TimothyD. Reynolds! NancyD. Ertler DerekK. Broemeling Biology Departnent Boise StateUniversity Boise,Idaho 83725 and Richard P. Howard U.S. Fish and Vildlile Service Endangered Species Program 11620Overland Road, Room 209 Boise, Iclaho 83705 Winter Distributionof Bald EaglesAlong a Segment of the Boise River, ldaho Abstract Balct Eagles I tjdliaect/ts le*cocephala:) rvere observed from mid-Dccember 1982 duoush m;d March 19Bl to determire their winter clensitvdensity and distribution alongalonp therhe BoiseRoise RivcrRiror extenclirg-vren.linc 1,i km upstreamIrom Boise, Ada County, Idaho. A minimum of 10 inaiinaividual caeles *'ere obscrveJ r tot^l of 148 timcs. EarlvEarly in the winter-winter, eaglesaeles were ratherrarher.venlv,li cvenly distributetl along thc 14 km stretch of river. Thercafter.'after. 60 Dercentpercent of thcrhi sishtinssrigtrrings were within*ithrn thcrt,c firstfir{r kilomererkilomer.r of rivcrriv.r bclow Lucky PeaL reservoir. Introduclion The Boise River, upstreamfrom Boise,Ada County, Idaho, provides the requiremenrsof open v,rater,suitable habirat, aod adequateprey for Bald Eagleswintering in rhe western Unitecl Srates(Steenhof et r/. 1980), and is annuallyused by eaglesfor J or more monrhs eachwinrer (Jensen1981). Ownership of rhe land adjaceorto the strerchof river from Boise to Lucky Peak Dam (Figure 1) is a mosaicof privarg industrial, and state inter- ests.The developmenrof the bo tomland within the floodplain is under the supervisioo of the U.S. Army Corps of Engineers.Currently there is only limited human acrivicy in this area.Nearly 75 perceot of rhe lancladjacenr to the river is either undisturbedor is pastufeiand.However, severaiplans for developmentof sites along rhis segmenrof the river have beeo proposed. Proposalsrange in scopefrom high density housing tracts ro parks with iittle ot no anticipatedhabitat changes.This mixture of lao<lownership and planning for development,coupled with the endangeredspecies sranrs of the Bald Eagle (USDI 1978), and Jensen's(1981 ) unpublisheddata on wiotering eagles,prompted this study. Our objectiveswere to quanrify rhe remporal and spatial density changesof the winterirla eagle population, and to identify specific habitat use areasancl possible foost sltes. rPrcscnt address: Radiological and Environmental Scienccs Laboratorv, U.S. Departmen. of Encrt!y. i50 2ncl Street.Idaho Falls,Idaho 81401. NorthwestScience, Vol.59, No. 2, 1985 93 Figure1. Locationof the 1982,83wintering Batd Eaglesrudy area atong the BoiseRiver, Ada Counw-Idaho. Study Area The study area extended 14 km upstream from ITalnut Street in Boise to Lucky Peak Dam ( Figure 1 ) . The lower 7 km of the river, from Valnut Srreet to Barber Dam, is relatively shallovz and fast flowing, bordered by native riparian habitat interspersed wirh pasrurelaod and residenrial, cofporale, aod municipal developments, Extending approximately 1.5 km upstream from Barber Dam to Diversion Dam the river is braided, slow flowing, and known locally as Barber Pool. Several islands in this area, aod the adjacentriver banks,support standsof mature cottonwoods(Popuh6 sp) . Imn.rediately above Diversion Dam the river valley abruptly narlows to a 45 m d,eep canyon which extendsupsffeam to Lucky Peak Dam.'With the excepriooof the dischargebasin imme- diately below the dam, this upper 3.) km reachof the river is mostly slack water. Mater;alsand Methods Surveysfor Bald Eagleswere conductedevery third day from ll December 1982 chrough 19 March 1983. Morning and afrernoonsurveys, starting at dawn and rhtee hours prior 94 Reynolds,Broemeling Ertter, and Howard to clark.respecrively, were conduced oo aiternaresampling days.Three rypesof surveys rvere conducteclbecause no single methocl provicled complere coverage of the study aren: 1) a i,ehiclesuLvey ioute alongHighway 21 (Figure 1) s,'hichparalleled the study area; 2) statiooaryobservations from norh of the river betweenLm 3 and 4 (Figure 1), ancl from the bluffs sotith of the river overlooking Diversion Dam in krn 11; l) float rrips by canoe. Lach survey lastecl2-l hours. Observationsfrom the bluffs above Diversion Dam rvereconducted every fourth samplingday. Caooetrips were mken at 3-1+week inrervals. Data recordecl for each eagle sighted iocluded the location, habitar, tirne, acriliry (perchint,feeding, soaring or flying), and rhe age (aclultor immature). Results During the 11 surveys (21 by vehicle, seven srationaryobservarions, ancl rhree canoe floats) a total of 148 sightings of Bald Eagleswas recorded.The maxirnum number of ea.qlesobsetved simultaneously, hence the mioimum known to use rhe study area,was 10. This occurredon 2! January and 18 February.Four of the birds observedon 29 January rvere identified as adults,and 6 as immatrre. Of the 148 rotal sightings,fifty- onc ( 31rpercent) were of adult bnds, 62 (42 percent) were of immatures,^r\d 35 (24 percent) n'ere of unknowo age. Temporal and spatialuse of the river environsby eagleswas not uniform (Table 1). Eagleswere observedalong the lower 3 km of the study area only during the first rwo weeksof the study,and rhen orrly in very limited numbers.Eagles seemed to prefer the Barber Pool area (specifically river segnlent8 and 9; Figtire 1) during late December trnd early January.Thereafter, the majority of the eaglesightings occurred directly below Luck Peak Dam in river segment 14 rvhere no eaglesn'ere recorded during the first nronth of the study.The sightings in rhe Barber Pool areawere equally divided Lretween perching and flying birds. Immediately below l-ucky Peak Dam, over 90 percent of the eaglesobserved rvere airborne, either soaring over the cliffs or circling low over the water. The maximum number of eagle observationsoccurred duriog late January and eatly Febtuary.By mid-Match a mioimum of four Ralcl Eagleswere observedbetn'een Diversion and T,uckyPeak Dams. Jensen(1981) reportedthar Baid Eaglesused the islandsin Barbet Pool as night 'l']\lll,lt L NuDnier .n .|serralions oi Rald liasles rlone ea.h 1 km seBneDt 1)l the Boise lliror', Idrh(), study {rer, ahnjrs thc 11rS219rli \yilrler. 1 2 it 4 ; li i I il 10 11 L9 l:l tl 1l -: it Der. 0 t1ltL0 '.t4221 11 2, nc(. 0 r11220 2 it1221 0 20 l)00012 l0 i1011 0011iril l ii1a02l0 2! 0|2100 l01II 00011 1r,r)11 2() Ireb 5 ,\hr'. 1101o0 1 ll 0 0lr 1 12 0000i' 001ltl jl 5 (irand T()lal 1l,Lxl Der s.ADenL r t;:l 12I la {i ! ii;tiJ 14s N(inter Disttibution of Bald Eagles loosr sites cluring winter. From the onset of this srudy until 14 -lanuarywe regulady recorded eaglesperched io the cotronwood trees ovedooking rhe river at Barber pool during the day, as rveil as at bodr sunriseand sunser. Sunsetand sunriseobservations in-rplythat eaglesroosted at Batber Pool. After rnid-January,rvhen rhe maximum eagle activiry shifreclto the upper reachesof the stuclyarea, eagles were consistentlyobserved soaringover the foothills on lhe ooth sicleof the river iate eachafternoon. Rarher than lnoving downriver toward Barber Pool to roost, these birds would fly in a northerly clirection.Although we did not determinethe specificnight roostarea, rhe birds appeared to roost in the cooiferousforesrs near SchoonoverGulch or the Robie Creek,:lrainage 10-12 krn due north of Luck Peak Dam. Discussionand Conclusions The populatioo of Bald Eaglesrvintering aiong rhe Roise River is unusual becauseo[ its proximity to a metropolitan area.The minimum of 10 eagleswe observedis similar to lhe rnaximum of 12 reported by Jensen (1981) overwinterinq in rhe samearea rwo yearsprior to this srudy. Alrhough this 14 km segmentof the Boise Rivet appearsto support 10 or more Bald Eaglesduring che n'ioter, ir is not kno,nn exacly how rnany eaglesconsistently use the area.The Lucky Peak Reservoir,directly upstream from our study area, and Arowrock Resetvoir above thar, both support wintering populations of Bald Eagles.A regular exchangeof birds betweenrhese areas is quire possible. Until iate January,Barber Pool appareotly provided a goocl area for hunting and day perching, ancl quite possibly serveclas a night roost. After rnicl-winrereagles were observedmore frequently in rhe uppet srretchof the srudyarea, and beganusing a nighr roost 10-12 km distant {rom and the river. This trend continued until the end of the strLdywhen eagles$'ere leaving their rvintering habitat. The reasonfor the appareotchange in night roosrsis Linknowo,bur daily movements of this sort are nor particularly novel for eagles.Bald Eagleswinterirls near rhe Anerican Falls Reservoirin southeasternIdaho make daily movesin excessof 15 km ro and from a foost io rhe BannockMountain Range (C. H. Trost, Biology Dept., IclahoState Univer, sity, personalcommllrrication ) . Sirniiar movementswere reporreclby Sn,isher(1964) lot l population of eaglesin nonhem Urah. LileratureCited -ienscn,S. \f. 1981. lrintering Bald [ag1e' oi rhc lorer Boi:c Rircr. Uopub Danuscript. USDI, FishFi,h ar,anct lvilctiife!(/rl.rlih Servi.r,Service, DivisionDivi.ion, of f EndangercdEndanr, cd Sp<.,rsSpecies.Boise, Bcise. Idaho.tdino. Steenirof, K., S. S. Berlinger, aod I- H. Fredfickson. 1980. Habitat use by wintering tsalci Iaglcs cuurl- inin Sourh Dakota.Drkcr". J rJflildl.\ ildl. Manase.Vnnr8c. ,1.1:798-80j.|:-ll8-s0r. S*'isher.Ssishtr.. J. T. I.h.I., Jr.lr. 1964. A roustineroosting area,,farea of thc Bald EasleEagEagle in oorrheronorrhern Utah.Ihal, V/ilson\X/ilson Bull.Rrll 76:186-76 186- 187. U.S. Dept. Intcrior. 1978. Dctcrminati,rn oi etrtein BaL'] Faglcs pupulrtrons as roclangcrcd or rh--aefle.r.l.'rral R.gi.Lrri{ (l't(r./.'.'. lfeceined15 Decetnber1983 tlccefited.Iol pnblica an 5 Ap l 1984 96 Reyooids,Btoemelirli, Enrer, aocl Horvard.