclll has resulted ill a total installed energy Mountains of Nlll'lb America led 1.0 growing COllcen; over the eapacllY 01' about )21,000 megaw<lus (MW-Wor\d Wind problem (Kunz et al. 200711). Since then, multiple ~tudies have Energy Associalioll, www.wwindea.org, accessed 15 May consistently found bat fatalities at wind energy sites in NOlth 20(9). with the average turbine now contributing between 1.5 Ameriefl, and unprecedented fatalilY rates of certain spccie,; at and 2.5 MW. Typical wind turbines currently have 10\.\--<:f ,ome felcilities (Amen cr al. 2008: Johnson 20(5). The heights of 60-100 nl. blade lengths of 30-50!D, and cumulative impact, of slich mortality on affcClC'·d spccics of maximum hlade tip speeds ot' about 240-300 ~m/h. Jn the bULS could have long-term population effecl.~ (Kulv d aL coming decades, the demand for elecnical energy is expected 2007b). Before ihis recent prohlem of bal fatalitie.' al wind

turbines. collision fatalities of bals at other tall anthropogenic structures were rmcly reponed, and the number of carca~se~

recovered after fatality events was consisLent!y low (Anony­mow, 1961, Avery and Clemcnl 1972; Crawford and Ba~,~r

:r) 2009 American Soeict} of i\1ammalogists 1981: Elder and Hansen 1967; Galuc:r J962; Ovcring 1936; www.mllnunll!ogy.org Saunder:; 1930; Terre~ 1956; Timm 1989; Van Gelder 19.%;

I.no

1331 DC( emher 2()09 SPECIAL FEATURE-CAUSES OF BAT FATALrnEs AT TURBINES

Zinn and Baker 1979). Unlike the situation with birds and lheir collisions with tall structures (Avery el al. 1980; Kuvlesky ct al. 20(7), there is no evidence or human-indllt:ed mortality to dny of" the affected bat species that is of similar magnitude to mowility at wind turbine". Mortality of bats at wind turbinc~

thus wmrant~ additional study and explanaLion in an cffort to dctcrnllne the bil)logicill consequences and develop mitigalion n1ca!,LJr~.~.

Several eonsi"em patterns haVi: emerged from the fatality c1ata on bats at wind Imbines. The species consistently

alTected, in terms of both overall numbers and geographlc

dIstribution, arc thosc that rely heavily on trees as natural r()(wls througbout the year--tree bats (Grirrin 1970). In North

i\rneriea. these species include hoary bats (Las/Ul'lIs cmereus), ca\lcrn rcd bat;; (L. horealil·). and silver-haircd bats (La"io­

TI)'Cleris Ilocllvagalls). A 41h North American species, the tricolored b;1I (Perimyur,s subfEm'lts). also mighl he charaCler­

i/.ed as a tree but (Findley 1954: Veilleux et al. 2(04). bUl its ,eaSl)na! whereabout, duri ng mueh of [he year relnain obscure

(Barbour and Davis 1969; Fujita and Kunt. 1984). in North Americ,L Irec balS comprisc aboui 75(~, of documcntcd

rataiitits to date, and hoary bats make up about haIr of all fatalities (Arnett et al. 20(8). European species of bats mosl

affected by wi nd lurbines include the noclu!c bat (Nyctalus

flocwlll), Lci~lcr's bat (lV. leisleril, Nalhusius's pipist.rcHc (FI].JIS/rdllis nathusii), common pipistrelle (P. pipisrrellw;). S0prano pipislrelle (P. pygmaeu,I), and particolored bm (Vesperlilio J1'Iurill/.lS), all or which rely on (rees a$ roosts, but also sometimes use human-madc s(IlIUmes (Bach and

Rahmel ~004; Di..irr and Badl 2004; Rodrigues et a1. 20(8).

!\Iwthcr <,;onsistcn! eharadcrislic or species of bats most affected by turbines is their propensity ror long-distance, ]uliludinal migrarion (Cryan 2003; Hullerer e[ al. 2005;

Sld'fens ct al. 20(7). HowevCi', I ~orth Amcricall species

afTeetcd by turbines currently is not eonsidered a migrant (P. Mdifla>'us··-Fujitu and Kunt' 1984. bUl see Jone' and Pagel, 1968: LaVal and LaVal 19l5lS; Reynolds 20(6), and certain

e~)nsi,tently affected European ,pecies are not thought to

migratc eXlensively in areas where thcy arc often found beneath lurbinc~ (e.g" P. pipi:itre/luS' in Gcrmany-Diirr and Bach ~004; Hulterer (;1 al. 2005; Steffell,~ el al. 2007). Another consistent paLlern at wincl turbi ncs in both North America and Europc is that most fatalities occur during latc sllJnmer thmugh autumn each year. and tend to peak during the period

t.hat coirll:ide,' with autumn migration (Arndt el a!. 2008; Bach and Rahmcl ~0(]4; Cryan and Brown 2007: DllH and Bach 2004; Johnson 2(05), The latter trend ha~ led many

rt',searehcrs 10 bclit'vc thaL migration plays a key role in thc p,if1icu]ar :;llsceptihiJity of bats 10 wind turbines,

COllsidcrable progress has becn made during rcccm years in .,ummarizmt~ pallel'llS or bal fatalities at wind turbines, eSllmating fatalily rates. studying behavior or bats around iurhines, describing physical condition or fatalities, and

discu"ing tbe p,)tcmial impacls of lurbines on "ffecled populati\)f[s or bats. However. there bave bee.ll fewer errons to unambiguously eSlablish lile underlying cau.~es of bat

fatalities at wind turbincs, although numerous hypothescs havc b~cn advanced (e.g., Arnelt et al. 2008; Johnson 2005; KunL ct al. 20mb). Tn this synthesis, we review hypothesized caUSi:S or bal fatalities al wind turbines. make predictions regarding the cvidence neccssary to support the various hypotheses, al1ll summarize exisling II1fonnation relevant to each predicllOl). \Ve see the articulation, orgdnizalion. and priorili/ation of

such hypotheses and pr~dieLions as a necessary S[CP ill

determining the underlying causes or bat mortality at Wind

lUrbines, and in stimulating appraprial~ research, including

bOlh observational and experimental studies.

Vo,!e base Olir review on hypotheses of cause published in

peer-revicwed scientific literature, but also includc informa­lion rrom seleCl unpublished sources. We con~alidati: and rename hypotheses to beller characterize themes, as well as to

highlight trends and major differences among. e:<-planutions. We divide causal explanations far mOrlality into 2 broad categories-proximate and ultimale causes. HYP\JUle,eS oC

proximate cause are those that addres~ the factors diro::cLly responsible For the deaths of i.ndjvLdual halS. For eXUlllple. Ihe proximate cause of the bal's (kiith al lhc wind turbine lOay b,'

that it, skull was crushed when it Will> hit by a moving blade.

Hypotheses or ultimale cau<,c are those that allelIC$<, why bals arc in the vicinity of turbines in the ISt place, For example, the

ultim~lte cause or (he bat's death may be that it perceived the lurbine as a potential placc to roost. For mosl hypothescs, we

propose I or more prediction, intended to serve as examples

oC the type or supporling evidence needed. We assess the Slatus of each prediction and provide relevant citation. again ba,cd on our revicw of published and unpublished papcrs. SLatus of" predictions is ba>ed on the limiled available infonnalion and our aSSC."ITlt'lIb should be considered preliminary in almost every case. Evidence penment to rno."t predictions is weak, at best, and additional ~tudy is needed,

We also believe that.. at least for the presclll, hYPOLhc,,~s and prediClion~ should be applied al tbe species level and tlial II i, likely that causal explanations will vary among '·pecies. For example, the susceptihility of liltle brown bats UYlyoris

/IICijilgUS) and hoary bats to wind turbines lllay be very different and driven by completely diffcrent causes. a, lllay differences among tree bars. We encoumge readers to take these hypotheses and predictions as sLaning points for con,ideration rather lhan a comprehensive lisL of possibilities.

HYPOTHESES, PREDTCT10~S, ANO DATA

Proximate causes of bul fmalirif's.-We identified 1 hypotheses ror explaining proximate causes of bat faralilies at wind turbincs (Table I). These involve bats colliding wiLh turbine lOwers, colliding with moving blades, or sufrering

intemal injuries (barolrauma) after heing exposed to rapid pressure changes near the trailing edges and tips of moving blades (Baerwald et al. 2(08). While flying at nighr. hirds occasionally collide with lall slnlClurcs ,uch as buildinp and communi caLion LOVier, (A very et a1. 1980; Gehring eL al. 200Y), and the samC may be true for the lowers 01 wind

1332 JOCRNAL OF 1'.'lAMMALOGY Vvl. \)0, Nv. 6

T\lIu: l.---Proposed hyporhese, ft)r expiaining (he proxllnate and uJtimm.e C,lll,es of bat fata.litie' at wind turhines. Here We deflne proxilT!illc causes as those lhill :iddres" the mechanistic (11ctOro direcily responslhJe for the deaths of individual hat,. Ultimate causes are {hoi>e {hal aJdre,s why bals are in the vicinity of mrbillc, kforc death. Hypothese., are nOt mUlually exclusive and arc intended to he applied at the species leYel. Ilallci,ed hYpOThese, ofuitimale cause are lhose we consider lSI dc!,ree (Fig, I), the dispHlval or which would elilliinare associated 2nd-degree hypothc',e' (i.e .• those 11m it;;lici7-cd: Fig. I).

H~polhcses PredictIOns

ColJi,ion with turblile lowers

Collision wilh movHlg bl?(ks

;'\lulllcroos C:lLa.11tic:s aL nonmoHllg lUrblnc~

Numerou, r'ltalilje, at other t"ll .")"uCtUre, 'Trau)l\[ltic ~XlCrnJ) injl\nl~S

No fatalliJcs [l{ nonmoving lurbincs [nlcmal injurlcs

Ralldorn

Coincidc-ni(tl

A{cracnon

Fllflll/L!(:s pru/w(((oIJ(1! (.() 17111,\ pI (' rei'll

~'llg.ranlS: clump In lIme': ,\nd ~racc

.\lIS(t!rllb/c whi.'n no! m,graf'illg Greatcr f.;cdlHg activity r~;,ulL~ lIt gf~ala mOJ1al:ity Grettler ~n:il:JJlg actlvity Tt'Slllt., ill gre~ller lnOTidlily

La~k of nlg:lt ~xp~nen(.~e re~uh~ ),1\ greatel rnoTtalH)'

?rf'J' (11.\"1171>/1/1011 1I1/lu('"e<> !l1ta{Il,'

G('n~ra/ artracllon to TurhinE's

Al\rac\~.d 10 lights Aaracl~d co l.iound or movwg bl[\des 01 generator

ALL(3Cled to l)lad< mow.",

ralilJJl\eS reflcd indices of aCl}\,ity or ;lbl.lndancc DClllographics oi faralitics retlecllvc of rhc popll!altOIl prcselll Most f,l\alilie, involYe migrants ,.,lIher th,Ln re·"dents

N0l111ugnnory speclc, or popnlatioll' lea'! afJccied Fala.-iHle:"l p(.~ak uunng lrllgmlhln pcnotl.....

fatahllC~ pca.k carh~f at highcf I<HltutlC,o.:.

D';b"ibuLwn or bat, more dump"d dunng nllgrillion ;\tlol'c 1l);griliHlg b81"S OCI,,;Ul In \ ... indy .nrc('l~

;\'lnre I"illalili~, with pa."age oi q"rm (ronl, Most I"alalltics ounng llIi:;mtion periods .MlJre rnigrlnh at hi~her "lIiludc' Mor,' ralalIlJ"~ .;1 taller lurblnc, No eCholocation uelecleU <H na,'e1k hei!?ht Mon' bats prcsent than echolocatIon passcs dCIl'.clel1 Atypical "choloc;)[liln caih (e.g.. "tall]e) ~'10,I.:.L fala.lilie~ involve re ... idenl~ J'aLhL~r 1han migTilTIt:-;

COllteldemal ,ea50nal mcrc"sc, In fec<iing 1:>Inzc, 01" ~iT"ct~d hat, CO!ncldenlal :-;ea.'onat tncrease~ in C'lpHm;':-. 01 mating bd~

Adult b"I' t,) fatalitlcs j\:lllre young, inexp~n~llced bal.~ umon~ farahl.l~~

Faialil} tate cOrrel.aieo WJlll rcgiollill or allitudinal prcy ~bundaTicc

!Jat, more likely 10 fly roward Ihall pa,t lurblLle,

Mor~ r"talltlcs ar Imbmcs with nYiarion llghlS More aelivlly at moymg than at nonmoving blade" Playback 01" souno allraClS bab Bat!o- I't'spond 111CYc (0 ~ounu nn IOW-IWilld nighl~

Turbines emilting p:u1icuklr noi.~s kiJJ mOTl' bal> More ac(ivit~ al nlovillg than at nllllmming blade, Devices s1Jnulaling. bindc ff10vemeni, will aUraeL bill;'. F~dmg bU77~S tll0re comll1on tU\Jund turbine':>'

A!lractcd to modified land,capc !ealurc,

A{\raCI~O to lurbl ""S ,1~ n>O"S

.Anrau~J to lurloJlue", a~ mallng or gaihering .;;iCe~

InSC·Cl abundance grealer ,lfound lu~'hJne~

COll,')lSlCTll pallcrn~ of 'illsect aCIJ\·II.y .arrHilld l.lirbmcs

Dead bats ha\·~ f,,11 <!orl1(;"h, Consj"tcnr prey In bal s(()mach~

DClid b;us havt:: in"e,-~(., In moulh

1\:Lorc ra[alll~c!lo neal I1CW~)' cfo:aicd edge 11?bllat~. rOiJ.d~, or w~ilaJlds

High!)' vi,ible lurbliles kill more bats j>.'lore ratalJti~s on moonlit nighl> More dcti\ it} al 1~lk,L treelike ,qructU1<'.' Low nCIi\ll} or :spC\:'les. that dv not roost III .r:rL~~S

\1L1S\ faralilles occur a~ l1i)(htly a<'rivn)' ends Equal actl\'ll)' al IllovllIg and L)ollmov,ng turbines Mos{ activit), 'It tallesl rrc'es or treelike ,Iruclure·s in :an,hc"pe h-1atiag 3Cllvity at mo\mg and nonmoving 'urbrn~s

:f\t1ale tcrntoriJl oehavlOr at lHo"Vrng and nOnm('lVlllg wrh:ne,," Group f()mla~lOn at moving and n(lnmovlllg lUl'b\ll~:-:

Sp.::rm tn IHales and jcm.:lJc~

rvbl~ bi~ls io r(lfatlliC'~ Lhal di;o:;apP~41r:.- oyer lIrnL>

SOCial c,11h derecled at n:'cclk hcight NTore ~()cHll InleraCllon itl llJ(oine:-. in 10'0 wwds

Equal acllvllY al mov1n!t ilnd I\OIl1l10HllP: rurblllcs

Deccmbcr ~()09 SPEGAL FEATURE-CAUSES OF BAT FATAUTIES AT TURBINES

turbincs (Kuv)e,ky Ct al. 20(7). If bals also collide with

lurbine rowers, this could c"plain at least some fataliLies at wind turbines. If Lhis were the case, then hat fatalities should occur when turbines arc nonoper3tional, and at olher tall strurtures ~uch as the InetcorologicaJ towers oftcn associated

with wind turbines. As noted above. bal fatalities arc rare al .'Utiionury struCi.ure" lind ill wind energy facilitie.-; bat fatalities

hal enol been reported al nOJ1operalioJ1aJ lurbillt:s or meteorologICal IOwcr~ (Arnctt et al. 200S). III addition,

reduction in l.urbme operation is associated with a redurlion

in bat fatalities lBaerwald et al. 2009; E. B. Amelt, Bal Conservation InLelllalil)n~ll, pas. comm.). There is little evidence that bms consislently collide wi th nonmovi ng turbine

blades or towers. If bat ratalilies occur due to strikes from moving turbine

blade~. Lhen carcasse,s should show traumatic injuries. Indeed, many c;u'cas,;es hayr such injuries, including broken or

;unput:lid wing~, cru"hcd ."KuIJS. broken vertebral columns. and ,ever..: laceratlolb (e.g., Baerwald eL al. 2008: Johnson el al.

2(Xl]). In addition, th,'rlrutl irnaging devices have been uscd to

dO,:dJnClll bats b(;'ing struck by moving turbine blades (Hom cl

al. 20(8). Despite dear cvidence that some bats arc hit by moving hlades, other bats round dead at wind turbines have no external injuries <Baerwald et ~Il. 20(8). These carcasses, as well

as many of those wirh exrernal friJuma, have internal injuries to

the thoracic and abdomimtl cavities that are consistenl with rapid decompression. or harotrauma (Baenvald et al. 2008: DUIT and Bach 20()4: von Hen!\Cn 20()4). Like other airfoils, moving wind Lurb!I1C hlacks havc areas of high and low prcssure around them.

In particular. a relcllively ';lllalJ vun.cx at lhe tip of cao.:h blade invoh-n a drop iu atmospheric pres~lIre ~urflcient 10 eam,e Internal injllrie<, when the ail' in the lungs of bats rIyiilg Lhrough

rhe vortex expand;. rapidly. Small blood vc~scls in the lungs are damaged causing bleeding into the thoracic cavity (Baerwald

el ;II. 2(08). This suggesls that even if bats can avoid direct

contact with moving turbine blades, they nlay ~ufter sufficient n0lleOlltae, injuries l~) eause death.

l./lrunale callses of bar j{)lali{i('~.--Hypotheses of ultimate cause are numerous hUl Lend to fall int0 3 general categories (Table I: sec also Horn et ,,1. 200g; Kunz c[ al. 2007b). These calCgori,~~ arc random colliSIOns, coincidental collisions. and cnll;siol1~ rcsulling [[,()1T1 auracLiOll of b<llS lO turbines. V>ic

dd'inc random coliisions as those that occur due to chance alone. This hypoLhesis involves no assumptions of circum­sLanc,;; l)[ attracLion. Unde! the random collision hypothesi.,. all

individuals of a particular specie, are equally vulnerable whcn occurring ncar turbines, regardles.s of sex, age, reproducti ve

condition, 01 time d year: turbi nes randomly sample bats t1ying in the airspace around them. Hypotheses of coincidental

collision involve bats being victims (;'1' unfortunate behavioral circumstances-··cenain aspecls of their behavior pul them al grc:aLer risk of colliding with llirbines. Hyp0theses of attraction ale .IW;tllJat: ,Oille a!tr;t(lor or combination of allrac!Ors draws b,llS to wind turbine). Although a ["C\>,. or lhe coincidental hypoLlle,C\ lllvohe a r¢gional -"patial ~cak, (e.g.. regional dilTerenc<:s 111 diSlribution of migrant, or prey). all 0tbel's are

intended to be applied at the scale of individual turbines.

These variou~ hypotheses need not be mutually exclusive. For example, specific behaviors may increa~e the challcc~ of bah heing near turhille facilities during particular times of year (coincidental), bnt then altractono could draw cenl!1l1 individ· uab closer or, alLematively. bats present could randomI)' collide with turbine blades .

Bat fatalitil'S at wind turbincs may be the result of random

collision~. JI is possi ble thaI tbe ,paLial, Lemporal. and

demographic variation observed in fatality rales of hats lI[

di ffercnt wind energy sites (Arnett et ai. 200H; Baerwald and

Barclay 10(9) merely retlects the rdative abundance of bats in

thc vicinity of wind lllrbines.lf tbis is true, then the compo~itioil

of fatalities in lemlS of timing. sex, age. and reproductive condition should match the composition of bats in the area (Table l). Such measures are diftieull to obwin for haLs (O'Shea and Bogan 2003), but a disl.inctlackofcorre~ponden,:c

between regional species composition and fataliLies na" been 0bsen'ed aL several sites; reside·nt spez:ies lend to bl' killed at

wind turbine, lcs, frequently than migmwcy specie" even in

areas where the rc,ident species arc common throughout the

summer (Amell ct a!. 2008: John~on el al. 2003: KunL. <;( ill.

20()7b). Most fatalities usually occur during the lale summer

and autumn, although activity levels of species Il0t regularly

killed b)' Lurbines can be high throughout the summer (ArneLt er aJ. 2008; Johnson et al. 20(3). Examination of some fatality

data also suggests biases between sexes, with adult males dominating samples fol' several afte.cled ,species in :.iorth America (Amen et al. 2(08). Such trends .)uggest nonrandom

imeraction~ between bats and wind turbines.

Several hypotheses of ultimate calise involvc colli~ions

occurring under coincldelltal circumstances. Bat falalIlle.~

observed at wind turbines tllll' far have involved many speci<~.s

that migrate long dislanees l>250 kOl), and because most

fatali ties occur dming a period that coincjde~ with their autumn movements, migration has been invoked as a primary

cxplanation for the particular ~usceptibility of bats lO wind

turbines. Hypotheses lhaL invol ve the coincidental susceptihility of migrating bats seem weJljustified: however, \l;e caution thar even the most ba~ic among them. sueh as that most fal<llities involve indiviclu<lls that arc migrating rather than tho,,~ that <ire: resident tn the Mea of the turbine,. need to be "~;4ed and not taken for granted. Stable isotope analysis and other lc('hniq\lc~

for assc.Nng the geographical origi ns of bats kdkd at \\·:nd turbines may prove useful in tcsling the migram hypolhe,>is

(Hobson and Wasseuaar 2008). Studying the phenology or raralities at contiueutal scales also lllay hdp u'; betler undeNaud the role of migration (Cryan and Diehl 20(9). As detailed beh)\v

and ill Tllhlc 1, there arc several possible behaviOr> associated with migmtioll that could result in fatalities, but that do noL

neeessatily requi re the act of migration. Certain risky behaviors might have evolved with a migrawry lil'eqyle. yet nm be dependent on migriJtion. Is ilthat migmting bat.... arc particularly -;usceptibk or simply that OLher aspects or these ~peclc,'

behavior c1Jdnge iu aulLlTnn (e.g .. mating 0r feedlllg) and makL' them su ...ceptible. whether tbcy are migrating or not')

)33·\ JOCRNAL OF MAJ\:IMALOGY Vol. 90, NO.6

Sorne of the coincidental migration hyporheses pertain to a regional ~patial ;calc and involve migranLs being cluSLered ncar lurbinc~ in lime and .~pace. Foremost among pred.ictions bt're i, thaL bats concentrali." in cerlain regjon~ during mIgration, sudJ as in "corridll]'s" or aL "stopover" habiLaLs, or in areas whcre (upography or prevailing winds. or both. force them inlo close pro\imiiy ,1S tbt~y travel. Such landscape fealures mighl include windy pl~ce~ such as rnountain ridges. passes, eoa~tal areas, and river va)Jeys (Cryan and Diehl 200\): Funnankiewkz and Kucharska 2009). LillIe is known abollt Lhc Lemporal and spatial dislrjbtllioL1 or den:>ily of bat:> during migralion, particularly the species most affe,~ted by wind Lurbines. Tn AlbeJ1a, Canada. al1tumn aetivily anti fatality of migrating species is concentrated ncar the foothills of the Rocky \1ounwins, suggesling that rnigrating bm, CalloW parlicular route~ on lheir way south (Baerwald and Barclay 2(09).

Certain wcalher condiLion, may fllrthn inllucncc migrants to cluqer in time. Fatality rales of bal~ at turbines orten increase with the passage of s(Orm fmllls (Amen et a!. 2(08), These observations. combined wilh the fact that most fatalities occur during a few weeks in Jate summer and autumn. suggesl lhal migrating bals n.ploil certain weather conditions during LHe sumlner and ,[ulumn that put them at risk. Very litrle is known about the effects or weatber on the behavior of l1ligraLi ng bats (Cryan and Brown 2(07) and this seems like a rich area em cooperaLion bd ween biologists and almospheric 'ciellusi~ il1l,:rested in formj ng collaboralions wilhin the enlC) ging f'icld of acroceoJogy (Kunz et al. lOOg).

Other coincidemal explanations for the role of migrmion in fatalitic'> apply at the spatial scale of individllal turbines and poslu]arc lhal migrating bats behave in distinclive ways rhat put lhem at grealer rLsk. These hypotheses include migrants flying higher above the ground than other bat~ and bigh·nying migranl" being less likely to echoloente and dereCL spinning lurbi!l(, blade;, (Tahle I: Kunz et £11. 20mb). Predicrions a,>~ocJaled with these ex.planations include more fatalities at talkr lurbincs, dctedion of more migrants high above Lhe !:!yo\J1:d, and no ccholocaLion detccled arnull(i the blades of turbine'>. FUlaliLy rales of bars are greater at talier turbines in North America (llarclay et a1. 20(7). and studies have rc\'eulcd gre,ltcr echolocation activity of migralory ~pecics

higher above Ihe groul\(l compared with olher species (Baerwald and Barclay 2009: E. B, Amett, in !itL), Sueh activllY rrHlY be indicatjve of high-flying migrants. bUL al~o

could rcJkcl ,;pccic~ dilTclence, in night and foraging ~tylc

,e.g., :"orberg j 994), Cenain almospheric conditions, such as tcrnreriiture inver;'JOll;; or overcasl skic.~, al~o may cause high­r1YUlg migrants to drop to lower allirudes where they are more likcl; to cnc:ountcr turbines I,Knllz et al. 2007b). If nighL alllludL' alone is a primary cau\c of bal f"taJitics, delection of sp.::cies k.illed by turbines "honld he greater al the heighl of turhine hlades during Lhe lare summer and aUllllTln, bUL not during other scasons when fatalilies arc less frequent. Few data ex,iSI ror assessing whether migrating bals me less likely to cc!loiocale, hUI il may be possible lo tesl this hYPolhesis hy comparing ~he lltl1nher of hars detected with thermal imaging

devices LO the numbn of echolocation passes picked up by echolocation detecLors (f-llll'n et al. lOOg; Hristov Cl a!. 200g). It is dear lhal at least some high·llying migranls echolocale, because they are regularly detected well above the ground (e.g., Baerwald and Barclay 2(09). However, some recordings invol ve abrupt sequences of calls suggestive of noneeholOCal­ing individuals being ~larlled by the presence of turbine blade~

and eeholoealing in re~ponse (J. Reimer, UniversilY of Calgary, pel's, comm.).

ESlablishing rnigratlon a~ a coincidental causal .::xplan::ition for bat deaths <Il turbines will require explaining the apparelll lack of fatalities assoeialed wilh spring migration. or lhe high numbers of fatalities of ,>pecies or sUbpopulations thaL are r:ol known lo migrate (Arnett el at. 2008; Dun and Bach 2004j. Almost nOLhing is known about the specific behaviors of bats during migration (Cryan and Diehl 2(09). If migration is a primary driver of fataIitie.' at turbines, it is possible Lhot behaviors of bals during migration di ffer enough between spring and autumn to explain the autlllnnal bias w fcltalili.::s, bUL we clo n()l yet know enougb Lo assess Lhal possibility. Investigating diiTerences in general migraLion behavior 01' bals between spring and autumn may be a useful way of narrowing down tbe possibk causes of bat fatalities at turbines.

Given lhat bird migralioll has been sLUdied for milch longer than bat migraLion, \Ve suggesl tbat much lllay be leamed from data regarding birds (Larkin 2006). For ex.ample. many migratory pa"serine birds Ily at night (Aler~lam 1990), yet they appear less affecled by wind turbines [han bats (Bmday ct a!. 2007 and references therein). The timing of bird fataliries at turbincs also frequently differ:> from that of baL~. and bird falalities are not as consistent in terms of (heir seasonalily (Kuvlesky el a!. 2(07), Does this sugge';L [hat behaviors, olher than simply migrating, di ffer between bats and birds and result in different susceptibility') Therc is a wealth of inConnallon in Lhe scientif]<: literature regarding the migralory hehavior of bll'ds (e.g.. Alerstal11 1990~ Berrhold el al. 20(3) and these methods need to be incorporated into studies or bat migration (McGuire .:tnd Guglielmo 2(09).

Bats may be vulnerable to wind turbines for coi ncidenlal n:asons thaI arc not as~ociaLed with migration. The faL:t thai lTIOSl fatalities involve species thaI migrate may nol be due to migratory behavior, but lO some other behavior eomrnon ro these sp.::cies when they are sedentary. Hypotheses in Ibis caLegory include tht' pos,~ibililie., Lhat sea,)onid ch:!Tlgc, 111 the hebavior llf hals lead to c()itll~idelHal increaSe.' 111 their susceplibilny (0 lurbines.

Energy reLjUiremelJlS of baL, change Ihrough(lllL the year. and seasonal increases in energy demand assocJated wirh producing young, mating, or preparing I'DI' hibernalion could lead to seasl)nal increases in Coraging time or feeding ,Ireil~

(Kurta et al. 1989; Lacki el a!. 2007: Speakman and Thomas 20(3), Late-summer increase~ in faLalities at turbines mighL be atlributable to an increase in foraging activity Lo allow bals Lo meet tbe eballenges of overwintering, migraling, 1)1' maling. If lhi~ was the case, coincidental seasonal increa,es in echolo­cation calls assoeialed with feeding ("bu:l.7..es"-Gnffiu i 958)

1135 LJeumbcr ~()09 SPECIAL FEATURE-CAUSES OF BAT FATALITIES AT TURBIl\£S

,11Culd bc dci\.'(;[<lbic around lurbi ne·, during periods of high fmalll)'. Such dalH likciy CXi~1 but ha~c not bccn n:ported In 1~l1'; cunlexl. The pOlclltiul innll~lJCC of ~easoHal changes in for"gJl1g di~pcrs,,1 "nd timing on susceptibility of bats to llirbine~ also could be investigated by rHdiotr"cking affcctcd

spc(;ic) HCroSS " as.on~.

Species of tcmperatc-lone in;,eetivorou, bats u~u"lly begm maling in autumn (Racey and Entwi;,tlc 2000). Increa;,cd Oight activity a.,sociated with finding males also could lead [0

coincidental increases in the probability of bats coming into eomaet with turbi nes. Many of the species affected by wind [Urbllle,; enga~c 1\1 mating activit)' during: the same pcriod whGn thcir C:lrc,',,;, Jrt: ,'l)umi In the gredlc,t l1umhers oeneath lurbines [Cryml 2(08). This synchronlcily between maling aCilvily <{nd fatalilic;, could renecr a cause rhal i, independc:Jll of I1llgr,i1ion or attraclion. B<t(5 in search of males lTIay simply fly in place~ wkre lhey do nOl regularly fly at other times or year. A prediction under this hypothesis would he an increase in eaplUres or olher observations of reproductivcly active bats, with a bias toward adulls. Goineiding wilh falality peaks al wind wrbincs, Exisllng ditla for assessing this hypothesis are Cew and anecdot"l I. Cryan 2(X)8). During an 8·year swdy of hals in the Ouachita Moullwins of Arkansas, R. Pcrry (Southern Research Station, Uruted 5t,ll<:5 Fnrest S(~rvicc, pcr,. coml1l.1 observcd large ii1Ci'ea.'G' in CilPl'..Ii'C rullOS uf male L !Jor('o!r, cuell year in Augu,t and 5cpternbt:r. ()nd reglilarl)· ohsc:rved pursuil. aggrL'S~i<ln, and cOjJulmil1n behaviol of eastern red bats around Ilets thai suggested lhal maling activity led 10 greater su,ccplibllity llf' caplure. AhJwugh long-lerm .spenn storage in male and female hats makes the precise IJllling of mating events difficuit to disccfll bil,cd 011 an"wmy alone. quantifying levels of circulating honnoue~ a~socialcd with male libido (c.g., andro· gens) may be a userul ledmiquc (GuSlabon 1979; Mnnin and Bernard 2(00). Seasonal changes in nighttime night activity of c1enwgraphic groups and species most affectcd by turbincs also could bc as,~.,)o:d thing radiotelemetry.

Anolher l'oiilcicklltal hypolhesis IS thm tbe laIC-Slimmer and ,IUIUl\ln pC'lk In t';i1alilit:s i~ attribulable to inlroduction of inexperienced young illlo bill populatlom, and lhal Juve!liJc~

lack the night skill, to avoid lurbinc hlades. However, existing data on the <tge composirion of fatalities do not support this hypothesis (Ameli el al. 2008; Baerwald 2008).

Regwdkss of whethcr bats arc migrating or sedent:'lfy, dilTcrcnces in distribution or their inseci prey also may bc ;1

eOlJ1cid<:nlal cause of f"lalilies (KuJlz et al. 2007b). For exarnpk, if in,ecls upon which bacs fced aJe mor<: likely to oceur in r,'gions, habitats, or air layers where turbines arc built. the bat~

l!l:ll e>.ploi[ them may be at greatcr risk. i\ prediction a.,~ociatcd

,\i,11 thi~ Ilypol.hesi, is that (ai.aJjty ral<: would he corrdm.ecl ",.'ith regionaL hahil,u-Sp,::clf'ic, or allitudinal prey abund<lnce·--a r)()~sibilrly thai remains untested.

There Me a rerndrk<,bly I"rge number of plausible hypothe~es invohlng. atlraction of bat, to wind turbines Cfable 1). These hypotb..:se~ generally involve bal~ being <lllraeted 10 lurbines OUI of curio.~ity or misperceplion, or as potential fet~diilg, I'<)osting, flocking. and mating opporwnlties.

Thus far, the only ilttraclion hypO(hesi.~ with sufficient evidencc to reject may bt' lhe allraction of bal.s 10 aviation warning lighl~ on lOp or turbines. Numerous slUdies have found no difference in the number or fatalities recovered nnder lurbincs lit and unlit in thi~ way (Arnetl et ai. ]008; Baer",·ald 2(08). ('\hhough olhe!' sludic;,; indicalc lhat biltS are able 10

perceive and re,pond 10 cues f'rom artificial lighL~ (Childs and Bucbler 1981; Cryan and Brown 2007), we are nOl aware of any evidence io sugge~t tbai lights on tall structures inercuse lhe probability or bat collisions, as is known 10 OCClir wilh birds at communicalion lowcrs and buildings (Avery ct al. 1980; Gehring el a!. 2(09). However. it is ~1I1i plausihle lhat hghts illurninating largc part, of turbine lowers and blade', may influence hltalitics in olhcr WilyS, as diseus~cd below

Bal.~ may be attracted to the sights, sounds, or movcments of Wind turbines. Accounts of bats possibly being atlracted to the '·swi.~hing sound" of sticks waved through the iiir (Barhour and Davis 1969: 153), or unknown cues at roosts previously used hy congeners (Constantine 1958; Downe~

1964). as well as thermal images of bars apparently chasing moving turbine blades (Horn et. al. 2008), highlight the plansibility of such attraction. Predictions associated ""llh these hypotheses include more activity around moving thiln nonmovi ng blades, variation in faIaI it)' fitlG w;sociated wi III variation in cmi~sjon oj sounds by turbines, and altraction or bal' to playbad.. of turbine sounds or devices th,lI ,imulcltc blade movement. To our l-..nowledge, these predictiuns and hypotheses remain untested, although they seem especially well suited to experimental stUdies.

II is possible lhal hunger hrings tree bllts into close proximity of turbines because they arc attracted to insects IhaL gather around lurbines. Thc talk;,;t ~tJueturcs in a landscape are us<:d as gathering point.s during the daytime by some tlytng insect., ("hilllopplllg" behavior reviewed by Thornhill and Alcock r1983]) and lnsecls may he attracted to avjation lights or the warmth or color of turbinc~" In turn drawing in hungry bats (Kunz e( 'II. 2007b). Aggregalion, of in,ccls around turbine, could sef\'c a.~ easily karned and consislenl food sources 1'01' bats, panicularly for hungry migrants travcling through unramiliar areas. Migrating bat,> were observed feeding on insccts around offshore turbine, in Scandinavia (Ablen e[ al. 2009). Predict.ions associaled With the feeding-attraction hypothesis include detection of more feeding buncs around turbines than elsewhere, con~istel1l

prcsence and uctivilY of insects a.round turbines. and comlslel1l pres::nce or in~ecls lhal oecur around turbines in tile gaslrninl::stinal lracts of balS killeu al lUrhines. III Alberta, Canada., bals killed at turbmes had full stomachs, ifldlcati ng that they had rcd the night lhcy were killed. but many <i( [he in,el'ts detccled in the stomach~. such as <lljualic hcmlplcrall~,

were unlikely 10 have com<: !'rom around turblnes [J R<:imer, pel'S. comln.). Tn addition, few feeding buues were recorded al bladc height There i~ currently no evidence (hat nocturnal insects upon which bats feed cngage in hi lliopping hehavior. and the henetJts to the insects of such behavior in the dark arc not obvious. Considering the many dirrcren{ regions and lypes

JOURNAL OF MAMMALOGY Vol. ()(), No.6

of habitat~ in which bat fatalitic<; at turbincs have been found,

a feeding explanation would require cvidence that some type of prey wnsistenlly occurs around turbines, thaL affected bats regular!y feed on that [ype of prey. and that such a relationship occurs ~crm<; a wide variely of locations and situarions,

cWt:elalty during autumn. Laild cleared dunng the: cOllstruction of access roads,

iurbine foundations, and power transmission lines migbl aUract hal., by rnirmch.ing natural linear landscape features,

such as Ilatural foresL edges. along WhlCh foraging and comtlluti ng bals lIlay regularly travcl (Kunz ct al. 20mb;

Vnboom and Huilcmu 1997). A predi.::tion associated with Lhc

Iillear-clerncnt hypothesis is t1l,1I more fatalitic" should occur at tUI biues Ilearer fore.;;t edge,~, newly created clearings, and other linear landscape fealllres. Howcvcr. publishcd swdies in

:\or(h Amcrica reveal a surprising lack of correlation beLween k)caJ lancl.,cape features and fata!itie, at wind energy site~

le'VEeli el al. ~008.L Also, rclauvely high I'alality rates of bats at ,ILl'S in open, treele's. scarc!:']y modified landscapes (e.g, Albc:rta. Canada-·-- Baerwald ~OOS) suggest that this explana­

tion may be I)C limiLed applicalicl J1 at best. I fypoll:e,,:.) involving aHraCLion 01' balS to wrbinc:; as roosts

.,eem piau)ible eonsidcring Lhiltlhc species of bals killed most

onen by wind turbines lend 10 rdy on trees as their primary natural j'OOq ,tructures. BaLs visually orient across land~cape"

(Griffin 1970) and the silhouettes of turbi nes mighl attract

lhelll ffOm considerable distances (> 1 km). II' [hi, is the case, visibiliiy or turhiTlt's to bats should Innuence latality rates. HlJIH'\Cr, ,ueh ditfercnce, likely involve visibiliLy of large

arcus of the llIrbinc qructure, rather Lball small parts of it. such as dyi,ltion light, Predictions under this hypoLhe,;i~ Includc

equal aCLivity al 'TloYing aflJ nonmoving lurbLne~, more <Il'l.iVlly and ratalidcs at higber-visibility turbll1e~. higher rawlil)' ralc, on moonlit lllghts. highcr falilhty rates al

experimcl1lally i11umin:Jled turhine" and activity around turbines peilh.ing Ht dawn v,,·hen bats Rre likely to invesliga,e

pOlential daytime roo~ts, Contrary to the last predi.::tion, bat

activity at turbines in southern Alberta occnrred tbronghout

the night with no peak near dawn (Baerwald 2008). Howcvcr.

Cryan and Browli (2U07) pre~t'llted evidence thai responses of migrating boary bu{s 10 vi,ible lundscape cue~ (il lighLhou~e)

varied \\'tlh cloud cover and moonlight. Baerwold (2008) lound ,hat mOllnlig.hi had an effect on latality rates of bab at turbines, wllh highcr fatalily rates on moonlil nigtHs. Thesc J\~"ldts <,ugge'.( ,'I ~'oul1ecli()n between the visibiliLy of turbine

,MucL\lre'i "nd bat fatalities-a possibility thaI warrants udditional rescan::h.

:Vlany specie~ (,f bats favor talier trees as roosts lKalcounis­Ruppel I et al. 20(5) alld fatalities a[ turbincs appear to be

correlated with IUrbinc height tBan;lay el al. 2007). Turbinc~

rar ell.eeed the heighl of Inn,t trees and are approaching the height<, or lhe LalkS[ lrees on Earth (ahOul 112 01). If baLs

simply inve~tig.ute the (allc~sl trce~ they encount(:·r for po:;;;ibk roosting opporLunillcs, thl'n ~uch behavior ~hould be ob,crv­

ar.k aL thc ta!ie:;t tree, and OLher Lall landsclpe struCl.ures (e.g., ccllulal phone towers and v"ater lOwers). as w,,",il as at

experimcntal structures lhat can be manipulatcd to as:;cs~

relatiouships between structural height, visibility_ and bat activity (Cryan 2008: Cryan and Brown 2007). To our

knowledge. the Lallest-tree hypothesis has not been tested.

Bats alw) mighL use the lalk:;t trees in a landscape dS

gathering [Joints dnring the breeding scason and visually miHakc turbines for [he talkst [rces, particularly during or before migralion (Cryan 20U8; Cryan and Brown '2007) Irlhie,

hYPolhesis is correct, group f0n11alion and maling activily should be observable at nonmoving Lurbines and surrogate structures, and evidence of reproductivc aetiYity should be

apparent in carcasses, Predktions under [his hypmhesi,

includc equal activiLy at moving and nonmoving. Lurbine", mosl aci iYity occurring al the taJlesttreee, or treelike ~tfllctllres

in the landscape, mati.ng activity (e,g., aerial copulaLion or

malc tcrritorial behavior) or group formution occurri ng al nonnwving turbines, more social interaction al turbinc" In low winds. sperm in the reproduclive tracl, or both male~ and

females or honnone levels indicative of active maLing. social

call~ detected al the heights or turbine blade" and adult male bja;, Lo ratalities (associated with thc early escablishment or Ick, and mating territories) that disappears as autumn

progresses. No sLudies direcLly assessi ng the mating bypoth­

esis have been published, but several of the pr~dictions may be testable llsing exisling carcasses found 3[ turbinc~, by analyzing: bal culls rccorded around turbines and olhcr tall landscape structures, or by meta-analysis of demographic and

temporal patterns in e~jstlng faLaliLy data (Tabk l). l.n n \tudy conducted in Canada, no social calls welT rccorded rrom hoary or ,ilver-haired bals flying at roior height ,Hound turbinc,

where ru[alilie~ we're subsequenlly recovered (1. Rcnner. pel". comrn,). eaits associated with mating have nol been characterized for North Amcnean species affected by wind

turbinc" although lhc prcvuicncc and disllllCllleS& of maLing calls among European species with similar life-hbLory charactenSLjc~ (Pfalzer and Kusch 2003) lead u" to believe

that North American ~pecies likely produce "uch calls.

Althongh evidence ,uggests thaL some juvenile males or

genera nrfected by tU rbines are reprouuctiyely active in thclr

l'>t autumn (e,g" Lmiurus, Lasionyclcns. ,md Nyualus·-­Druc'.::ker 1972: Myel" 1977; Racey and Enlwis(\e 200!)).

support for the mating hyporhesi, also must include eslab­li.shing that juvcnile bats killed by lUrbincs arc involved in ITIming ac[ivi(jes.

PRIORITIZING HYPOTHESES FOR FUn;RE RESEARCH

Identifying, prioritiZing, and tesling the hypothesiz.ed causes of bat fatalities at wind turbines are vital Slcps IowaI'd

developing practical solutions to the prohlem, There are rnd.jor implieaLions if the ultimate causes of lurblne collision.) are random. coincidental. or che resull of aterac[ion. Foremosl. if there is an atcractant, it might be possible to cither eliminatl'

the attraclion or exploil iL Lo draw boh away from wlIld energy facilities. DespiLe a few year,~ ()f ~tudy inlo the problem of bal ratalities at Wind turbines, if is surpnsing Ihat '0 few or the

DC("f!m!Jcr 2009 SPECIAL FEATURE-CAUSES OF BAT FATALITIES AT TURBINES 1:137

TURBINE SCALE

Bats attracted to turbines?

(Attraction 1°)

Yes

Focus research on determining and eliminating source of attraction (Attraction 2Q

)

Yes

Focus research on determining how monitoring general presence and activity (e.g., echolocation detectors) can be used as a means of assessing risk

No

Bat coilisions with turbines

random? (Random 1°)

No

Focus research on determining and monitoring behaviors (e.g., mating, feeding, migrating) or habitat features that increase risk (Coincidental 2°)

REGIONAL SCALE

Bats susceptible while migrating? (Coincidental 1°)

Yes/~NO Focus research on understanding how migration patterns and behaviors (e.g., use of corridors) relate to seasonal and regional differences in mortality of migrating bats (Coincidental 2°)

'--------./../

Focus research on understanding how habitat use and resource distribution (e.g., insect prey) relate to seasonal and regional differences in mortality of bats that are not migrating (Coincidental 2°)

"-------------'

1-'1(:. I.-·-Ueci~ion trees ~l1g66ted for r('';ting hypothesized cause, of bat faralirle,; at wind turbiucs and for focu,;H1g on a,'eilS of rc'earch. D~cl>JOn' lree' "dtlrc'>' qlle.'IJ'''l' :,1 tile ,>palial ,coIn. of individual l"nrbinc, ond largeL region,. Ll\;h r~ctJngle repre;,enls it di,provable I-f ­

c!eglCl: ( i . J hYI'otllC,i,. \>;ilh hi ~'UI cdlrng outcome, [0 [be llexl ] ,t-degree hyp<)[he'l'. group of 2nd·degr~e (2 ) hypollw,e" or rccomillended r,>("Ll.- ,llca, I(li' Iy,edrdl. Dcldlkd h)POlhc:,,,,, ;Lre (·,.lcgolil~d and li-,red in T"ble L

a[trncllon hypothe~(;;, have b..:en le5li:d. l:kc<lllse the eomer­\·alion implications of bat amaetwn to \VInd turbines are by far lhe mo,1 \erious. we "uggc~t Ihal tesLing thc general hypothesis of aHraction be a top priOlity (Fig. 1). Existing lTlethods for following the nighl path" or indi vidual bats, such as radar and radiotelemetry (Cryan and Diehl 2009l, could be used to dClermine 'whether bats are more likely (0 fly toward rather ihan past turbinc.'> (Table J). Evidence or general ,l(lractlon would 'Warrant additional Jesting of ,econdary hypolhe~t:~ addrcs,ing specifiC cJu.,>e(5) of :lllractilln, We abo rccummcnd pr:Olili7:ng the g~ncrJ! llypoLhe,is thar haLs are more ,u~ccpllbk to turbine:; while migrating (Fig.. 1). Populatiom of migratory bats may be particularly vulnerable 10 the detrimental jlT1paet~ ul- human activities, bCCJLlse they rely 011 widely dispersed babit<tts and lJrge proportions of Lheir popUlations cluster ilHo restricted spaces during migratinn. In addi lion. conservation strategics direcLed towl1rd migmtory bats musr be intt'gratcd across huge spada) scales (Fkming and Ell)" 2003). If il is determined (hal bat fatJJilies at turbines lTlUSily in'vol\'c "cli've migmllls, research could be focused on underslilnding huw 11llgr:1lioll pattt'nls rt'late [0 mortality and i(\'lliding high-w,c are,LS when building new turbInes. or cunaiJinf! OpCriliIOn, or existing Lurbines during LJl1gralion c~\'CI1(S (Fig. 1). II is currently assulned thut tile cildang.;:rcd H'lwaiian hoary bat (L cine,ells .\emofU.I) is kss '·ulncrabk f<)

Lurbines on Lhe Hawaiian blands because IL does not migrme long distal1ce~, e\·en thougb the ~pecies (L. cinereus) l1lake\ up about on<:-half of aU fatalities in continclllaJ North America where it is known to mig.ra[e long distances (Arnell et al. 2008; Cryan 2003), ProporLionally high ffltality rJtes or purportedly scdent,lry spc(;ics <lnd populations elsewhere stress the need Lo tesc. c.he migration cOllileetion.

If it is detcrmined that bat~ arc not <1\lraeted (0 turbines. (hen 'We suggest tesring the generai hypothesis tbat fJ[flilries are fl

random sample of bats prC~el1l Jround lurbilles (Fig. J 1. If collisions are determined Lo be the result 01 random prncc~se,.

[hen research could focus on monill1ring general aClivlly or bat;, as a means of as,e~sing risk, because activity and fatality should be correlaled. We are aware 01 no published studie\ ,howing a correlation between <letivity of bats at a site before turbines Me

built and fatalities there after mrbines are operational. However, the inere:tsingJy common practice of prceonstIllctiol1 acoustic moniroring for bats at proposed wind energy faeilities as .1

ITK,thod of risk assessment seems to rely in large parl on an assumption that eollisiom of halS with wine! turbines are random processes (e.g., they do not aceoul1\ lor dirferelKe~ in sex.. ago:. 1'1

reproductive status. or anmelion). Considering evidenn' thai fatalitk~ at turbine, are ~e;'(-bia~ed, the a"umpUoll of r,Jndolll collision, i, questil11l:lbJc If il is dctermined Ih<ll ~'JlaJilics are not the result ot" attraeliOIl Jlld are nOi random proces~e,. then

'3:10 JOCRNAL OF MAMMALOGY \iol, GO, No. 0

re~carch could I'()cu~ on dctClTIlining and monitoring spcCI11c bch,\Yior) that incrca~c ri,k (Fig. I), such a~ ~casolliJJ change, in l'.'cding Of mating activiliCS.

Dctnmining. whelher the causcs oC bal fatalilic~ <Irc ihe fl'."tdl oC attraction. coincidence. or nllldorn processes may not influcnce the effectlveness or current tcchfllques for monitof­ing exi,til1g turbine sitcs for fatalities (Klinl. ct al. 2007a). but IL will (cnainly have implication" ['or assessment of lisk and popuialion impacts. For example, if mating individuals are dispro]1ortil)nalc!} altracted 10 turbines ill places where their speei,;s passes through geographic bottlenecks during migra­1it1ll, thc chan,'cs o[ rapid and irrcvcrsible populatioll decline \\·111 be r-reaLer Iban if cOHisjOll~ involve turbine, randOlnly killing bar;; rhar are nOl I1ligrating. If collisions arc random or linked 10 coincidental phc~nomcn" .'uch a) migration or incrca.,cd ~ea,ol1al actlyity, then the prospcCb Cor prcconstJlJC­tion lhK a)sLc"srncnl and sik ,,~lecti()11 <ire g.ood, b<::cau~c ,ucb eY(,I1I, ill\~ I'red.icLabk <11 a ~itc. However. if bats are attractcd to ruriJilll':s. paJ'liculariy Cram the long di,lances over which tll(:y <1re capabk of ~ecing at night. c'(isting prcconstruction Icchniqucs of ri~k as<,CSSlTlcnt may be lcs:s usefuL because bals might not be active aL Ihe site, and thus d~lectable. unlil turbine~ Ihal attracl Ihem arc huilt.

Tcsiin~ hypolhese~ 0f C~.lU,C will be" Iremcndou'> challenge. Bat fc"cai'(:;hcr~ are no",,' faced with the difficult ~ilUati()n of ~tud)' ing nIghltimc phenomena occurri ng a.' high <is 40-slry buildings and pOlentially across kilometers of ai rspa(c. For -:xall1plc, il will b<: important 10 rnea,me random and coi ncidenlal hypnlhe,es at spatial scales beyond the po~sible

influence of attranor~, Th.:: difficulty will be in detcrmining what the range of aHraCllon might be, and such distanceS' likcl>· differ for diCferent aUfC1c(ive clle~ and potenlially for different specle~ and landseapes. Techniques ex),t for monitoring [he movements of bats aero~s large areas of air<;paee. as well a" their illleraction, with turbines, but some mav nm he ,uited for inve,tigaling hypothe,e" of cause. and additIOnal experimental methods arc needed (Kunz et aL 2007a, 2008). TLll'bine racilit((~'> urI.' well ~\Iired for both nb,crvationa! and manlpulmivc expcrimcnLal studics into h)'p()(hesc~) t,f ,:ausc, ,0 industry coopcration seem'> essenlial.

..\ p.ll"licuJady promising ah'IlUe of research Inlo hypolbese, of cause is the ana!vsi, of carcasses recovcred benealh turbines. There i, much LO be learned fl'(llTI analyz.ing fresh carcasses rccovered in the vicinity of' wind wrbincs (e.g .. Baerwald el at ~OOX) but ptoportj(lually few are collected or saved for wch purpo~cs. Analysis of ~t()mach contenLs. reproductiYe status, in,lurie" and geochemiL:<'] ,;gnature, 01' body tissues ,U'c just a fe"" c\<ll1'lpks of how analysis or carcasses hilS and \\'ill ,'Ol1tilllle to reveal lmporLant dues about \.... hy bar~ die at (UrhllC~, bUL aho ,Iboul ihe basic biology of bats. Addition<J1 ilblght aho ll\,{y corne Crom closer qudy oC "Hected speclcs wil!; life-history charnctcrI:;Uc< th,lt diFfer from (rends discussed abo\·c, parllClliarly trec' roosting and long-distance migration. For e ....ample. I' subtlal'w and Pipistre/lus piPlSl1 cllHs roost in trees bUI arc. nOl considered to be migratory in area, where they collide With turbines. and the eaverniculons species Tadarida

fJi"c1si!lensis might account for nlost f'ILa!itics at ,,>Ol1le site<, in souLhern North America (Arnell el al. 2008: KUlll cl al. 2007b: Miller 20(8). These exarnpk.~ empha,ize that the same U!tlfllilte call~e or caus~s of Cmality may nO\. apply to all specie" or subspeCIes in the case of the L c;neuus semOlus. and researchers conducting observational and cxperimental ,'ludies need to keep thi" in mind.

If the ultimate cause or causes of bat fataJi[ie<, at wind turbine,; are not established. it may never be possible to accurately assess risk [() bats before Lurbines arc buill, mcasure Lhe true impacts of turbines on affected population" or come tIp with the most efficienl ways LO avoid or minimi/e fatalities. Although mitigation techniques such as operational curtail­ment are Itkely ro be an effective way of rninimi7.ing fataiitic., at c;xlsting tlIrbines (Baerwald et a1. 2009: E. B. Arnell. pef). comm.), land managers, conservationist~, and wlI1d cncrgy developers abo want to know in advance, before lurbirll':s are builL how they can avoid bal fatalities. Establi,bing the underlying causes of bat fataJiucs at turbines may he the most likely way of preventing them, starring at rhe earliest planning stagcs of wind energy development.

ACKNOWLEDGi\1ENTS

We thank T. J. O'Shea, E. Baer~,ald, C. K. R. W111l~, and J. Hayc, f,)]' di~clIs~ion, t!l,ll helped us with the org,mi7.ation of rhi, paper. E. Arnell, E. Baerw;;sld, J H:iyC'. T. Kunz. C. K. R.. Willi" and L. EIII~on provided helpful comment" on earhcr dr,]th of thiS manu~cripL We also thank M. R. Cannon. S. Mlslry. and C, A. Jlldi~a for organizing a symposillrl1 on bab and wmd enagy ~I the 38th annllal meeling of Ihe North American Society for Bat Research in Scnmton, Pennsylvania, that inspired thi~ collaboration. a" well a; tbe Bars and \'iind Energy Cooperutive nnd Bat COllServhil,)ll Jnternational for continuing to fmter di,J!og on rhi, ImpOflant "UbjcCI.

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