j. RaptorRes. 39(3):264-273 ¸ 2005 The Raptor ResearchFoundation, Inc. NORTHERN GOSHAWK DIET IN MINNESOTA: AN ANALYSIS USING VIDEO RECORDING SYSTEMS BRETT L. SMITHERS 1 Departmentof Range,Wildlife and FisheriesManagement, Texas Tech University, Lubbock,TX 79409-2120 U.S.A. CLINT W. BOAL U.S. GeologicalSurvey, Texas Cooperative Fish and WildlifeResearch Unit, TexasTech University, Lubbock,TX 79409-2120 U.S.A. DAVID E. ANDERSEN U.S. GeologicalSurvey, Minnesota Cooperative Fish and WildlifeResearch Unit, Universityof Minnesota, St. Paul, MN 55108-6124 U.S.A. ABSTRACT.--Weused video-recording systemsto collect diet information at 13 Northern Goshawk(Ac- cipitergentilis) nests in Minnesota during the 2000, 2001, and 2002 breeding seasons.We collected4871 hr of video footage, from which 652 prey deliverieswere recorded. The majority of prey deliveries identifiedwere mammals (62%), whereasbirds (38%) composeda smallerproportion of diet.Mammals accountedfor 61% of biomassdelivered, and avian prey items accountedfor 39% of prey biomass. Sciuridsand leporidsaccounted for 70% of the identifiedprey. Red squirrel(Tamiasciurus hudsonicus), easternchipmunk (Tamiasstriatus), and snowshoehare (Lepusamericanus) were the dominant mammals identified in the diet, while AmericanCrow (C0rvusbrachyrhynchos) and Ruffed Grouse(Bonasa umbellus) were the dominant avian prey delivered to nests.On average,breeding goshawksdelivered 2.12 prey items/d, and each deliveryaveraged 275 g for a total of 551 g delivered/d. However,daily (P < 0.001) and hourly (P = 0.01) deliveryrates varied among nests. Delivery rates (P = 0.01) and biomassdelivered (P = 0.038) increasedwith brood size.Diversity and equitabilityof prey usedwas similar among nests and was low throughout the studyarea, most likely due to the dominance of red squirrel in the diet. KEYWORDS: NorthernGoshawk; Accipiter gentilis; dieP,, Minnesota; prey diversity; red squirrel; Tamiasciurus hudsonicus. DIETA DE ACCIPITER GENTILIS EN MINNESOTA: UN AN•LISIS BASADO EN SISTEMAS DE GRA- BACION EN VIDEO RESUMEN.--Empleamossistemas de grabaci6n en video para recolectar informaci6n sobre la dieta de Acdpitergentilis en 13 nidos ubicadosen Minnesotadurante las temporadasreproductivas de 2000, 2001 y 2002. Obtuvimos4871 hr de grabaci0n,a partir de las cualesregistramos 652 entregasde presas.La mayoffa de las presasentregadas que identificamosfueron mamiferos (62%), mientrasque las aves (38%) representaronuna proporci6nmenor de la dieta.Los mamiferos y lasaves representaron el 61% y el 39% de la biomasaentregada, respectivamente. Los scifiridosy lep6ridosrepresentaron el 70% de las presasidentificadas. Los mamiferos predominantesidentificados en la dieta fueron Tamiasciurus hudsonicus,Tamias striatus y Lepusamericanus, mientras que las aves11evadas a los nidos predominante- mente fueron Corvusbrachyrhynchos y Bonasa umbellus. En promedio, los individuosnidificantes entre- garon 2.12 presas/d,y cada entrega tuvo un promedio de 275 g, para un total de 551 g entregados/d. Sin embargo,las tasasdiarias (P < 0.001) y horarias (P = 0.01) de entrega de presasvariaron entre nidos. Las tasasde entrega (P = 0.01) y la biomasaentregada (P = 0.038) incrementaroncon el tamafio de la nidada. La diversidady equitabilidadde las presasconsumidas fueron similaresentre nidosy bajas a travfs del firea de estudio,probablemente debido a la dominanciade T. hudsonicusen la diem. [Traducci6ndel equipo editorial] Presentaddress and correspondingauthor: P.O. Box 1363,Meeker, CO 81641U.S.A.; email: [email protected] 264 SEPTEMBER 2005 BIOLOGY 265 The Northern Goshawk (Accipitergentilis) is a eastern Minnesota (46ø50'N, 92ø11'W) as describedby large, forest-dwelling raptor generally associated Boal et al. (2001) and Roberson (2001; Fig. 1). The study area elevation ranged from ca. 200-400 m. Mean summer with mature deciduous, coniferous, or mixed for- and winter temperatureswere 18øC and - 11øC,respec- ests (e.g., Bright-Smith and Mannan 1994, Siders tively, and maximum and minimum temperature records and Kennedy 1996, Beier and Drennan 1997, for the region were 40øCand -46øC, respectively(Daniel Squiresand Reynolds1997). Goshawkresearch in and Sullivan1981). Annual precipitationaveraged 60-70 cm. The studyarea was dominated by pine, mixed-hard- North America has been conducted primarily in wood, boreal, and second-growthforests with wetland the western half of the continent (Boal et al. 2003). community typesinterspersed among forest stands(Tes- Consequently,there is little published literature ter 1995). describingecology of the speciesin the Western GoshawkNests. Nests included in this studywere con- Great Lakes Region (WGLR) of North America, sidered as sampling units and were selected from all known occupiednests in the studyarea (Boal et al. 2001). where it is currently listed as a Migratory Non- With the exception of one nest, where few data were col- game Bird of Management Concern by the U.S. lected during 2000, diet information wasnot collectedat Fish and Wildlife Service (Region 3) and as a sen- any nest for more than one breeding season.Nests were sitivespecies by the U.S. ForestService (Region 9) selected randomly within the constraintsof accessibility due to lossof habitat (Reynoldset al. 1992). and to include different land ownerships.Thus, our sam- ple is not truly random and may not be representativeof Depending on region, season,and availability, the goshawkpopulation of our study area. However, to goshawkscapture a wide variety of prey and are examine the applicabilityof our diet data to the goshawk consideredprey generalists(Squires and Reynolds population as a whole, we examined prey diversity and 1997,Squires and Kennedy2005). Althoughbreed- overlap among nests. High overlap and low diversity would suggestprey use wassimilar among goshawkpairs ing-seasondiet composition has been studied for and that our data were representativeof the population many populations (e.g., Meng 1959, Grzybowski in general. and Eaton 1976, Boal and Mannan 1994, Younk Video Recording. We used VHS (Model SL 800, Se- and Bechard1994, Lewis2001), site-specificstudies curity Labs©, Noblesville,IN U.S.A.) and 8-mm video re- of diet are necessaryfor developingmanagement cording systems(Sony © Model M-350, Fuhrman Diversi- fied, Inc., Seabrook, TX U.S.A.) with color or strategiesfor goshawkpopulations at regional and black-and-white cameras (Model CCM-660W, Clover Elec- local levels (e.g., Reynoldset al. 1992). A number tronics©, Los Alamitos, CA U.S.A.). Cameras were m- of records exist of prey items collected opportu- stalled on nest trees within 0.6 m of the nest or, for cam- nisticallyat goshawknests in the WGLR (Eng and eras with zoom lenses, on an adjacent tree up to 9 m from the nest. Video recorders were placed in weather- Gullion 1962, Apfelbaum and Haney 1984, Martell proof casesca. 30 m from the base of each camera tree. and Dick 1996), but these reports are anecdotal Coaxial-video cables were used to convey power to and and provide a prey list rather than a quantitative transmit images from the cameras. Recorders were pro- assessmentof food habits (Roberson et al. 2003). grammed to record from 0530-2100 H (15.5 hr of foot- Methods used in goshawkfood habits research age) at the 48-hr (1.3 frames/sec) or the 72-hr (0 8 frames/sec) setting to optimize the amount of tape used have included indirect (i.e., identification of prey per samplingsession and battery life. We replacedtapes remains or contents of regurgitated pellets) and and batteries every 3-4 d. direct observationsof prey deliveries to nests Prey Identification. To identify prey delivered to nests, (Meng 1959, Grzybowskiand Eaton 1976, Bosa- we reviewedvideo footage until a prey deliveryoccurred, kowski and Smith 1992, Boal and Mannan 1994). then advanced frame by frame and freeze-framed to fa- cilitate prey identification.We identified avian and mam- Indirect methods of assessingraptor diet can lead malian prey by morphologicalfeatures and developed a to biased results (e.g., Bielefeldt et al. 1992), list of prey speciesdelivered by goshawksto all nests(Ta- whereasdirect methods should provide the least- ble 1). Goshawksmay cache prey and retrieve cached biasedresults (Collopy 1983, Marti 1987,Boal and prey items (Boal and Mannan 1994), which could bias estimatesof deliveryrates and proportional useof species Mannan 1994). During the breeding seasonsof in the diets. We attempted to identify cached prey on 2000-02, we usedvideography as a modified meth- basisof a successive,iterative processthat included com- od of direct observationof prey deliveriesto ex- paring prey itemsusing flesh color,pelage or feather con- amine diet of Northern Goshawks in northern dition, and time of delivery from review of video footage, Minnesota. and then remove those items thought to be cached from analysis. METHODS Age and Biomass Estimation. We assignedavian prey to age categories(e.g., adult, juvenile, or nestling) based StudyArea. The studyarea waslocated in the Lauren- on plumage (e.g., feathers and down) and amount of tian Mixed-Forest Province of north-central and north- sheathing on flight feathers (Reynolds and Meslow 266 SMITHERSET AL. VOL.39, No. 3 •V•AO ßLSP eDEE4 _ 'DIll2. MCD3 - 'PMT,, 3 2/ 1 . 1 WRI• STE eSH2 . ß Ne• Loc•i on • 25 50 i00 150 200 Figure 1. Study area and distribution of Northern Goshawknests in Minnesotawhere food habitsinformation was collectedduring the 2000-02breeding seasons. The three-letterdesignations indicate individual nests. Breeding seasondiet informationcollected