J. RaptorRes. 32(2):111-115 ¸ 1998 The Raptor ResearchFoundation, Inc. FOOD HABITS AND HUNTING RANGES OF SHORT-EARED OWLS (ASIO FLAMMEUS) IN AGRICULTURAL LANDSCAPES OF SOUTHERN CHILE DAVID R. MARTINEZ, RICARDO A. FIGUEROA AND CARMEN L. OCAMPO Laboratoriode Ecologia, Departamento de CienciasBeisicas, Universidad de LosLagos, Casilla 933, Osorno,Chile FABIANM. JAKSIC Departamentode Ecologia, Pontificia Universidad Cattlica de Chile,Casilla 114-D, Santiago,Chile ABSTR•CT.--The diet of the Short-eared Owl (Asioflammeus) was quantified by analyzing 400 pellets collected in two agricultural landscapesof southern Chile (Osorno and Chahuilco). Diet composition fluctuatedseasonally and included severalspecies of small mammals,birds, and insects.Almost 80% of the annual biomassconsumed was from two rodent species(Akodon olivaceus and Rattusnorvegicus) and of a bird (Vanelluschilensis). No differencesin the compositionof the winter dietswere detectedbetween the two studysites, and the latter were similar in landscapestructure and use by humans.Also, the size of hunting ranges used by Short-earedOwls was similar between the two sites (ca. 250 ha), with a prevalence of landscape elements such as meadows,wetlands, and agricultural fields. Nevertheless, Short-earedOwls concentrated their hunting activity in areas with little human disturbance,such as vegetation fringes along roadsides,ungrazed meadows,and untilled lands. Even though Short-eared Owls perched on the ground, they also used postsalong roads and between properties as perches. Although suboptimalor marginal habitat for most other raptors, these human-dominatedlandscapes appeared to be valuable for the survivaland persistenceof Short-earedOwl populations, as long as their food and shelter remained unaffected. KEYWORDS: Short-earedOwl; Asio flammeus;prey selection; hunting range;, Chile;, temperate agroecosystems. H/tbitosalimentarios y/tmbitos de cazade nucos (Asioflammeus)en agroecosistemasdel sur de Chile R•SUMEN.--Secuantific6 la dieta del nuco (Asioflammeus)analizando 400 egagrtpilas recolectadasen dos agroecosistemasdel sur de Chile (Osorno y Chahuilco). La composicitn de la dieta fluctu6 esta- cionalmentee incluy6 variasespecies de mamiferos,aves e insectos.Sin embargo, casi el 80% de la biomasatotal consumidaestuvo constituida por s61odos especiesde roedores (Akodonolivaceus y Rattus norvegicus)y de un ave (Vanelluschilensis). No existieron diferencias en la composicitn de la dieta invernal entre los dos sitiosde estudio,y ambosagroecosistemas fueron similaresen cuanto a estructura y a uso por humanos.De igual modo, el tamafio estimadodel /trea de caza utilizada por los nucosfue similar entre los dos sitios (ca. 250 ha), predominando en ambos elementos del paisaje tales como praderas,humedales y cultivos.Sin embargo,los nucosconcentraron su actividadde cazaen/treas con nula o escasaintervencitn humana, talescomo franjasvegetadas a orillas de camino,juncales, praderas no pastoreadasy terrenos baldios. Si bien los nucos se posabanen el suelo, tambi•n utilizaban como perchaslos postesde cercosa orillas de caminosy los existentesentre predios.Concluimos que estos ambientes antrtpicos, inadecuadospara muchas rapaces,permitirian la subsistenciay residencia de parejasde nucos,siempre y cuando suspresas y sitiosde crianza no fuesen afectados. [Traduccitn Autores] The Short-eared Owl (Asioflammeus) occurs on (Glade 1988). Populationsare declining in most of all continentsexcept Australia and Antarctica.De- the country but are increasing in southernmost spite being extensively distributed throughout Chile (JaksicandJim•nez 1986). Rau et al. (1992) South America (Clark 1975), literature available on reported the diet of Short-earedOwls in mainland the natural history of this speciesis chiefly anec- Chile basedon 53 pellets and Fuentes et al. (1993) dotal (e.g., Housse 1945, Borrero 1962). In Chile, reported the diet of a population inhabiting Juan Short-eared Owls are regarded as poorly studied Fern•tndez archipelago based on 20 pellets. Thus, 111 112 MARTINEZ ET AL. VOL. 32, NO. 2 this speciesis one of the leastknown owlsin Chile of birds were obtainedfrom Morgadoet al. (1987). We (see reviewin Jaksic1997). estimated the total biomassof each prey speciesin the diet by multiplying the number of individualsin the pel- Short-eared Owls in Chile occupy a variety of lets by the mean body massof each species.We assumed open habitats.Urbanization has had negativeim- that massesof unidentified prey were similarto the mean pactson many Chilean raptors,which are decreas- massof the most closelyrelated identified taxon. We an- ing due to illegal hunting, habitat alteration, and alyzedthe Osorno diet on a seasonaland year-roundba- sis, and compared the results obtained for winter 1995 prey reduction (Jaksic and Jim•nez 1986). Al- with those from the samplefrom Chahuilco with Pianka's though the habitat and prey requirementsof most symmetricalniche overlapindex (Ojk),using programs raptors are generally not met in urban environ- listed in Ka'ebs (1989). ments (Martinez and Jaksic 1996, Petty 1996), the Concomitantlywith pellet collections,we estimatedthe Short-earedOwl is an exception. At least in south- hunting area used by a pair of Short-earedOwls in each of the two study areas. To the best of our knowledge, ern Chile, this owl opportunisticallyuses suburban these were the only pairs present in each site. With a environments such as airports, pasturelands,and hand-heldglobal positioningsystem (GPS) receiver(Gar- golf courses.Here, we report the prey identifiedin min GPS 38), we determined the location of each pre- 400 pellets of Short-eared Owls collected in two viouslyknown roostingplace, asevidenced both by sight- agricultural areas of southern Chile, and provide ing and pellet collection at roosts.To determine the size of the area used by Short-earedOwls, the data were ex- preliminary data on the size and landscapefea- pressedin UTM coordinatesand analyzedusing the min- tures of their hunting grounds. imum convex polygon method (Jenrich and Turner 1969). For Osorno and Chahuilco,we pooled the spatial STUDY AREA AND METHODS data availablefor autumn and winter 1995. The percent- We collected 336 Short-earedOwl pellets on a seasonal age of landscape elements of each range was estimated basis from April (autumn) 1995-February (summer) from 1:30000 aerial photographs,which were resizedto 1996 in an area located on the outskirts of the city of 1:15000 with a scanner.Ranges of Short-earedOwls, re- Osorno (40ø35'S,73ø05'W), in southernChile. The study sizedaccordingly, were overlaidand the area of each cov- area included a golf course, a web of fallow vegetation er categoryincluded was estimatedusing a Placom KP- stripsbetween agricultural fields or along roads,an apple 80N digital planimeter. Statisticalsignificance was set at orchard, a sedge-rush(Carex-Juncus spp.) marsh, pasture- P • 0.05 for all tests unless otherwise stated. lands (some abandoned), an airport, and lawns sur- rounding the main campusof Universidadde Los Lagos. RESULTS AND DISCUSSION From June (winter) 1995-February 1996, we alsocollect- ed 64 Short-eared Owl pellets at Chahuilco (40ø42'S, The 241 whole pellets we measured averaged 73ø09'W),an area that included pasturelands,a marsh, 41.6 _+ 0.072 mm x 21.4 -+ 0.043 mm and had a fallow vegetation along roads and agricultural fields, as mean dry massof 2.8 -+ 0.080 g (2 -+ SE). All three well as berry farms. The climate of thesetwo studyareas measurementswere slightly lower than those re- •s within the oceanic region with mediterranean influ- ence of di Castri(1975), whichis characterizedby heavy ported by Holt et al. (1987) for Short-earedOwl rainfall (200-300 cm yearly), mostly during winter and pellets in North America. decreasing in summer. The 336 pellets from Osorno yielded 812 prey Only pellets with identifiable prey remains were con- items (Table 1), of which small mammals were nu- s•dered.From the Osorno sample (336 pellets),we mea- sured and weighed 241 intact pellets. We identified and merically the most frequent, followed by insects quantified most vertebratesin the pellets on the basisof and birds. Olivaceous field-mice (Akodon olivaceus) skulls,beaks or dentary pairs (Reise 1973), which gave were the most frequent prey in the diet year-round, the highest count. For remains such as hair and feathers, although they were somewhatmore frequent dur- we used referencecollections and quantified theseprey ing winter. This is in close agreement with the au- assuming the smallest possible number of individuals (e.g., hair or feathers of a given specieswere deemed as tumn-winter peak of these vole4ike mice in prai- representing only one individual). For insect identifica- rie-scrublands of southern Chile (Murfia and Gon- uon, we followed Pefia (1986) and quantified theseprey zalez 1986). Long-tailed rice rats (Oryzomyslongi- by counting head capsulesand mandibles.We identified caudatus) were also eaten relatively frequently. prey items to the finest possibletaxonomic categoryin all cases,as recommendedby Marti (1987). Although this speciesis as abundant as the oliva- In Osorno, we evaluated the relative abundance of ceous field-mouse in southern Chile (Meserve et small mammalsfrom May-July1995 (autumn to winter) al. 1991), only its winter consumptionby Short- by live-trappingat an abandonedpasture located inside eared Owls coincided with their autumn-winter the owls' hunting area. The massof most prey species was determined by weighing individuals captured in peak abundance