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Juvenile Grouse in the Diet of Some Raptors

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Juvenile Grouse in the Diet of Some Raptors j. RaptorRes. 38(3):243-249 ¸ 2004 The Raptor ResearchFoundation, Inc. JUVENILE GROUSE IN THE DIET OF SOME RAPTORS VITALI REIF 1 AND RISTO TORNBERG Departmentof Biology,University of Oulu, P.O. Box3000, FIN-90014 Oulu, Finland KAUKO HUHTALA Tehtaanpuistokatu3, FIN-85310 Sievi,Finland ABSTRACT.--Grouse(e.g., Tetrao)constitute a significant part of the diet for some raptors. Especially grousechicks are important for severalavian predatorsincluding speciesthat normally prey upon small mammals. To evaluate the impact of avian predation on grouse, we need to know which raptors are hunting which speciesand when juvenile grouse suffer from the heaviest predation. Becausegrouse chicks are difficult to identify in prey samples based on morphological characteristics,we made an attempt to addressthis problem by measuringthe humerussize of grousechicks found in prey remains of Northern Goshawks(Accipiter gentilis), Eurasian Sparrowhawks(Accipiter nisus), Common Buzzards (Buteobuteo), and Northern Harriers (Circuscyaneus). Then, we plotted humerus sizesfrom prey remains againstgrowth curvesof chicksof Capercaillie (Tetra0urogallus), Black Grouse (Tetra0tetrix), Willow Grouse (Lagopuslagopus), and Hazel Grouse (Bonasabonasia). We found that the size of grousechicks in the diet wasbest explained by the massof raptors, but not the fledging date of their young. Ranges of grouse size in the raptor diets were overlapping,suggesting that all four raptor specieshunt grouse chicks at about the same dates. Pressureof the avian predator assemblageon juvenile grouse does not appear to be uniform; smallergrouse species sufibr from heavierpredation during a longer period than larger grouse. KEYWORDS:Northern Goshawk; Accipiter gentilis; Eurasian Sparrowhawk; Accipiter nisus; Common Buzzard; Buteo buteo; NorthernHarrier, Circus cyaneus;grouse,, predation; Finland. LAGOPODOS JUVENILES EN LA DIETA DE ALGUNAS AVES RAPACES R•SUMEN.--E1lagopodo (Tetrao)constituye una parte significativaen la dieta de algunasrapaces, en especial los pichones del lagopodo son importantes para muchos depredadores de aves incluyendo especiesque normalmente se alimentan de pequefiosmamiferos. Para evaluar el impacto de la depre- daci6n sobre el lagopodo,necesitamos saber cualesrapaces esfftn cazado a cualesespecies y cuando los juveniles del lagopodoseven afectadospor la mayor depredaci6n.Debido a que los pichonesde la- gopodo son dif/cilesde identificar en los restosde presasbasados en caracteristicasmorfo16gicas, hici- mos un ensayopara resolverel problema mediante la medici6n del tamafio del numero de los pichones de lagopodoencontrados en los restosdel azor (Accipitergentills), el gavil/tneuroasi/ttico (Accipiternisus), el ratonero comfin (Buteobuteo), y el aguilucho nortefio (Circuscyaneus). Posteriormente ploteamos el tamafio del numero de los restos de presasversus las curvas de crecimiento de, los pichones de Tetrao urogallus,Tetrao tetrix, Lagopus lagopus, y Bonasabonasia. Encontramos que el tamafio de los pichonesde lagopodo en la dieta se entiende mejor a partir de la masa de las rapacesy por la fecha del emplu- mamiento de los juveniles. Los rangos del tamafio del lagopodo en la dieta de las rapacesse sobre pusieron, lo cual sugiereque las cuatro especiesde rapacescazan pichones aproximadamenteen las mismasfechas. La presi6n del ensamblajede los depredadoresaviares en losjuveniles de lagopodo, es aparentementeuniforme; las especiesmenores de lagopodosufren una mayor depredaci6ndurante un periodo mas largo que las especiesde lagopodo de mayor tamafio. [Traducci6n de Cfisar Mfirquez] The tetraonidsconstitute a significantportion in (e.g., Northern Goshawk [Accipitergentilis], Gyrfal~ the diet of some birds of prey. For some of them con [Falcorusticolus], Golden Eagle [Aquila chrysae- tos]) small game are commonly-taken prey (Huht- ala et al. 1996, Tornberg 1997, Sulkavaet al. 1998, E-mail address: [email protected] Nielsen 1999). For others, such as those that feed 243 244 REIF ET AL. VOL. 38, NO. 3 mainly on small mammals (e.g., Common Buzzard time period this occurred. We compared the ap- [Buteo buteo], Northern Harrier [Circus cyaneus], pearance and the growth of grouse chicks in the EurasianEagle-Owl [Bubo bubo] , Ural Owl [ Strixur- field with the size of juvenile grousein the diet of alensis]), the tetraonids may become an alternative thesefour avian predatorsduring the late nesting prey in poor vole (Microtusspp.) years (Angelstam period in July,when mostprey remainsaccumulate et al. 1984, KorpimSki et al. 1990, Redpath and in raptor nests. Thirgood 1999, Reif et al. 2001), when they can be MATERIAL AND METHODS an important sourceof food to support breeding. Depending on their size and hunting capacity, The prey material was collected in Finland in the fol- lowing areas:Oulu (goshawk,sparrowhawk, harrier), Ku- raptors kill both juvenile and adult grouse (e.g., P. usamo (goshawk), and Central Ostrobothnia (harrier, Sulkava1964, S. Sulkava1964, Tornberg 1997, Sul- buzzard). Most of the material was collected in Oulu kava et al. 1998, Reif et al. 2001). There are few (65ø00'N, 25ø30'E) and Central Ostrobothnia (64ø00'N, quantitative studiesregarding raptor predation on 24ø00'E). These areas represent coastal lowland with many small•ivers and lakes.Roughly 67% of the area is grouse chicks (S. Sulkava 1964, H6glund 1964, coveredby mosaicof spruce (Piceaabies) and pine forests Gronnesby and Nygfird 2000, Thirgood et al. (Pinus sylvestris)mixed with birch (Betula pubescens). 2000), in part because the latter are difficult to About half of the forested area includes bogs, a large Identify to the specieslevel in prey remains. Five number of them being drained. Cultivated areaswith set- grouse species (Capercaillie [Tetrao urogallus], tlements are situated mainly in river valleys.Kuusamo (66ø00'N,29ø00'E) is a rolling highland area, the highest Black Grouse [Tetrao tetrix], Hazel Grouse [Bonasa hilltops reaching 450 m above the sea level. Area is also bonasial, Willow Grouse [Lagopuslagopus], and characterizedby large lakes.Coniferous forests comprise Ptarmigan [Lagopusmutus]) found in Fennoscan- more than 70% of the area, with fewer bogs than the d•a come in a variety of sizesfrom 350 g (Hazel previous area. Due to lower population density,the area cultivated is less in Kuusamo than in the Oulu-Ostro- Grouse) to 4000 g (Capercaillie cock). The growth bothnia area. rate of their chicksvaries correspondingly (e.g., Se- The prey remains analyzedwere collected during dif- menov-Tian-Shansky1959, Lindfin 1981, Klaus et ferent years between 1966-2003 (Table 1). Prey •emains al. 1990). Large raptor speciesmight prey relatively and pellets were collected from the nestsafter fledging little on grouse chicks generally (e.g., Golden Ea- once a year. Prey remains for goshawkswere collected alsofrom the area surrounding the nest during the post- gle; Sulkava et al. 1998) or mainly on chicks of fledging period ('after the young leave the nest, but stay large grouse species(e.g., Northern Goshawk;S. in the nesting area). The total number of collected sam- Sulkava1964, Tornberg 2001). However,the latter ples varied among the speciesas follows: goshawks,32 soon escapepredation of small raptor species,such (and 14 from nestareas); buzzards, 74; sparrowhawks,14; harriers,7. We assumedthat most collectedprey remains as Eurasian Sparrowhawks(Accipiter nisus) and the were accumulatedin the nests shortly before fledging, Northern Harriers. becausefemales often clean their nestswhen they rear To assessthe predation impact of the whole small chicks (Dement'ev and Gladkov 1951, pets. ob- communityof raptorson different grousespecies, serv.). we need to know the variation in the sizeof grouse To determine the size of grouse chicks in the diet we measured the length of hu•neri. The humeri of juvenile chicks in the diet of different avian predators. In grousewere distinguishedfrom the adults' by the degree our studyarea, goshawks,sparrowhawks, buzzards, of ossification.There are four grouse speciespresent in and harriers hunt small game on a regular basis. the study area: Willow Grouse, Black Grouse,Capercail- The aim of this studywas to determine how these lie, and Hazel Grouse. However, the bones of grouse chickswere not sortedby speciesbecause they could not predators partition this common food resource-- be identified with certainty. When two opposite humeri grouse chicks. We used a large existing prey-re- of the same size were found, we counted this material as mains collection of the Zoological Museum, Uni- one a•fitnal. versityof Oulu, and published data. We expected We compared the size of grouse chicks (expressedin that the selectionof juvenile grouseby these pred- humerus length) found in the diet of each raptor at the end of its nestling period (and post-tledgingperiod for ators would depend: (1) on the size of the raptor the goshawk)with sizesof grousechicks of different spe- that generallyaccords with the sizeof the prey and, cies derived from growth curves (Fig. 1). Given the scar- (2) because of rapid growth of grouse chicks and city of grouse remains in many nests, we were not able temporal variations in breeding times of the rap- to consider each nest as a unit for analysis;therefore, each grouse chick was consideredan experimental unit tors, on the date of capture. In addition, we at- instead.Fledging dates of raptorswere calculatedby add- tempted to assesswhich speciesof grouseare most ing the length of the nestling periods (Cramp and Sim- vulnerable to raptor predation and
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