j RaptorRes. 31 (4):370-375 ¸ 1997 The Raptor ResearchFoundation, Inc.

SELECTIVE PREDATION ON SCANSORIAL AND ARBOREAL MAMMALS BY RUFOUS-LEGGED OWLS (STRIX RUFIPES) IN SOUTHERN CHILEAN RAINFOREST

DAVID R. MARTINEZ Laboratoriode Ecologia, Departamento de CiendasBtisicas, Universidad de Los Lagos, Casilla 933, Osorno, Chile

F• M. JAKSIC Departamentode Ecologia, Pontificia Universidad Catdlica de Chile,Casilla 114-D, Santiago,Chile

ABSTRACT.--Weestimated the annual mammalian diet of Rufous-leggedOwls (Strix rufipes)in three temperate rainforest sitesin southern Chile (Anticura, Rupanco and San Martin), and mammal abun- dance at San Martin and Anticura. Diets were estimatedby analysisof 894 pelletscollected from 1988- 95. In all sites,arboreal (arborealmouse, Irenomys tarsalis and colocoloopossum, Dromiciops australis) and scansorial(long-tailed mouse, Oryzomyslongicaudatus) mammals occurred significantlymore fre- quently than those mammals with cursorial habits, such as the olivaceousfield mouse (Akodonolivaceus) and long-haired field mouse (A. longipilis).Although the body size and abundance of the olivaceous field mousewere similar to thoseof the long-tailedmouse, the former specieswas seldomtaken by Rufous-leggedOwls, likely becauseit wasassociated with vegetationalfeatures that offer overheadpro- tection.

KEYWORDS: Strix rufipes;Rufous-legged Owl; prey selection; Patagonian Wood Owl; Chile,,temperate forest.

Depradaci6nselectiva sobre mamlferos escansoriales y arbor/colaspor concones(Strix rufipes) en la pluviselvadel sur de Chile RESUMEN.--Estimamosla diem anual del bfiho de bosqueo conc6n (Strixrufipes) en tres localidadesde pluviselvatemplada (Anticura, Rupanco y San Martin) y la relacionamoscon la abundanciaanual de micromamlferospresentes en San Martin y Anticura. La dieta se determin6 mediante el anfilisisde 894 egagr6pilascolectadas entre 1988-95. En los tres sitios,los micromamlferosde hfibitosarbor/colas (Ir- enomystarsalis y Dromidopsaustralis) y escansoriales( Oryzomyslongicaudatus) fueron significativamente milsdepredados que los de hfibitoscursoriales tales como Akodon olivaceus y A. longipilis.A pesarque A. olivaceusexhibi6 abundanciaen terreno y tamafiocorporal similares a los de O. longicaudatus,casi no fue depredadopor losbfihos. Esto puede atribuirsea su asociaci6ncon variablesvegetacionales que le ofrecerian protecci6n desde un plano vertical. [Traducci6n de autores]

The Rufous-legged Owl or Patagonian Wood sidered to be declining in southern Chile because Owl (Strix rufipes)is one of the least known noc- of decreasinghabitat brought about by logging turnal raptors of South America, inhabiting the (Jaksi• and Jimfnez 1986). This was confirmed temperate rainforest region shared by Chile and when Martinez andJaksi• (1996) reportedthe first Argentina (Jaksi•1997). What information there is quantitative information on the relative abundance about this species is either anecdotal (Housse of Rufous-leggedOwls in forest remnantsof south- 1945, Johnson 1967) or consistsof brief accounts ern Chile, and described forest stands used by on taxonomy and distribution (e.g., Humphrey et these owls. al. 1970, Vuilleumier 1985,Jaksi• and Feinsinger The first quantitativereport on the diet of the 1991). Rufous-leggedOwl wasbased on an analysisof 161 From a conservation viewpoint, Glade (1988) pellets (Martinez 1993). Although the study con- listed this speciesas inadequately known. While the cludedthat Rufous-leggedOwls are generalistfeed- Rufous-leggedOwl is relatively common it is con- ers, the most frequent vertebrateprey were arbo-

37O DECEMBER 1997 SELECTrv• PREDATION BY RUFOUS-LEGGED OWLS 371 real and scansorialsmall mammals. Cursorial spe- equipped with one trap for a total of 144 traps covering cies, despite their similar or higher diversity(Mes- an area measuring132 X 132 m (1.7 ha). Each census lastedsix consecutivenights every month for a full year erve and Jaksi• 1991), were poorly represented. The mean annual abundance (recaptures were not con- Here, we report additional data on the diet of Ru- sidered) of each small mammal speciestrapped wasused fous-legged Owls that inhabit three rainforest to estimate the expected frequency of prey consumpuon stands in southern Chile and test the assumed se- by Rufous-leggedOwls (e.g., proportion of mammal spe- cies i in the field with regard to the abundance of all lective predation noted by Martinez (1993). speciestrapped). The actual prey identified in the d•et

STUDY AREAS AND METHODS constituted the observed frequency. For Anticura, we used data reported by Rau et al. (1995), who assessedthe We studied Rufous-leggedOwls in three sites in the abundance of ground-dwelling small mammals with two Valdivian Rainforestregion of southern Chile (sensuVeb- trap lines 2 m apart with 30 stationsset at 12-m intervals len et al. 1983). The first site was the San Martin Exper- and equipped with one medium-sizedSherman trap per imental Forest (39ø38'S, 73ø11'W; 20 m elevation) in the station for a total of 60 traps in an area measuring 348 coastal ranges near the city of Valdivia. This 150-ha site X 4 m (0.14 ha). Rau et al. (1995) simultaneouslyesu- was covered by a second-growthmultistoried forest rem- mated the abundance of tree-dwelling small mammals nant dominated by Aextoxiconpunctatum and Podocarpus with 60 hair-samplingtubes nailed to trees along the trap salignawith substantial Gevuinaavellana and some scat- lines. An empirical estimate of abundance was obtained tered old individualsof emergent Nothofagusdombeyi. The by pooling these data, given that the trapping effort was understory was sparse,and the soil was covered with a the same for ground- and tree-dwelling small mammals. thick layer of mossesand litter. A Chi-squaretest was performed to test the goodness- The second site wasa 180-ha second-growthforest near of-fit of the frequency distributionsof prey in the d•et Lake Rupanco(40ø45'S, 72ø38'W; 150 m elevation)that and in the field (Zar 1984). Nonsignificant values were consistedof an almost pure stand of A. punctatumwith interpreted as if Rufous-leggedOwls used prey in pro- some Laurelia sempervirens,G. avellanaand old N. dombeyi portion to their field abundance, whereas significant de- trees. viations suggestedthat Rufous-leggedOwls "preferred" The third site, Anticura (40ø40'S, 72ø10'W; 600 m el- or "avoided" some small mammal species,thus appear- evation), was an extensive preAndean forest located in ing to select prey. Puyehue National Park, near the city of Osorno. It wasa Becausewith the Chi-square test it is not possible to multistoried old-growth stand with emergent trees (N. determine which prey specieswas individually avoided or dombeyi)35-40 m in height; also present were L. phillipi- preferred, we constructed confidence intervals for each ana, Eucryphiacordifolia and shade-toleranttrees such as prey using the Bonferroni adjustment(Neu et al. 1974, Weinmania trichospermaand A. punctatum.Shrubs were Byerset al. 1984): scarce and the understory was comprised of saplingsof Pi-- Za/2k%/Pi(1 -- Pi)/n --•Pi --• Pi + Za/2k%/Pi( 1 -- Pi)/n, shade-tolerant trees, ferns, a few bromeliads and both bamboo (Chusqueaspp.) and Ribesmagellanicum clumps. where Pi is the proportion of consumptionof prey spe- The ground was covered by mosses(Sphagnum sp.), and cies i, k is the number of prey species,n is the total num- •n damp areas ferns reached tree-like sizes. ber of prey and Za/, is the upper standardnormal vanate Owl pairs were resident in all three sitesyear-round corresponding to a probability tail area of a/2k. The 2k and their pellets were collected under roosting trees. In denominator under a is used because multiple confi- SanMartin, we collected72 pelletsfrom April 1988 (early dence intervals were computed simultaneously.If the autumn)-February 1990 (late summer). In Rupanco, we confidenceinterval included the expectedproportion of collected 347 pellets from February 1993-September consumption(Pio), then we did not reject the hypothes•s 1995 (early spring) and, in Anticura,we collected475 that prey specieswere preferred or avoided. If the ex- pellets from September 1994-October 1995 (spring). We pected proportion of consumption was not included •n •dentified and quantified most vertebratesin the pellets the interval, we concluded that observedand expected on the basisof skulls and dentary pairs, whichever gave consumption differed significantly.Statistical significance was set at P -• 0.05 for all tests unless otherwise stated. the highest count. For other remains, such as hair, we used reference collectionsand quantified these prey as- RESULTS suming the smallestpossible number of individuals (e.g., presence of hair of a given specieswas deemed as rep- In the three sites,both by number and biomass, resenting only one individual). We estimatedthe biomass the scansoriallong-tailed mouse (Oryzomyslongicau- contribution of each prey type in the diet by multiplying the number of individualsin the pellet by the mean body datus)was the staple prey for Rufous-leggedOwls mass of that prey item. We assumedthat massesof un- (Table 1). Although the biomass contribution identified prey were similar to the average mass of the made by the arboreal mouse (Irenomystarsalis) was most closelyrelated identified taxa. We did not consider lower, it was consistentlyeaten at all sites.This was nonmammalian prey items, becausetheir contribution to also true of the scansorial black rat (Rattus rattus). the biomassconsumed was minimal in all sites (•10%). In San Martin, small mammal abundance was evaluat- Only in San Martin was the consumptionof the ed from 1988-89 in a 12 X 12 grid of medium-sized Sher- arboreal colocolo opossum (Dromiciopsaustralis) man live traps. Trapping stationswere set 12 m apart and important. 372 MARTINEZAND JAKSIC VOL.31, No. 4

Table 1. Vertebrateprey of Rufous-leggedOwls (Strix rufipes) in Anticura,Rupanco, and SanMartin (Chile).Below eachprey species are denotedtemporal (N = nocturnal,D = diurnal) and spatial(F = fossorial,C = cursorial,S = scansorial,A = arboreal)activities, and massa in grams.B% is percentof preyby biomassand N is frequencyof prey by number.

ANTICURA RUPANCO SAN MARTIN VERTEBRATEPREY B% (N) B% (N) B% (N) Akodonlongipilis (D-N; C; 41) 29.7 (155) 0.8 (2) 0.0 (0) Akodon olivaceus (N; C; 23) 0.8 (7) 1.7 (8) 1.4 (1) Auliscomysmicropus (N; C; 58) 1.6 (6) 1.1 (2) 3.4 (1) Geoxus valdivianus (D-N; F; 25) 2.1 (18) 0.2 (1) 0.0 (0) Irenomystarsalis (N; A; 42) 10.0 (51) 6.8 (17) 5.0 (2) Oryzomyslongicaudatus (N; S; 26) 50.0 (411) 56.2 (228) 35.7 (23) Phyllotisdarwini (N; C; 66) 0.0 (0) 1.2 (2) 0.0 (0) Rattus rattus (N; S; 40b) 0.4 (2) 4.2 (11) 12.0 (5) Unidentifiedrodents c 5.3 (31) 25.5 (67) 11.2 (5) Dromiciopsaustralis (N; A; 34) 0.0 (0) 2.3 (7) 30.4 (15) Rhyncholestesraphanurus (N; S; 29) 0.1 (1) 0.0 (0) 0.0 (0) Unidentified bat (A; 15) 0.0 (0) 0.0 (0) 0.9 (1) Total prey items 682 345 53 Total biomass(g) 21 379 10 546 1677 Total pellets 475 347 72 • Massesfor mammalswere obtained from Martinez(1993) and references therein, and from D.R. Martinez (unpubl. data) for the remaining taxa. bJuveniles. • Meanmasses of unidentifiedprey for eachlocality: Anticura: 37 g; Rupanco:40 g; SanMartin: 38 g.

In general, the mostfrequent prey were arboreal nights and detectedduring 8095 tube-nightscon- and/or scansorial small mammals. In Anticura, ductedsimultaneously (Table 2). While the traps theywere 60% of the biomass,whereas in Rupanco in San Martin capturedfive small-mammalspecies, theyreached 70% and >80% in SanMartin. Only Rufous-leggedOwls captured six species(Tables 1 in Anticura was the cursorial long-haired field and 2). In Anticura,six specieswere capturedby mouse (Akodonlongipilis) heavily preyed upon. To- Rau et al. (1995), but Rufous-leggedOwls caught gether,the remaining cursorialrodents, olivaceous eight species(Tables I and 2). field mouse (A. olivaceus),austral greater mouse At both sites,the frequencydistributions of small (Auliscomysmicropus), and Darwin's leaf-eared mammal speciesobserved in Rufous-leggedOwl mouse (Phyllotisdarwim), contributed <5% of the pellets (Table 1), compared to those estimatedin biomass consumed in all sites. the field through trapping (expectedvalues, Table In SanMartin, 837 individualsbelonging to five 2), were significantlydifferent. In San Martin, the speciesof small mammals were caught during olivaceous field mouse was "avoided," whereas the 20 736 trap-nights,whereas in Anticura59 individ- long-tailedmouse and the pooled species(due to uals of six specieswere caughtduring 358 trap- restrictions of the Chi-square test) were "pre- DECEMBER 1997 SELECTIVE PREDATION BY RUFOUS-LEGGED OWLS 373

Table 2. Proportion of captures (Pi) calculated from mammals constituted the core of the Rufous- live trapping (recaptures not included) of small mam- legged Owl diet, whereascursorial small mammals, mals present in San Martin. Data for Anticura were despite their similar field abundance (Martinez rained from Rau et al. (1995). 1993), were poorly representedin the diet.

ANTICURA SAN MARTIN DISCUSSION SMALLMAMMALS (Pl) (Pl) In all three sites,the frequency of the scansorial Akodonlongipilis 0.571 0.120 long-tailed mouse in the diet of Rufous-legged Aulis comys micropus 0.000 0.007 Owls was high. The low consumption of the cur- Akodon olivaceus 0.368 0.461 sorial olivaceousfield mouse was noteworthy be- Geoxus valdivianus 0.003 0.000 causethis specieswas numerically codominant with Irenomystarsalis 0.000 0.007 the long-tailed mouse, at least in San Martin (Mu- Oryzomyslongicaudatus 0.049 0.405 rfia and Gonzfilez 1986). Considering their similar Dromiciopsaustralis 0.003 0.000 body masses(23 g for olivaceousfield mouse and Rhyncholestesraphanurus 0.006 0.000 26 g for the long-tailed mouse, this trait may not Total individuals 59 837 explain avoidanceor preference. Total trap-nights 8453 20 736 Alternatively,different escapetactics or detect- ability of these two speciesmay be involved.Murfia and Gonzfilez (1982) attributed the different mi- ferred" (Table 3). In Anticura, the long-tailed crohabitat selection of olivaceous field mice and mouse accounted for more than 80% of the Chi- long-tailed mice to predation pressure.The former squarevalue, suggestingthat, as in San Martin, it speciesis associatedwith vegetationalvariables that was the preferred prey species. provide greater cover from above implying avoid- In Anticura, the expected proportions of use of ance of aerial predators. The latter speciesis as- three prey speciesexceeded the Bonferroni confi- sociatedwith dense areaswith understory that of- dence intervals (Table 4). The cursorial long- fer protection from a horizontal viewing implying haired and olivaceous field mice were consumed avoidanceof terrestrial predators. lessand the scansoriallong-tailed mouse was con- In addition, different antipredatory behaviors sumed more than expected. In San Martin, as in have been documented for those two rodents, as Anticura, the olivaceous field mouse was consumed well as for other rodent speciesconsumed by Ru- lessand the long-tailedmouse more than expected fous-leggedOwls. Mann (1978) reported that, be- (Table 4). Overall, arboreal and scansorial small cause of its long hind legs, the long-tailed mouse

Table 3. Absolutefrequencies of observedand expectedconsumption of smallmammals by Rufous-leggedOwls in Anticura and San Martin (Chile).

ANTICURA SAN MARTIN

SMALL MAMMALS OBSERVED EXPECTED OBSERVED EXPECTED

Akodonlongipilis 155 372.0 1 21.6 Akodon olivaceus 7 239.0 0 5.6 a Auliscomysmicropus 6 0.0a 1 0.3a Geoxus valdivianus 18 2.0 • -- -- Irenomystarsalis 51 0.0• 2 0.3• Oryzomyslongicaudatus 411 32.0 23 19.1 Rattus rattus 2 0.0 • 5 0.0 • Dromiciopsaustralis 0 2.0• 15 0.0• Rhyncholestesraphanurus 1 4.0• -- -- Pooledspecies • 78 8.0 23 6.2 Total pellets 475 72 Chi-square(dr) 452.9 (3) P < 0.001 65.69 (2) P < 0.001 Pooled becauseexpected values were <5.0. 374 MARTINEZAND JAKSIC VOL. 31, No. 4

Table 4. Bonferroni confidence intervalsfor the proportion of utilization of prey types,Pi, by Rufous-leggedOwls in Anticura and San Martin (Chile). An asteriskindicates a difference at the 0.05 level of significance.(+) eaten more than expectedby chance;(-) eaten lessthan expectedby chance.

EXPECTED OBSERVED PROPORTION PROPORTION BONFERRONI'S INTERVALS SMALL MAMMALS OF USE (Pio) OF USE (Pi) FORp•

Anticura Akodonlongipilis 0.571 0.297 0.248 --birds (Furnariidae) and insects climbs up shrubs as an escape response.In con- (cockroaches, tree crickets and beetles), are trast, the short-legged rodents (Phyllotisspp. and strongly associatedwith temperate rainforests.By Akodonspp.), make only short quadrupedal runs eating these organismsfound in the forest canopy, toward the nearest shelter (Mann 1978) or exhibit Rufous-leggedOwls may exploit resourcesnot uti- "freezing" behaviorwhen presentedoverhead rap- lized by other predators,thus reducing the poten- tor silhouettes in laboratory experiments (Simo- tial for competition.Although other owl speciesin- netti 1986). habit southern Chile, their smaller sizes or differ- The microhabitattype used by theserodent spe- ent habitats may reduce food competition. Thus, cies and their antipredatory behaviors,coupled the Austral Pygmy Owl (Glaucidium nanurn) is a with the hunting mode of Rufous-legged Owls smaller forest speciesthat specializeson birds, and could lead to differential predation pressures,re- its preferred hunting groundsare forest edgesand gardlessof the local abundance of these prey. Ru- parklands. The sympatricBurrowing (Speotytocun- fous-leggedOwls are sit-and-waitpredators closely icularia), Barn and Short-eared (Asio flammeus) associatedto old-growthstands (Martinez andJak- Owls nest and hunt mostly in open areas (D.R. si• 1996). Hence, prey that venture on branchesor Martinez, unpubl. data). climb on trees, or move from one spot to another, Because scansorial and arboreal small mammals leaping on the ground, may be more easilydetect- are closely associatedwith forest landscapes,the ed by this owl than sympatriccursorial rodents that accelerated loss of southern Chilean temperate commonly move along runways. rainforests is reducing the survival odds for both Similar habitat use and antipredatory behavior Rufous-leggedOwls and their prey. Even in the could also explain the high incidence of long- best possible scenario where there is an increase tailed mice in a dietary study of other owlsinhab- of suitable small mammals in cutover areas, the ste- iting central Chile. As shownbyJaksi• et al. (1981), reotyped hunting mode, a reduction of roosting both Barn Owls ( Tytoalba) and Great Horned Owls and nestingsites and the increasedpresence of po- (Bubovirginianus) consumed 70% more long-tailed tential competitors (open-field owls) may eventu- than olivaceous field mice, whereas the incidence ally lead to the extinction of this forest-adaptedowl of long-tailed mouse in the diet of terrestrialpred- species. ators such as foxes was consistentlylow (Iriarte et ACKNOWLEDGMENTS al. 1989). Specificallyfor San Martin, Martinez et Martinez thanks S. Chaplin for sponsoring him as a al. (1993) reported that gray foxes (Pseudalopexgri- member of RRF, TJ. Cohn for a sponsoredsubscription seus)preyed preferentially on olivaceousfield mice to the WilsonBulletin and RJ. Guti6rrez for sending the and long-tailed mice. last "cri de la mode" on owl traps. ER. Gehlbach, RJ. DECEMBER 1997 SELECTIVE PREDATION BY RUFOUS-LEGGED OWLS 375

Gutierrez, and an anonymousreviewer greatly improved supiales,quir6pteros, edentadosy roedores. Gayana the contentsof this paper. This studywas supported by Zoologia(Chile) 40:1-342. grant FONDECY-F194-1256 from Chile's Fondo Nacion- MARTINEZ,D.R. 1993. Food habits of the Rufous-legged al de Investigaci6nCientifica y Tecno16gica. Owl (Strix rufipes)in temperate rainforestsof south-

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