Prey of Breeding Northern Goshawks in Washington

Home , American crow, Northern goshawk, Pika

j. RaptorRes. 32(4):297-305 ¸ 1998 The Raptor ResearchFoundation, Inc.

PREY OF BREEDING NORTHERN GOSHAWKS IN WASHINGTON

JAMESW. WATSON,DAVID W. HAYSAND SEAN P. FINN1 WashingtonDepartment of Fish and Wildlife,600 CapitolWay N., Olympia,WA 98501-1091 U.S.A.

PAUL MEEHAN-MARTIN 2 ResearchExperiment Station, 3625 93rd Ave. S.W., Olympia,WA 98502 U.S.A.

ABSTRACT.--Weidentified 936 prey from food remains and pellets collected at 82 Northern Goshawk (Accipitergentilis) nest sitesin Washingtonfrom 1986-96. Mammals and birds constitutedhalf of the prey by frequencyand biomassthroughout Washington, although birds were more prevalent(P = 0.050) in the diet of goshawksnesting in the Olympicand Cascademountains of westernWashington (53%), than in the Cascadesof easternWashington (47%). Douglas'squirrels (Tamiasciurus douglasii), grouse (Dendragapusobscurus and Bonasaumbellus), and snowshoehares (Lepusamericanus) were jointly the most frequentlyrepresented prey on the westside (41%) and eastside (54%). Grouseand snowshoehares accountedfor the overwhelmingmajority of prey biomassin theserespective areas (76% and 80%). Relativeto other Northern Goshawkpopulations, goshawks in Washingtonappeared to prey on species from a similar number of genera,but they had a smallerfood-niche breadth and they took larger-sized birds primarily due to their high consumptionof grouse.Northern Goshawksin westernWashington took prey in more equal numbersthan thoseon the eastside. Potential bias from examinationof prey remainswhen comparedto pelletsreinforced the need for inclusionof observationson prey deliveries at nestswhen determining the diet of nestingNorthern Goshawks. KEYWORDS: NorthernGoshawk; Accipiter gentilis;diet;, food habits;grouse, hare,, Washington.

Presasdel azor en reproducci6n en Washington RESUMEN.--Identificamos936 presasde restosde comida y egagr6pilasrecolectadas en 82 nidos de Acdpitergentills en Washingtondesde 1986-96. Los mamiferos y las avesconstituyeron la mitad de las presaspot frecuenciay biomasaen Washington,aunque las avesprevalecieron (P = 0.050) en la dieta de las montafiasOlympic y Cascadedel oestede Washington(53%), al contrario de las Cascadedel esteWashington (47%). Tamiasdurusdouglasii, Dendragapus obscurus, Bonasa umbellus y Lepusamericanus fueron en conjunto las presasmas representadasen el oeste (41%) y este (54%). Dendragapusobscurus y Lepusamericanus representaron la mayoriade la biomasade presasenestas areas respectivas (76% y 80%). Con relacitn a otras poblacionesde azoresdel notre, los azoresde Washingtonaparentemente depredaron a especiesde un nfimero similar de gtneros, pero tuvieron nichos de alimentacitn de menor tamafio y una media mayor en el peso de las aves,lo cual es el resultadodel alto consumode Dendragrapusobscurus y Bonasa umbellus. Sin embargo,los azoresdel norte en el oestede Washington tuvieron un uso mas equitativode presasque los del este. E1 sesgopotencial del ex/tmen de los restos de presascomparado con las egagrtpilas refuerza la necesidadde incluir observacionespara la deter- minacitn de la dieta de los azores del norte en anidacitn. [Traducci6n de Ctsar Marquez]

In the Pacific Northwest, the association of 1994, Woodbridge and Detrich 1994) may be re- Northern Goshawks(Accipiter gentilis) with mature lated to the structural characteristics of stands that forests (Bull and Hohmann 1994, Hargis et al. optimize the availabilityof goshawkprey (Reynolds et al. 1992). Even where preferred prey are abun- dant, structural characteristics of habitat such as • Presentaddress: Raptor ResearchCenter, Department of Biology,Boise State University,1910 UniversityDrive, tree density and understory may reduce prey avail- Boise, ID 83725 U.S.A. ability thereby affecting habitat selection and dis- 2 Presentaddress: Snohomish County Public Works, 2930 tribution of nests (Beier and Drennan 1997, De- Wetmore Ave., Suite 101, Everett, WA 98201 U.S.A. Stefanoand McCloskey1997).

297 298 WATSON ET AL. VOL. 32, NO. 4

In Washington,nesting Northern Goshawksare sampled>2 yr). Most nest treeswere in late successional distributed east and west of the Cascade Moun- forests and were located in national forests (N = 58), national parks (N = 11), private timberland (N = 9), and tains, where there are different climates, forest state land (N = 4). In ownershipsother than national communitiesand potentiallydifferent prey species parks, landscapessurrounding nests and within nesting and prey abundance.In westernWashington, mar- territories were potentially subjectto forest management. itime influences at nest sitesin the Olympic Moun- The actual degree of timber harvestwithin these territories was unknown. tains may further influence prey and diets of nest- The climate in westernWashington is characterizedby ing Northern Goshawks.Diets in this area couldbe mild, wet winters and warm, dry summers. Forests are very different from that of goshawksnesting inland predominantlyDouglas-fir (Pseudotsuga menziesii), western in the west CascadeMountains. In the CoastRange hemlock ( Tsugaheterophylla), and westernredcedar ( Thu- ja plicata).Sitka spruce (Piceasitchensis) is presentat a few of Oregon, for example,dense understory vegeta- lower elevation nest siteson the west side of the Olympic tion and high rainfall are believedto contributeto Peninsula. Forests in the Cascadesof eastern Washing- low populationsof nestinggoshawks due to their ton, a region with cool wintersand hot, dry summers,are negativeeffects on prey availability(Reynolds and dominated by standsof Douglas-fir,ponderosa pine (Pi- Wight 1978, DeStefano and McCloskey1997). nusponderosa), and westernhemlock. For each collection of prey remains,we attempted to There is also the potential for dietary differences identify the minimum number of individualsrepresented in Northern Goshawksnesting in managed stands to specieslevel from pooled occurrencesin food remains in national forestsand private timberland, where and pellets. Matched hair and feather samplesfrom prey timber harvestmay influenceprey availability,and and pelletswere consideredto representthe same individual, whereas counts of pooled bones and flight feath- in national parks,where there is no timber harvest ers allowed for identification of >1 individual. When we (Crocker-Bedford 1990). found skeletal remains of snowshoehares (Lepusameri- Prey frequency,prey biomass,and food-niche canus)and grouse (Dendragapusobscurus and Bonasaum- breadth are commonlyused to quantify raptor di- bellus),we estimatedthe age of individualsthrough size ets (Marti 1987). Methods of identifying raptor comparisonswith museumspecimens and the degree of bone fusion at joints. Ages of these specieswere used to prey, including direct observation,examination of estimate their respectivecontributions to prey biomass. prey remains,and pellet examinationprovide data Solid remains (e.g., skeletal remains, beaks) were iden- on diet, but each is subject to potential biases tified from museum specimens(University of Washing- (Marti 1987). Here, we examine prey speciesiden- ton, Seattle, or University of Wisconsin,Madison) or tified in prey remainsand pelletsof nestingNorth- identificationkeys (Olsen 1964, 1972). Fur and feathers were matched to museum specimensor descriptionsin ern Goshawksthroughout Washington,with specif- field guides (Peterson 1947, Burt and Grossenheider ic objectivesto contrast prey frequency,biomass 1976). Arthropods were excluded from data analyses. and food-niche breadth for populations of North- Our suspicion was that many insectsfound in prey re- ern Goshawks east and west of the Cascade Moun- mains, particularlybeetles and ants,were consumedin- directly when small mammals and grouse were eaten. tain crest; contrastmajor prey groups among sub- Presence of ants was correlated (r2 = 0.18, P = 0.001) regions of these populations, and in areas with the occurrence of Douglas' squirrels (Tamiasciurus potentially subject to timber harvest and those douglasii)and chipmunks(Tamias spp.) in prey remains, without harvest;identify differencesin prey species and stomach contents of three whole carcassesof Doug- or food-niche breadth peculiar to Washington las' squirrelsand chipmunkscontained numerous beetle Northern Goshawks relative to other areas in shell fragments. Several pellets contained insect fragments mixed with fir needles,seeds, and grouseremains. North America; and identify biasesassociated with We reported mammalian and avian prey by frequency identification of prey from remains or pellets. and biomass. Biomass estimates were derived from average weightsof speciesfrom Reynoldsand Meslow(1984) STUDY AREA AND METHODS and other publishedsources (Table 1). Weightswere de- Preyremains and pelletswere collectedat 38 Northern rived for juvenile and adult age classesof snowshoehares Goshawk nest sites in western Washington (16 in the and grouse. For prey for which we could estimatebio- Olympic Mountains,22 in the CascadeMountains) from mass(i.e., not including unidentified birdsor mammals), 1986-96, and at 44 nest sites in the Cascades of eastern we calculated the mean weight of avian prey (MWAP), Washington(17 in the centralCascades, 27 in northern mean weight of mammalian prey (MWMP) and mean Cascades)from 1992-96 (Fig. 1). Prey and pellets were weight of total prey (MWTP). Food-nichebreadth was collected from nests, under nest trees, and at plucking calculatedfor prey genera using the following equation posts.Most remainswere collectedincidentally during (Levins 1968): breeding surveysfrom the nestling stage through post- 1 fledging, and nestswere not sampled equally among B- years (65% sampled1 yr, 24% sampled2 yr, and 11% DECEMBER 1998 GOSHAWK PRgY IN WASHINGTON 299

o o q• I o OL • Okanogan I

Lewis

Yakima Cowlitz I SYMBOLS I * + West of Cascade Mountain Divide

D $ 1D152025 Mountain Divide .,,•s Clark l - Klickitat o EastofCascade •.. , ....._ ••• .

Figure 1. Locationsof Northern Goshawknest sitesin WashingtonState where prey were collectedfrom 1986-96. where Pi was the proportion of prey in each taxon. In and avian prey were excluded from analysesinvolving order to compare food-niche breadth with other north- prey species,and prey with combined frequencies --<1 ern goshawkresearch, breadth was standardized(Reyn- were pooled into a miscellaneousspecies category. olds and Meslow 1984) using: B5 = (B- l)/(n- 1) RESULTS where n = the total number of taxons.Values approach- We identified 936 prey at 82 Northern Goshawk ing 1 were indicative of relativelymore equitable use of nest sites.West side prey collections (N = 38 sites) prey, with lower values indicative of narrower diet breadth. accountedfor 57% of identifiedprey, and eastside We used chi-squarecontingency tests at the P = 0.05 collections (N = 44 sites) accounted for 43% of significancelevel for frequency comparisonsof prey spe- prey. An average of 11.0 (SD = 10.5, range = 1- cies and classesby region (east side and west side, and 53) individual prey items were identified per nest four study areas) and collection type (prey remains or pellet), agesof major prey species(adult or juvenile) by site. Prey remains were not identified beyond class region,and major prey speciesby forestmanagement sta- for 11% of 465 mammalian remains and 17% of tus (managedor unmanaged). Unidentified mammalian 471 avianremains. These remains typically consist- 300 WATSONF.T •. VOL. 32, NO. 4

Table1. NorthernGoshawk prey assessed from prey remains and pellets at 44 nestsites in theOlympic and Cascade mountainsof westernWashington, and at 38 nestsites in theCascade mountains of easternWashington from 1986-96.

WESTERN EASTERN WASHINGTON WASHINGTON

WEIGHT % BIO- % BIO- SPECIES COMMONNAME (g)a NO. %b MASSc NO. %b MASSc

Mammals Tamiasciurusdouglasii Douglas'squirrel 201.2 80 15.1 9.6 78 19.2 10.0 Lepusamericanus snowshoehare d 47 8.9 32.6 57 14.1 40.6 Tamiasspp. Unidentified chipmunk 80.5 31 5.8 1.5 26 6.4 1.3 Unidentified mammal n/a 27 5.1 n/a 19 4.7 n/a Glaucomyssabrinus northern flying squirrel 167.0 28 5.3 2.8 12 3.0 1.3 Clethrionomysgapperi red-backedvole 27.0e 7 1.3 0.1 9 2.2 0.2 Unidentified vole 25.0 13 2.5 0.2 I 0.3 <0.1 Peromyscusspp. Unidentified mouse 19.5f 6 1.1 <0.1 6 1.5 <0.1 Thomomysmazama Mazama pocket gopher 103.0e 5 0.9 0.3 0 0.0 0.0 Unidentified smallmammal n/a 2 0.4 n/a 3 0.7 n/a Scapanustownsendii Townsend'smole 140.0• I 0.2 <0.1 I 0.3 <0.1 Thomomystalpoides northern pocket gopher 104.0e 0 0.0 0.0 2 0.5 0.1 Ochotonaprinceps pika 146.5• I 0.2 <0.1 0 0.0 0.0 Sorexcinereus masked shrew 4.5 • I 0.2 <0.1 0 0.0 0.0 Neotomadnerea bushytailwoodrat 396.0• 0 0.0 0.0 I 0.3 0.3 Martesamericana marten g 0 0.0 n/a I 0.3 n/a Subtotal 249 47.0 47.1 216 53.5 53.8

Birds Unidentified grouse h 73 13.8 35.1 74 18.3 35.8 Unidentifiedbird n/a 49 9.2 n/a 32 7.9 n/a Cyanocittastelleri Steller'sJay 106.6 46 8.7 2.9 16 4.0 1.1 Colaptesauratus Northern Flicker 148.8 26 4.9 2.3 23 5.7 2.2 Ixoreusnaevius Varied Thrush 79.3 25 4.7 1.2 4 1.0 0.2 Turdusmigratorius American Robin 81.2 18 3.4 0.9 1 0.3 <0.1 Dendragapusobscurus Blue grouse i 11 2.1 7.1 5 1.2 2.7 Picoidesvillosus Hairy Woodpecker 48.3 3 0.6 <0.1 8 2.0 0.2 Unidentified woodpecker 165.2J 7 1.3 0.7 3 0.7 0.3 Perisoreuscanadensis GrayJay 89.4k 9 1.7 0.5 0 0.0 0.0 Unidentified passerine 168.7• 3 0.6 0.3 6 1.5 0.6 Bonasaumbellus Ruffed Grouse 550.0m 3 0.6 1.0 3 0.7 1.1 Varied Thrush or American 80.0 I 0.2 <0.1 3 0.7 0.2 Robin Spinuspinus Pine Siskin 13.0 I 0.2 <0.1 3 0.7 <0.1 Corvusspp. Northwestern or American Crow 460.0f 2 0.4 0.6 I 0.3 0.3 Unidentified owl 259.0 n 0 0.0 0.0 3 0.7 0.5 Bombycillacedrorum CedarWaxwing 33.5 I 0.2 <0.1 I 0.3 <0.1 Glaucidiumgnoma Northern PygmyOwl 42.8ø 1 0.2 <0.1 0 0.0 0.0 Juncohyemalis Dark-eyedJunco 17.6 I 0.2 <0.1 0 0.0 0.0 Loxialeucoptera White-wingedCrossbill 24.1P I 0.2 <0.1 0 0.0 0.0 Sturnusvulgaris EuropeanStarling 74.5 I 0.2 <0.1 0 0.0 0.0 Dryocopuspileatus PileatedWoodpecker 282.0 0 0.0 0.0 I 0.3 0.2 Sphyrapicusvarius Red-breastedSapsucker 48.3q 0 0.0 0.0 I 0.3 <0.1 Anas platyrhynchos Mallard 1185.0f 0 0.0 0.0 I 0.3 0.8 Subtotal 282 53.4 52.6 189 46.9 46.2 DECEMBER 1998 GOSHAWK PR•¾ IN WASHINGTON 301 ed of hair samplesand feather shafts,but no skel- to other birds, proportions of grouse,Steller'sJays, etal remains. Varied Thrush (Ixoreus naevius), American Robins At least 13 speciesof mammals and 18 species (Turdusmigratorius), and GrayJays (Perisoreuscana- of birds were identified in prey remains (Table 1). densis)were greater in westside than eastside avian Douglas' squirrels, grouse (unidentified grouse, remains (X• = 38.89, df -- 5, P = 0.001). Converse- Blue Grouse [Dendragapusobscurus] and Ruffed ly, relative to other mammals combined, propor- Grouse [Bonasa umbellus]), and snowshoe hares tions of Douglas'squirrels and snowshoehares in were the most common prey species,and together east side remains were greater than in western accountedfor 54% of all prey in easternWashing- Washington(X• = 6.96, df = 2, P = 0.031). ton and 41% in westernWashington. They were Mammals and birds accounted for similar pro- alsothe mostwidely distributedprey, with Douglas' portions of prey biomassthroughout Washington squirrelsidentified at 69% of the 82 nestsites state- (51% and 49%, respectively).While Douglas'squir- wide, grouseidentified at 57% of nest sites,and rels were the most prevalent species in prey re- snowshoe hares identified at 61% of all nest sites. mains, they accountedfor only 10% of prey bio- Other speciesthat accountedfor >3% of prey by mass on both east and west sides (Table 1). frequencyin both easternand westernWashington Snowshoehares and combined grouse specieswere included chipmunks( Tamiasspp.), northern flying the most important to overall biomass.These tax- squirrels(Gtauc0mys sabrinus), Steller's Jays (Cyan- ons accountedfor 80% of all prey biomassin east- ocittastelleri) and Northern Flickers (C0laptesaura- ern Washington,and 76% of all biomassin western tus). Passerinesaccounted for 28% of west side Washington.Adult specimensaccounted for much prey and 18% of eastside prey. of the biomass; adult snowshoe hares composed Mammals and birds composed equal propor- 87% of 67 haresthat were aged, and adult grouse tions (50%) of goshawkprey throughoutWashing- composed75% of 113 grouseremains that were ton by frequency. However, proportions of mam- aged. There was no difference in age of captured mals and birds in prey remains differed between snowshoehares (P = 0.458) or grouse (P = 0.410) westernand easternWashington (X•= 3.81, df -- between eastern and western Washington. Other 1, P = 0.050). Birds were more prevalent than prey speciescontributed <3% of overall biomass mammals in prey remains of west side goshawks throughout Washington (Table 1). (53% vs. 47%), while mammals more prevalent Mean weight of mammalian (N -- 399), avian than birds on the east side (53% vs. 47%). Relative (N -- 390) and total prey was 411,415, and 413 g,

aWeight of individualprey item usedin biomassestimation. From Reynoldsand Meslow(1984) unlessnoted otherwise. bPercent of prey in overalldiet. c Percentof prey biomassto overallbiomass. dWeight of juvenile (150 g) and adult L. americanus(1500 g) from Forsmanet al. (1984); meanweight of adult andjuvenile used to estimateweight of unagedspecimens. Remains included 27 adults,3 juvenilesand 16 unagedspecimens. e Burt and Grossenheider (1976). f Steenhof (1983). gMass for this prey not usedin estimateof biomassdue to the inherent bias;guard hairsfound in pelletslikely from a scavenged carcass. hAverageweight of adult (1053 g) andjuvenile (909 g) D. obscurus(Zwickel et al. 1966) and adult (575 g) andjuvenile (550 g) B. umbellus(Bump et al. 1947); mean weight of adultsand juvenilesused to estimateweight of unaged specimens.Remains included 37 adults,9 juveniles and 30 unaged specimens. lAverageweight of adult andjuvenile D. obscurus(Zwickel et al. 1966); meanweight of adult andjuvenile used to estimateweight of unagedspecimens. Remains included 4 adults,1 juvenile and 7 unagedspecimens. JAverage weight of P villosusand D. pileatus. kAverage weight of unidentifiedjay from Reynoldsand Meslow(1984). • Averageweight of passerinesidentified to species. mAverage weight of juvenile B. umbellus(Bump et al. 1947). Remainswere of 1 juvenile. n Usedweight of medium-sizedowl (Asiootus) from Karalusand Eckert (1974). o Karalus and Eckert (1974). PAverage weight of White-crownedSparrow (Zonot•ichia leucophrys) from Reynoldsand Meslow(1984). qUsed weight of P. villosusfrom Reynoldsand Meslow (1984). 302 WATSON ET AL. VOL. 32, NO. 4 respectively.The standardizedfood-niche breadth = 0.001). Remainsof grousewere 15% more prev- (FNB) was 0.27, based on frequencies of prey alent among prey than pellets when compared to among 20 genera. However, goshawksin western other birds (X2 = 15.82, df = 1, P = 0.001). Washington had a more equitable use of prey DISCUSSION (FNB = 0.44, 18 genera) than those in eastern Washington (FNB = 0.31, 16 genera). Raptor diets are most accurately described Proportions of mammals and birds in the diet through the combination of observationsof prey were different (X9 = 17.67, df = 3, P = 0.001) deliveries at nests and prey collections (Marti among prey of goshawksnesting in the Olympic 1987). We identified biases associatedwith the Range, western CascadeRange, the central/south- identification of only Northern Goshawkprey re- ern east Cascades, and northern Cascades in east- mains that overemphasized snowshoehares and ern Washington(N = 16, 22, 27, and 17 territories, grouse,and believe that by includingpellets in the respectively).Goshawks in the central and south- analysisa more complete representationof the ac- ern Cascadesof eastern Washington ate the high- tual diet results,particularly in regard to the im- est proportion of mammals (57%), with fewer portance of small mammals. In western Washing- mammals eaten in the Olympic Range (54%), ton, the predominance of collected prey remains northern Cascadesin eastern Washington (44%), comparedto pelletsmay havepartly accountedfor and west CascadeRange (41%). Proportionsof the greater occurrence of avian prey, particularly Douglas'squirrels in prey remainsdiffered (X9 = grouse, on the west side. We were unable to iden- 14.79, df = 3, P = 0.002) amonggoshawks in the tiff/ biasesthat may have resulted from a lack of central and southern region of the east Cascades observationsat nests.This may have underestimat- (22%), Olympic Range (20%), northern Cascades ed the consumption of arthropods and reptiles as in eastern Washington (15%), and west Cascades was found for Accipitersin Oregon (Reynoldsand (11%). Proportions of snowshoehares also dif- Meslow 1984), and overemphasizedavian prey in fered (X2 = 11.73, df = 3, P = 0.008) in the central the diet as determined in several studies (Ziesemer and southern region of the east Cascades(16%), 1981, Reynolds and Meslow 1984, Boal and Man- Olympic Range (10%), northern Cascadesin east- nan 1994). We concluded that most arthropods ern Washihgton(10%), and west Cascades(7%). were eaten by goshawksincidental to the consump- Proportionsof grousewere different (X2 = 22.43, tion of other prey. Although we identified no rep- df = 3, P = 0.001) in the northern Cascades in tiles as prey, reptiles, notably garter snakes(Tham- eastern Washington (32%), west Cascades(20%), nophisspp.), were common in all forestswe studied central and southern region of the east Cascades throughout Washington (K. McCallister pers. (17%), and OlympicRange (12%). comm.). Other studies have not identified these We found no difference (P = 0.853) among fre- same biases from prey sampling methodologies. quenciesof Douglas'squirrels, snowshoe hares and For example, prey and pellet analysisof nesting grouse relative to each other on land ownerships Northern Goshawksin northeast Spain over-rep- with potential timber harvest (i.e., national forest, resented Leporids,and under-representedthrushes state, and private land) and without harvest (i.e., and small birds (Mafiosa 1994). The rank of prey national park). taxons assessedfrom prey remains, pellets and ob- We collectedpellets 17% lessoften (X9 = 5.34, servationsdid not differ for breeding goshawksin df = 1, P = 0.021) when sampling nests on the New Mexico (Kennedy 1991). These differences westside (N = 47 visits)compared to the eastside reemphasize the importance of observationsof (N = 76 visits).We suspectedthat some skeletal prey deliveries for determining diets of specific remains on the west side were misclassified as com- populationsof nesting goshawks. ing from prey remains rather than pellets due to Variation in the seasonaland annual timing of the rapid breakdown of pellets in the moist cli- prey collectionsamong nest sitesintroduced other mate. Twenty-eightpercent more birds than mam- potential biases in diet assessment.Seasonal mals were identified in prey remains when com- changes in diet composition of Northern Gos- pared to pellets (X9 = 81.59, df = 1, P = 0.001). hawks may include a shift to fledgling passerines Remains of snowshoehares were 24% more prev- and increased diet diversityas nesting progresses alent among prey than pelletswhen compared to (Squiresand Reynolds1997). Thus, prey constitu- all mammals of smaller size (X9 = 19.81, df = 1, P tion may change from the nestling to fledgling pe- DECEMBER 1998 GOSHAWK PREY IN WASHINGTON 303 riods of the nesting phenology. Our geographical ulations in California (Bloom et al. 1986, Wood- comparisonsof Northern Goshawkprey in Wash- bridge et al. 1988) and a greater consumption of ington were based on irregular prey collections snowshoehares relative to northern populations in over the 10-yr period and at different times of the southeastAlaska where hares are evidentlyuncom- nesting period; westside prey were collected mon (Titus et al. 1994). In California, primary prey throughout the entire 10-yr period, whereas east- specieswere Douglas' squirrels,Steller's Jays, and side prey were collected over a 5-yr period. These Northern Flickers; lagomorphs and sciurids com- collectionswere a compositeof severalstudies, and prise 66% of the total biomass(Bloom et al. 1986). each nest site was not sampled equally.It was not In the southern Cascades of northern California, known how, or if, cyclicchanges of major prey spe- Steller'sJays and four speciesof woodpeckersare cies throughout the state may have biased our an- the principal birds taken and sciuridsaccount for alysesbecause we did not monitor their spatialor over half of the total biomass (Woodbridge et al. temporal variability.In the northern boreal forest, 1988). In Washington, we found grouse accounted snowshoe hare and Ruffed Grouse undergo re- for 42% of total biomass and lagomorphs and gion-wide cyclic population fluctuations approxi- sciuridscomposed an additional 46% of total bio- mately every 10 yr (Keith and Rusch 1986, Doyle mass.Even assumingunidentified grousewere the and Smith 1994, Hik 1994). Populationsof other smallestjuvenile grouseand unaged hareswere all potential prey, particularlyDouglas' squirrels, may smallerjuveniles, these taxons still accountedfor alsobe subjectto periodiccycles depending on the 42% and 44% of total biomass, respectively.In annual production of cones by conifer forests southeast Alaska, goshawkseat high numbers of (Buchanan et al. 1990). However, Douglas' squir- Blue Grouse which were identified at 73% of 25 rels accounted for little biomass relative to other nest sites, but snowshoehare were found at only frequently eaten prey of goshawksin Washington one nest (Titus et al. 1994). Additionally, Steller's suggestinga lesserimportance of this speciesin the Jays,Varied Thrush, and red squirrels (Tamiasciu- diet overall. Reduced hare numbers in southwest rus hudsonicus)have been found at •47% of the Yukon resulted in dietary shifts to smaller mam- southeast Alaskan territories. Comparatively few malian prey and increased avian consumption small mammals are available as prey, which may (Doyle and Smith 1994). A sudden decline in Eu- limit populations (Titus et al. 1994). Queen Char- ropean hare (Oryctolaguscuniculus) in northeastern lotte Goshawkson Vancouver Island eat species Spain resulted in a reduction in rabbit consump- similar to those eaten by goshawksin Washington tion by nesting goshawksand increasedpredation including Steller'sJays, Varied Thrush, and North- on Red-legged Partridge (Alectorisrufa) (Mafiosa western Crows (Corvus caurinus) (Beebe 1974). 1994). Consequently,if cyclicpopulation phenom- Shorebirdsand seabirdsare alsocommon prey,but ena are similar for hare and grousein Washington, we did not record them as prey in Washington, our regional comparisonof prey could, at best, be most likely becauseno nestsin our studywere lo- interpreted to reflect actual differencesin state- cated near seacoasts. wide prey selection;or, at worst, to be merely the Even though regional and study area prey class identification of prey specieseaten by breeding proportions in Washingtonwere statisticallydiffer- goshawksin eastern and western Washington. ent (e.g., regional variation of 46-54%, studyarea The sameprey specieseaten by goshawksin east- variation of 41-59%), dietary class proportions ern and western Washington accounted for the were more similar to goshawkdiets in New Mexico, greatest biomass and frequency of prey in these Arizona and Oregon than to diets in New Jersey areas (i.e., snowshoehares, grouse, and to a lesser and California (Table 2). Goshawksin New Jersey degree, Douglas' squirrels, chipmunks, Steller's and California eat 10-15% fewer mammals and Jays, and Northern Flickers). These same species 10-15% more birds compared to goshawks in have been found to be important prey throughout Washington. More recent prey collections at gos- the goshawk'sNorth American range (Reynoldset hawk nests in three different areas of eastern al. 1992, Squiresand Reynolds1997). Compared Oregon found frequenciesof mammalian prey var- to goshawkdiets in other Pacific Coast states,the ied from 38-66% (Bull and Hohmann 1994, De- most pronounced latitudinal differencesappear to Stefano et al. 1994). These results indicate that be the prominence of grousein diets of Northern there can be as much variation in proportions of Goshawksin Washingtonrelative to southern pop- major prey species and classesof prey between 304 WATSON ET AL. VOL. 32, No. 4

Table 2. Comparative food-niche breadths (standardized) and prey classratios of Northern Goshawksin North America. Diet parametersare basedon prey remains and pellets collectedat nestsunless indicated otherwise.

FOOD-NICHE MAMMAL: BIRD BREADTH LOCATION NESTS RATIO (NUMBER GENERA) SOURCE NewJersey 16 30:70 0.26 (22) Bosakowskiet al. (1992) Arizona 20 53:47 0.29 (18) Boal and Mannan (1994) a EasternWashington 44 53:47 0.31 (16) this study New Mexico 8 49:51 0.36 (22) Kennedy(1991) b,c California 114 32:68 0.41 (21) Bloom et al. (1986) a,c WesternWashington 38 47:53 0.44 (18) this study Oregon 59 45:55 0.45 (30) Reynoldsand Meslow (1984)c Analysisof prey remains. Pellet analysis. Standardizedfood-niche breadth calculatedby Boal and Mannan (1994). studyareas separated by •100 km, as there can be speciesto buffer the effectsof specializedfeeding, in nestsseparated by severalhundred kilometers. goshawk productivity may mirror the changes in Dietary proportionsof mammalsand birds are re- cyclicprey populations(Reynolds et al. 1992,Doyle flective of the abundanceand availabilityof poten- and Smith 1994). Monitoring temporal changesin tial prey species throughout the range of the hare and grousepopulations to assesstheir cyclic Northern Goshawk(Reynolds et al. 1992), suggest- tendencies,and simultaneouscollection of prey at ing there wasconsiderable variation in local abun- the samegoshawk nest sitesover severalyears, both dance or availabilityof key prey in Washington.We in eastern and westernWashington, would provide did not find a relationship in the occurrence of an informative contrastas to the importance of cy- major prey typesamong managed and unmanaged clic prey to nest site occupancyand productivity. forests,which we hypothesizedmight be a factor ACKNOWLEDGMENTS influencing local prey abundanceor availability. J. 15egley,J. Fackler,S.P. Finn, CJ. Flatten,T.L. Fleming, Relative to five other breeding populations 15.Gaussoin, R.E. Lowell, D. Mueller, M.L. Nixon, and`i. throughout the U.S., goshawksnesting in eastern Swinglecollected prey remains. 15.1sicknell, I. Otto, 15.L. Washington had a low food-niche breadth while Cunningham, and C. Fletcher identified prey.We thank those in western Washington had a high food- L.E Ruggiero for logisticalassistance. C.W. 15oal,T. 15o- niche breadth. While similar specieswere eaten, sakowski,A.15. Carey, E.D. Forsman, S. DeStefano,and R.N. Rosenfieldcommented on earlier drafts.The proj- east side goshawkstended to eat large, cyclic prey ect wassupported by the PacificNorthwest Research Sta- such as hares and grouse more frequently. The tion and Randall Ranger District of the U.S. Forest Ser- high consumption of grouse throughout the state vice, in cooperation with the Wildlife Research and Diversity Divisions,Washington Department of Fish and resulted in a mean avian prey weight (415 g) that Wildlife. was higher than that reported in New Jersey (332 g) and Connecticut (337 g) (Bosakowskiet al. LITERATURE CITED 1992), and Oregon (195 g) (Reynoldsand Meslow 15EEBE,F.L. 1974. Field studies of the Falconiformes of 1984). The mean weight of mammalian prey (411 15ritishColumbia. Occasional papers of the 15.C.Pro- g) in Washingtonwas more similar to the range of vincial Mus. No. 17. Victoria, 15.C. Canada. the averagemammalian prey in thesesame studies 15EIER,P. AND.I.E. DRENNAN.1997. Forest structureand (423-445 g). The varietyof specieswe identified as prey abundance in foraging areas of Northern Gos- prey suggestedthat nest occupancyand productiv- hawks.Ecol. Appl. 7:564-571. 15LOOM,P.H., G.R. STEWARTAND 15J. WALTON. 1986. The ity of goshawksin Washingtonis not dependent on status of the Northern Goshawk in California, 1981- cyclicfluctuations in the populationsof grouseand 1983. Wildl. Manage. 15ranchAd. Rep. 85-1. Calif. hare alone, although eastside goshawks were more Dept. Fish and Game, Sacramento,CA U.S.A. specializedfeeders during this study. 15OAL,C.W. AND R.W. MANNAN. 1994. Northern Goshawk While the species is an opportunistic feeder dietsin ponderosapine forestson the Kaibabplateau. (Doyle and Smith 1994), without a variety of prey Stud. Avian Biol. 16:97-102. DECEMBER 1998 GOSHAWK Play IN WASHINGTON 305

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