J Raptor Res. 39 (4) :439-444 O 2005 The Raptor Research Foundation, Inc.

RED-TATLED DIETARY OVERLAP WITH NORTHERN GOSHAW ON THE KAIBAB PLATEAU, ARIZONA

ANGELA E. GAT TO^ Bureau of Land Management, Lake Havasu, 2610 Sweetwater Avenue, Lake Havasu City, AZ 86406 USA.

TERYLG. GRUBB Rocky Mountain Research Station, 2500 South Pine Knoll Drive, Flagstag AZ 86001 U.S.A.

CAROL L. CHAMBERS School of Forestry, College of Ecosystem Science and Management, Northern Arixona University, I? 0. Box 15018, Flagsta8 AZ 86001 U.S.A.

~~STRACT.-~~determined food habits of Red-tailed ( jamaicensis) for comparison with published information for Northern Goshawks (Accipiter gentilis) to evaluate potential competition on the Kaibab Plateau, Arizona. We collected prey remains and pellets from 42 Red-tailed Hawk nests at the end of the nesting season between August-October 1998-2001, and opportunistically from below nest trees during site visits, May-July 2000-01. We identified 478 prey items, including 17 mammal, 7 , and 2 reptile species. Prey species frequency did not vary among years (P = 0.3), across habitat types (P = O.8), or by collection technique (P = 0.4). Annual food niche breadth for Red-tailed Hawks averaged 0.57. Published mean niche breadth for Northern Goshawks was 0.32, supporting that Red- tailed Hawks were feeding generalists, while Northern Goshawks were more specialized. However, 48% of Red-tailed Hawk diet on the Kaibab Plateau consisted of species comprising a major portion of the documented diet of Northern Goshawks, including Nuttall's cottontail (Sylvilaps nuttallii), golden-man- tled ground squirrel (Spermophilus lateralis lateralis), rock squirrel (S. variegates grammums), and Northern Flicker ( auratus). Because raptor communities with high dietary overlap and lack of prey par- titioning show food-limited nesting success, greater agnonistic behavior, and territoriality, Red-tailed Hawks could be negatively affecting Northern Goshawks on the Kaibab Plateau.

KEY WORDS: Red-tailed Hawk; Buteo jamaicensis; ; Accipiter gentilis; competition; diet; food habits, food niche breadth; foraging.

SOBREWAM3ENTO DE LA DIETA DE BUTEO JAMAICENSISY ACCIPITER GENTILISEN LA MESETA DE KAIBAB, ARIZONA RESUMEN.-Determinamos la dieta de Buteo jamaicensis en la planicie de Kaibab, Arizona y la compara- mos con informaci6n publicada sobre Accipiter gentilis con la finalidad de evaluar la posible existencia de competencia entre estas dos especies. Recolectamos restos de presas y egagrbpilas de 42 nidos de B. jamaicensis a1 final de la temporada reproductiva entre agosto y octubre de 1998-2001, y de forma oportunista debajo de iirboles de anidacibn durante visitas a la zona de estudio realizadas entre mayo y julio de 2000 y 2001. Identificamos 478 tipos de presas, incluyendo 17 especies de mamiferos, 7 de aves y 2 de reptiles. La frecuencia de las especies de presa no vari6 entre aiios (P = 0.3), tipos de hiibitat (P = 0.8) y tecnicas de colecta (P = 0.4). La amplitud anual del nicho alimentario de B. jamaicensis promedici 0.5'7. Registros publicados indican que la amplitud del nicho aIimentario de A. gentilis es 0.32, lo cual sugiere que B. jamaicensis es una especie generalista, mientras que A. gentilis es m6s especializada. Sin embargo, el 48% de la dieta de B. jamaicensis en la planicie de Kaibab consiste de especies que forman la mayor parre de la dieta documentada para A. gentilis, entre las que se encuentran Sylvilagus nuttallii, Spermophilus lateralis lateralis, S. variegates gramm~~msy Colaptes auratus. Debido a que las comunidades de rapaces que presentan un alto grado de superposici6n en su dieta y en las que no hay repartici6n de presas rnuestran un bajo 6xito de anidaci6n debido a falta de alimento,

Email address: [email protected] un nlqor comportarniento an~ig6nicoJ. mayor territorialidad, B. jamaicensis podria estar afectando negativamente a -4. gmtilis en la planicie de bibab. [TraducciBn del equipo editorial]

Fire suppression, timber harvesting, and live- esized that Red-tailed Hawks and Northern Gos- stock grazing over the past 100 JT have caused hawks, because of their close proximity and use of changes in southwestern forests, including those similar nesting and foraging habitat, could also be on the Kaibab Plateau in northwestern Arizona using the same prey species, creating the potential (e.g., Weaver 1951, Cooper 1960, Covington and for competition. Moore 1994). Decreasing the quality and quantiq STUDY-4 of climax forest can favor habitat generalists (Carey The Kaibab Plateau, on the North Rim of the Grand 1984) such as Red-tailed Hawks (Buteo jamaicensis), Canyon, is located in northwestern Arizona within the which may out compete species dependent on late Kaibab National Forest. The plateau encompasses 2980 successional forests, such as Northern Goshawks km%bove 1830 m elevation. Our study area was confined (Accipitmgentilis; Andersen et al. 2003). Creation of to 1732 km2 above the 2075 m contour to be consistent with concurrent long-term Northern Goshawk research 259-ha buffer zones on the plateau by the Kaibab on the plateau (Reynolds et al. 1994). Goshawk and Red- National Forest, although maintaining the quality tailed Hawk nest sites are interspersed throughout this of old-growth for nesting sites, does not ensure suf- study area. Vegetation on the plateau consists of ponde- ficient quantity or quality of old-growth forest be- rosa pine (Pinus ponderosa) forest between 2075-2500 m; mixed-conifer forest (ponderosa pine, Douglas-fir [Psm- yond these protected areas (Reynolds et al. 1992). dotsuga menziesizl , white fir [Abies concolor], blue spruce Red-tailed Hawks often nest in abandoned North- [Picea pungens] , and quaking aspen [Pqbulus tremuloides] ) ern Goshawk nests after timber harvesting (Crock- between 2500-2650 m; and Engelmann spruce-subalpine er-Bedford 1990). Beyond old-growth-managed fir (Picea engelmannii-Abies lasiocarpa) forest between 2650-2800 m (Rasmussen 1941, White and Vankat 1993). buffer zones, open areas may favor the hunting style, and thus, increase foraging habitat of Red- METHODS tailed Hawks more than Northern Goshawks. Ex- Prey Remains and Pellet Identification. We collected ploring the potential relationship between these prey remains and reg-urgitated pellets from all known oc- two species facilitates a better understanding of cupied Red-tailed Hawk nests (N = 21, 24, 32, 11 for 1998-2001, respectively), from below and in the nest, at how upper level avian predators have adjusted to the end of the nesting season, August-October 1998- human-caused alterations in southwestern forest 2001, and opportunistically during the nesting season, ecosystems. Further, this understanding could aid May-July 2000-01. Collections from each nest were sep- resource managers in maintaining a viable North- arated by >3 d, and end-of-season samples were collected >30 d after previous collections. We treated each collec- ern Goshawk population on the Kaibab Plateau. tion of pellets and prey remains from an occupied nest Competition occurs when use or defense of a as one sample for that nest site. We assumed prey species resource by one individual reduces the availability identified in each sample had been consumed since the of that resource to other individuals (Gill 1990). last sample and represented new prey. To identify prey, we separated samples into bones/frag- Interspecific competition occurs when individuals ments, feathers, and hair. Bones and feathers were iden- of coexisting species require a resource that is in tified to the lowest taxon possible, through comparison limited supply relative to their needs such that sur- with S. Bayard's reference prey collection stored at the vival or reproduction of at least one species is de- Rocky Mountain Research Station, Fort Collins, CO. We identified hairs using keys (Williams 1938, Stains 1958, creased (Ricklefs and Schluter 1993). On the Kai- Moore et al. 1974) and by comparing hairs directly with bab Plateau, Red-tailed Hawks and Northern samples from the Northern Arizona University, Depart- Goshawks, based on their proximity and similar ment of Biological Sciences' collection and the private habitat use, could be competing for nest sites, for- collection of H.E. Graham (Flagstaff, AZ U.S.A.). The characteristics we considered included color banding, aging areas, or prey, despite potential partitioning shape, presence of a hair shield, ancl configuration of a resulting from morphological and behavioral dif- medulla if present (Moore et al. 19'74). ferences between species (Ballam 1984, Squires Determining diet via indirect means requires cautious and Reynolds 1997). In this study, we focused on interpretation because of inherent biases associated with determining the diet of Red-tailed Hawks for com- each method (Lewis et al. 2004); however, identification of a prey species in pellets and prey remains is an abso- parison with published information on Northern lute indication of presence. Therefore, we recorded one Goshawks to evaluate potential overlap. We hypoth- occurrence whenever a species was found in a sample. Itre then pooled samples across years for each nest and list based on pooled prey species present across all oc- summed the number of times a prey species occurred. cupied Red-tailed Hawk nests by year. This yielded a sep- This provided a conservative estimate of the relative im- arate food niche breadth measure for each of the four portance of prey species consumed by Red-tailed Hawks study years. We also calculated a 95% confidence interval at each nest. We also calculated the total number and for Red-tailed Hawk niche breadth on the Kaibab Plateau percent of nests at which each prey species occurred. for comparison with calculated niche breadths for North- Prey Species Dissimilarity. Dyer (1978) developed a lin- ern Goshawks from other studies. ear statistical analysis for comparing species dissimilarity that can be used in conjunction with any species dissim- ilarity index and is designed for data sets that involve both muItiple species and multiple environmental vari- Pellet and Prey Remain Analysis. Considering ables. Total species dissimilarity is divided into compo- nents with one component being assigned to each envi- each visit's collection of pellets and prey remains ronmental variable or interaction of environmental as one discreet sample, we obtained 140 prey sam- variables: ples (N = 80 end of nesting season, 1998-2001; N = 60 opportunistic during nesting season, 2000- 01), consisting of 1-10 collections from 42 nests (2 where Dii is the (dis)similarity between observations i and jJ 6$') is a known function of i and j which corresponds = 3.3). We identified 478 prey items, including at to the 2~ environmental variable or interaction, P1 is an least 17 mammal, seven bird, and two reptile spe- unknown parameter which represents the contribution cies (Table 1). The number of species at any one of the z?' environmental variable or interaction to the nest site ranged from 1-6. For all 4 yr combined, total dissimilarity, and eij is an error term with an ex- pected value of 0. mammals represented 72% of Red-tailed Hawk We used this linear model with the Jaccard dissimilarity diet and represented 27% (Table I). Only index (Krebs 1998) to estimate whether the species iden- two reptile species were identified; they did not tified in prey remains and pellets of Red-tailed Hawks contribute greatly to tlie overall diet (

Table 1. Frequency of prey species in 140 samples (478 identified prey items) of pellets and prey remaim collected from 42 Red-tailed Hawk nests, May-October 1998-2001, on the Kaibab Plateau, Arizona, U.S.A.

Nuttall's cottontail (SylviZaps nuttalliq Golden-mantled ground squirrel (Spermuphilus lateralis) Rock squirrel (Spermophilus vanegatus grammurus) Kaibab squirrel (Sciurus aberti kaibabmis) Chipmunk (Eutamias sp.) Northern pocket gopher (Thomomys talpoides) Long-tailed vole (Microtus longicaudus) Red squirrel ( Tamiasciurus hudsonicus) Mouse (Peromysms sp.) Shrew (Sorex sp.) Black-tailedjackrabbit (Lepus califmicus) Long-tailed weasel (Mustelaj-enata arizonensi) Mule deer ( Odocoihs hemionus) Plains pocket mouse (PerognathusJlavescens) Porcupine (Erethizon dorsatum) Ringtail (Bmsariscus astutus) Coyote ( Canis latrans) Northern Flicker ( Colaptes auratus) Steller's Jay ( Cyanon'tta stelleri) Clark's Nutcracker (Nucifaga columbiana) Unknown bird Common Raven ( Corvus corax) Western Bluebird (Sialia mexicana) Hairy (Picoides villosus) Common Nighthawk ( Chordeiles minor) Unknown snake Mountain short horned lizard" (Phrynosoma douglassi) " Mountain short horned lizard was positively identified as a prey species during observations, but was not found in the prey collec- tions. Table 2. Food niche breadth of nesting Northern Goshawks calculated from prey remains collected in Arizona and other western states.

NUMBEROF FOODNICHE LOC~TION NESTS BREADTH SOURCE Arizona 20 0.29 Boa1 and Mannan (1994) California 114 0.41 Bloom et al. (1986) New Mexico 8 0.36 Kennedy (1991) Oregon 4 0.38 Reynolds and Meslow (1984)

provide insight into any spatial or temporal parti- Our results on the Kaibab Plateau were consis- tioning of prey by these two species. However, our tent with Red-tailed Hawks being feeding general- research clearly shows Red-tailed Hawks are prey- ists and preying primarily upon rabbits (SyZzdagus ing upon many of the same species utilized by spp.), black-tailed jackrabbits (Lepus californicus) , Northern Goshawks. Thus, we believe the potential and ground squirrels (Spermophilus spp; Preston for competition exists. We further hypothesize that and Beane 1993); however, Northern Flickers and the more generalist nature of Red-tailed Hawk diet Steller's Jays were also frequent in this diet analysis. and nesting habitat, in combination with the de- Previous observation and prey remains analyses for terioration of late-successional forest habitat: and Northern Goshawks on the plateau indicated gos- concurrent creation of openings on the Kaibab hawks also preyed mostly upon rabbits and hares, Plateau, may tend to exacerbate any potential con- tree and ground squirrels, Northern Flickers, and flict. We suggest that additional research should be Steller's Jays (Boal and Mannan 1994, Kaufmann implemented to examine this potential competi- et al. 1994, and Reynolds et al. 1994). Bosakowski tion, better quantify numbers of prey in both spe- and Smith (1992) found that in eastern forests, cies' diets, and determine potential effects on such competition between accipiters and is usu- competition on Northern Goshawk management ally minimized by a difference in prey selection, alternatives. with buteos typically having a higher proportion of mammals in their diet, and accipiters more avian We thank R. Reynolds for first conceiving of this pro- prey- Red-tailed Hawks frequently exhibit switching ject; he, S. Bayard de Volo, S. Salafsky, and his summer crews on the Kaibab Plateau provided field and logistical behavior, which is the capability to utilize which- support. A. Hales, R. Lopez, and T. Rohmer assisted with ever species is most abundant at the time (Steen- data collection. R. Balda and P. Beier offered helpful hof and Kochert 1988). Predators that are gener- comments on study design, as well as the original man- alists often have weak and variable prey uscript. R. King provided invaluable statistical assistance. We also thank D. Andersen and S. Lewis for thorough preferences and will exhibit switching behavior, and constructive reviews of the manuscript. This research while specialists with strong or consistent prefer- was part of the senior author's Master of Science project ences do not (Murdoch 1969). The wider niche at Northern Arizona University, funded by U.S. Depart- breadth of Red-tailed Hawks on the Kaibab Plateau ment of Agriculture Forest Service, Rocky Mountain Re- indicated weaker preferences compared to the nar- search Station. rower niche breadth of Northern Goshawks. Red-tailed Hawks and Northern Goshawks fre- quently occupy similar nesting habitat on the Kai- ANDERSEN,D.E., S. DESTEFANO,M.I. GOLDSTEIN,K. TITUS, bab Plateau. Both species tend to nest in larger C. CROCKER-BEDFORD,JJ. KEANE, R.G. ANTHONY,AND R.N. ROSENFIELD.2003. The status of Northern Gos- trees (2DBH = 68.3-72.5 cm) and mid-slope in hawks in the western United States. Wildlife Society drainages (2slope position = 0.36-0.3'7; LaSorte et Technical Review 04-1. The Wildlife Society, Bethesda, al. 2004). Because they also nest in close proximity MD U.S.A. ((2000 m apart, USDA Forest Service, North Kai- BALLAM,J.M. 1984. The use of soaring by the Red-tailed bab Ranger District unpubl. data), we suggest that Hawk. Auk 101:519-524. they may also be utilizing the same foraging habi- BLOOM,P.H., G.R. STEWART,AND BJ. WAITON.1986. The tat. Unfortunately, pellets and prey remains do not status of the Northern Goshawk in California, 1981- 444 GATTOET AL. VOL. 39, NO. 4

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Princeton University Receivecl 15 Marc11 2004.; accepted 26 March 2005 Press, Princeton, NJ U.S.A. Associate Editor: Clint W. Boa1