Diet of Mexican Spotted Owls in Chihuahua and Aguascalientes, Mexico
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j RaptorRes. 31 (4) :376-380 ¸ 1997 The RaptorResearch Foundation, Inc. DIET OF MEXICAN SPOTTED OWLS IN CHIHUAHUA AND AGUASCALIENTES, MEXICO KENDAL E. YOUNO • Departmentof Fishery and WildlijbSdence•; New MexicoState University, Las Cruces;New Mexico88003 U.S.A. PHILLIPJ. ZWANK U.S. GeologicalSurvey Cooperative Research Unit, NewMexico State University, Las Cruce•New Mexico 88003 U.S.A. RAULVALDEZ, JEANNE L. DYe,AND LuIS A. TARANGO2 Departmentof Fishery and WildlifeSciences; New MexicoState University, Las Cruces;New Mexico88003 U.S.A. ABSTRACT.--Weanalyzed pellets of Mexican SpottedOwls (Strix occidentalislucida) collected at roostand nest sites in Chihuahua from 1992-94, and Aguascalientes,Mexico from 1994-95 to determine diet composition.We identified 647 prey itemsfrom 13 owl territoriesin Chihuahuaand four owl territories in Aguascalientes.Vertebrates constituted 64% of all preyitems and 99% of totalprey biomass. Woodrats (Neotomaspp.), mice (Peromyscusspp.) and cottontail rabbits (Sylvilag'usfloridanus)comprised 82% of total prey biomassin Chihuahua and 89% of total prey biomassin Aguascalientes. KEYWORDS: Mexicanspotted owl; Strix occidentalislucida; diet;,Chihuahua; Ag'uascalientes; Mexico. Dieta del Strix occidentalislucida en Chihuahua y Aguascalientes,Mexico RESUMEN.--Nosotrosanalisamos bolitas de Strix ocddentalisludda colectadosen perchasy sitiosde nido en Chihuahua de 1992-94, y Aguascalientes,Mexico de 1994-95 para determinar la composicionde dieta nosotrosidentificamos 647 articulosde presa en 13 territorios de buhos en Chihuahuay cuatro territorios de buhos en Aguascalientes.Vertebrados componieron 64% de los articulosde presay 99% del total biomassde presa. (Neotomaspp.), (Peromyscusspp.) y (Sylvilag'usfloridanus) compnieron 82% del total de biomassde presa en Chihuahuay 895 del total biomassde presa en Aguascalientes. [Traducci6n de Rafil De La Garza,Jr.] Mexican Spotted Owls (Strix occidentalislucida) viewed Mexican Spotted Owl diets and found that are consideredhabitat specialists(Ganey and Dick diet varied by location, but mammals generally 1995, Seamans and Guti6rrez 1995) that inhabit comprise a greater percentage of the diet than do mature mixed conifer and ponderosa pine (Pinus birds, reptiles or arthropods. Within mammalian ponderosa)forests (Ganey and Balda 1994) from species,woodrats (Neotomaspp.) and mice (Pero- southern Colorado and Utah, south to the south- myscusspp.) are taken more commonlythan other ern end of the Mexican Plateau (Ward et al. 1995). species.Although Mexican Spotted Owls are op- The Mexican Spotted Owl was listed as threatened portunistic, the predominance of woodrats and by the U.S. Fishand Wildlife Servicein 1993 (U.S. mice in diets suggeststhey may be more specialized Department of Interior 1993) and the Mexican in their diet than waspreviously thought. The stud- governmentin 1994 (Anonymous1994). ies that Ward and Block (1995) reviewed were con- Mexican Spotted Owls are perch and pounce ducted in the southwestern U.S., except for one predatorsof small-to medium-sizedmammals (Gu- study in Chihuahua, Mexico (Tarango 1994). As ti6rrez et al. 1995). Ward and Block (1995) re- such, the diet of the Mexican Spotted Owl outside the U.S. is largely unknown. Herein, we describe their diet in southwestern Chihuahua and north- • Present address: U.S. Forest Service, PSW, Redwood western Aguascalientes,Mexico. Sciences Lab, Arcata, CA 95521 U.S.A. STUDY AREA AND METHODS '• Presentaddress: Arizona State University,Tempe, AZ Regurgitated pellets were collected opportunistically 85287 U.S.A. from March-September at roost sites in the Sierra Fria 376 DECEMBER 1997 MEXICAN SPOTTED OWL DIET 377 in northwesternAguascalientes during 1994-95, in the Table 1. Social statusand number of prey items iden- Sierra Madre Occidental in southwestern Chihuahua in tified in dietsat Mexican SpottedOwl sitesin Chihuahua, 1992-93 (Tarango 1994) and 1994 (Young 1996). Mexi- Mexico. can Spotted Owl habitat in Chihuahua consistedprimar- ily of pine/oak (Quercusspp.) forests(71%), with Duran- go pine (P. durangensis),Mexican white pine (P. ayachui- 1992--93 1994 te), and Arizona pine (P. arizonica)dominating. Pure pine and mixed conifer represented17% and 12%, respec- # OF # OF tively,of spotted owl habitat in Chihuahua (Young 1996). SOCIAL PREY SOCIAL PREY In Aguascalientes,occupied habitat was primarily pine/ SITE STATUS ITEMS STATUS ITEMS oak forest. Dominant pine specieswere ocote (P. hairier), nut pine (P. cembroides),Chihuahua pine (P. chihuahuana) Agua Fresca Pair 78 Pair 72 and Michoacan pine (P. michoacana).Elevations of roost Arroyo Cuervo Pair 65 Pair 44 sitesranged from 2200-2600 m in Chihuahua, and from Arroyo Enmedio Pair 23 -- 0 2450-2560 m in Aguascalientes. Arroyo Sonorecomchi -- 0 Pair 9 Pellets were separated into bones, arthropod exoskel- Arroyo San Vicente Pair 13 Pair 0 etons, and fur and feathers following procedures out- Arroyo Hojasichi Male 21 Male 141 lined in Marti (1987). Prey remains were identified to Arroyo Hondo Pair 8 Pair 10 the lowesttaxon possibleby comparing skulls,chitinous Arroyo La Laguna -- 0 Pair 5 material, beak sheaths,claws and feathers to keys (An- Chimoto Pair 7 Pair 44 derson1972, Dunning 1993) and referencespecimens at New Mexico State University.We were unable to differ- E1 Yeposo Pair 23 Pair 3 entiate the yearsof origin of pellets collectedin Chihua- Guacayvo -- 0 Pair 10 hua during 1992-93, or in Aguascalientesduring 1994- Guaqueachi -- 0 Male 1 95. Diet was expressedas relative frequency (number of Puerto Pino Hueco -- 0 Pair 8 prey in each taxonomic category divided by the total TOTAL 238 347 number of prey collected) and relative biomass(number of individualsin each taxonomic group divided by aver- age body massfor the taxon). Averagebody masseswere estimated for mammals from Anderson (1972) and Ga- ney (unpubl. rep.), for birds from Dunning (1993) and for arthropods from Ganey (unpubl. rep.). A list of av- biomassin Aguascalientes(63%) than in Chihua- erage body massesused in the analysiswas recorded in hua (38 and 48%). Biomassof mice only differed Young (1996). Due to small sample sizes, no statistical analyseswere conducted. by 6% amongyears and locations.Peromyscid mice comprised95% of total mouseprey. Grasshopper RESULTS mice (Onychomystorridus) were absentfrom diets in Chihuahua in 1994, but were present in all other We identified674 prey itemsfrom MexicanSpot- locations and years,while harvest mice (Reithrodon- ted Owl pellets in Mexico. A total of 238 prey items tomysspp.) were present in diets in Chihuahua in were identified from seven pairs and one single 1992 and 1993 but were not found at other loca- male in 1992 and 1993 in Chihuahua,347 prey tions or during other years. Pocket gophers (Tho- items were identified from 10 pairs and two single malesin 1994 in Chihuahua (Table 1) and 89 prey momysspp.) comprised4-9% of total prey biomass in Chihuahua, but were absent from owl diets in items were identified from four pairs in 1994 and 1995 in Aguascalientes. Aguascalientes. Pellets from Aguascalienteswere collected from Relative biomassof birds in diets was greater in the following spotted owl sites:Cueva Prieta, Los Chihuahua (7 and 11%) than in Aguascalientes Pillares, E1 Carrizal and Barranca Los Laureles. (4%; Table 2). Bats (Eptesicus,Lasiurus, Myotis and Prey items represented 15 mammalian genera, five Pipistrellusspp.), diurnal sciurids( Spermophilusand avian families, one reptile genus and nine arthro- Eutamiasspp.), cotton rats (Sigmodonspp.), voles pod families. Vertebratesconstituted 64% of all (Microtusspp.) and shrews(Sorex spp.) accounted prey itemsand 99% of prey biomass.Woodrats (N. for <4% of the total prey and <3% of total prey albigulaand N. mexicana),mice (Peromyscusspp.) biomassin all locationsand years.Four lizards (Sce- and cottontail rabbits (Sylvilagusfloridanus) com- loporusspp.) were identified in the owl's diet in prised51% of total prey and 82% of prey biomass Chihuahua in 1994, but they comprised <1% of in Chihuahua,and 42% of total prey and 89% of total biomass.Arthropods contributed 31-54% of prey biomassin Aguascalientes(Table 2). prey items in diets,yet they accountedfor <2% of Woodratscomprised a larger proportion of prey total prey biomassin diets. 378 YOUNGET AL. VOL. 31, NO. 4 Table2. Relativepercent frequency and percent biomass (g) of preyin dietsof MexicanSpotted Owls in Chihuahua and Aguascalientes,Mexico. CHIHUAHUA CHIHUAHUA AGUASCALIENTES 1992--93 1994 1994--95 FREQ BIOMASS FREQ BIOMASS FREQ BIOMASS PREY GROUP (N = 238) (13160 g) (N = 347) (25308 g) (N = 89) (3493 g) Woodrats 14.3 47.5 15.0 37.8 13.5 63.2 Neotomaspp. a 14.3 47.5 13.2 33.4 13.5 63.2 N. albigula -- -- 0.6 1.4 -- -- N. mexicana -- -- 1.2 3.0 -- -- Mice 34.8 15.6 28.8 10.4 27.0 15.0 Onychomystorridus 0.8 0.4 -- -- 4.5 3.0 Peromyscusspp. a 31.9 15.1 28.8 10.4 22.5 15.0 Reithrodontomysspp. a 2.1 0.1 .... Rabbits Sylvilagusfloridanus 1.7 11.9 7.2 38.5 1.1 11.2 Gophers 3.8 8.8 2.0 3.6 0.0 0.0 Thomomysspp. a 3.8 8.8 1.4 2.5 -- -- T. umbrinus -- -- 0.6 1.1 -- -- Bats 2.1 0.6 3.5 0.8 2.2 0.3 Eptesicusfuscus 0.4 0.1 .... Lasiurus borealis -- -- 0.3 <0.1 -- -- L. cinereus 1.3 0.5 2.6 0.7 -- -- Myotisspp. a .... 2.2 0.3 M. lucifugus -- -- 0.3 <0.1 -- -- M. velifer -- -- 0.3 <0.1 -- -- Pipistrellushesperus 0.4 <0.1 .... Sciurids 0.8 0.9 0.3 0.6 0.0 0.0 Spermophilusspilosoma -- -- 0.3 0.6 -- -- Eutamiasspp. a 0.8 0.9 .... Cotton Rats Sigmodonspp. a 1.7 2.6 0.3 0.3 1.1 2.4 Voles Microtus mexicanus 1.3 0.5 0.3 0.1 0.0 0.0 ShrewsSorex vagrans 1.3 0.1 0.3 <0.1 0.0 0.0 Birds 7.1 10.8 6.3 7.1 1.1 3.6 Strigiformes 2.5 2.8 2.6 2.2 -- -- Trogonidae 0.4 0.7 .... Aphelocomaultramarina 0.8 1.9 2.3 3.9 1.1 3.6 Cyanocittastelleri 2.5 5.2 0.6 0.9 -- -- Psaltriparusminimus 0.4 <0.1 0.9 <0.1 -- -- Sayornisspp.