Prey Size Matters at the Upper Tail of the Distribution: a Case Study In

170 SHORT COMMUNICATIONS VOL. 33, NO. 2 j RaptorRes. 33(2):170-172 ¸ 1999 The Raptor ResearchFoundation, Inc.

PREY SIZE 1M_ATTERSAT THE UPPER TAIL OF THE DISTRIBUTION: A CaSE STUDY IN NORTHCENTRAL CHILE

D^vir) P. SANT•}•A•q}•ZANt) FA•L/tNM. j2ucsic Departamentode Ecologfa, Pontificia Universidad Catdlica de Chile,Casilla 114-D, Santiago,Chile

KEY WOP,DS: prey selection;Barn Owl; Tyto alba; Great remains in pellets (mosdy small mammals) were deter- HornedOwl; Bubo virginianus; Chil•, semiaridecosystem. mined to species level. More details about procedures may be found in Castro andJaksic (1995). Whole cranial remains of chinchilla rats found in owl Sympatric raptors are known to consume different pellets were set apart and measured.According to the prey species,often cueing on the abundance,size, mor- morphometric characters of each cranium, we estimated phology,or behavior of the prey (Kotler 1985, Kotler et the body mass by regressionanalysis. The relationship al 1988, Jaksic 1989). Less known is how raptor preda- between cranial measurementsand body xnasswas cal- tlon applies on different size or age classesof a given culated from specimensof known massin the Museo Na- prey species(Fulk 1976,Marti and Hogue 1979, Zamo- cional de Historia Natural (Santiago,Chile). Three cra- rano et al. 1986,Vargas et al. 1988, Longlandand Jenkins nial dimensionswere measuredwith calipersat 0.5 mm 1987, Dickman et al. 1991), but abundance, size, and be- precision: width of the zygomaticarch (cf. Green and Jameson1975), minimum distancebetween upper inci- havior of age classeshave also been postulatedas the cues sor and first molar (upper diastema,cf. Blem et al. 1993) used for hunting them. and length of the upper tooth row. Castro and Jaksic (1995) showed that sympatricBarn We used bilateral Kolmogorov-Smirnovtests (Sokal Owls (Tyt0 alba) and Great Horned Owls (Bubovirgini- and Rohlf 1981) to compare the sizedistribution of chin- anus)at a studysite in northcentral Chile (Auc6) did not chilla rats preyed upon by each speciesof owl. Although take different sizesof their most frequently sharedprey, estimates of body mass derived from cranial measure- the leaf-eared mouse (Phyllotisdarwini). The larger Great ments were computed to 1 g, we preferred to group Horned Owl (1200 g) preyed on averageon 50-g mice, individualsinto 20-g increment classesbecause of the in- herent statistical error contained in making extrapola- while the Barn Owl (300 g) consumed 54-g mice. The tions basedon regressions.We pooled data obtained dur- lack of statisticaldifference resulted from both owlsprey- ing the entire study period of 36 mo. (March 1993- ing acrossall size/age classesof their shared prey. February 1996). Becauseleaf-eared mice in Auc6 average47 g, which is closeto the mean prey size for the Barn Owl in Chile RESULTS AND DISCUSSION (45.1 g, Marti et al. 1993), we decided to investigatepre- dation on a prey speciesshared by both Great Horned The three cranial measurementswere good estimators and Barn Owls that exceedsthe mean prey size for the of chinchilla rat body mass (r > 0.949), but tooth row Great Horned Owl in Chile (72.8 g, Marti et al. 1993). length waschosen because of its better fit (r = 0.978, P The speciesstudied was the 182-gchinchilla rat (Abrocoma < 0.05), and because it could be measured in 97% of bennetti),the second largest rodent speciesat our study the cranial remains (256 out of 264). The equation was: sitein northcentralChile (Jaksicet al. 1992,Jaksic1997). body mass(g) = antilog (2.341953 + 3.386149 log tooth row length in ram). MATERIAL AND METHODS On average, Barn Owls consumed chinchilla rats Las Chinchillas National Reserve (31ø31'S, 71ø06'W) at weighing145 _+73 g (__ SD, N = 182), whereasthose in Auc6 is located approximately300 km north of Santiago, the diet of Great Horned Owlsweighed 178 -+ 70 g (N Chile. This site has a semiarid climate, mean annual pre- = 73). This differencein preyweight consumed was sig- cipitationof 157 mm, elevationsranging from 400-1700 nificant at P = 0.00119 (Kolmogorov-SmirnovD = m and slopeswith vegetationdependent on solar expo- 0.28005). Nevertheless,the preyweight rangesconsumed sure. On equator-facingslopes, vegetation is dominated overlappedconsiderably: 31-332 g for Barn Owl and 47- by cacti, bromeliads and a few evergreen shrubs;on po- 348 g for Great Horned Owl (Fig. 1). How a 300-g Barn lar-facingslopes, evergreen shrubs are the dominant spe- Owl can take such large-sizedchinchilla rats eludes us, cies. More details about this site may be found in Castro and Jaksic (1995). unlessour equation overestimatesprey weightsbased on From March 1993-February1996, we collectedpellets cranial measurements.We would like to emphasize that of Great Horned and Barn Owls under perches, roosts, chinchilla rats >290 g were preyed upon only sporadi- and nestsin Auc6. At least one pair of Great Horned and cally by Barn Owls (Fig. 1). On the other hand, it is not four pairs of Barn Owls inhabited the study area. Prey surprising that 1200g Great Horned Owls preyed on JUNE1999 SHORTCOMMUNICATIONS 171

12 BarnOwl •1o GreatHorned Owl

e 6 .>

30/ 50/ 70/ 90/ 110/ 130/ 150/ 170/ 190/ 210/ 230/ 250/ 270/ 290/ 310/ 330/ 49 69 89 109 129 149 169 189 209 229 249 269 289 309 329 349

Weightof Abrocomabennetti (g) Figure 1. Body massfrequency distribution of chinchilla rats (Abrocomabennetti) consumed by Great Horned Owls (Bubovirginianus; N = 73 rats) and Barn Owls (Tytoalba; N = 182 rats) in Auc6, northcentral Chile, March 1993- February 1996.

<49-g chinchilla rats becausethis owl is known to con- dor Phyllotisdarwini de pesopromedio 50 y 54 g, respec- sume even smaller rodents at the studysite (Castro and tivamente. Esta diferencia no era significativa.Debido a Jaksic 1995). It should also be noted that chinchilla rats que esteroedor esfftcerca del tamafio promedio de presa compriseonly a minor part of the diet of Barn Owls at calculado en Chile para Tyto(45 g) y lejos del calculado the studysite (• = 2.2% by prey numbersthroughout para Bubo(73 g), decidimosinvestigar qua ocurria con 1988-90; seeJaksic et al. 1992), whereasthey are the sec- la depredaci6n de estaslechuzas sobre un roedor mucho ond most common mammal consumedby Great Horned m/rs grande, Abrocomabennetti (182 g). Encontramos que Owls (• = 22.7% throughout1988-90;Jaksic et al. 1992). Tytoconsumia individuos de peso promedio 145 g y que Tyto(300 g) and Bubo(1200 g), which differ in body Buboconsumia aquellosde peso promedio 178 g, una weight by a factor of 4, and by 23% in mean prey size diferencia significativade 28%. Nuestra conclusi6n es (145 vs. 178 g, respectively),were able to exploit a single que cuando la presa es pequefia (Phyllotis)las dos lechu- prey speciesranging over one order of magnitude in zas no alcanzan a segregarseen cuanto a los tamafios mass (31-348 g). This suggestedthat the Barn Owl was consumidos,y que asto s01oocurre cuando la presa es able to handle, even if infrequently,prey of 50% its own grande (Abrocoma). body weight which is remarkable. The equivalentfigure [Traducci6n de Autores] for the Great Horned Owl would be 15%, well within its handling power (Marti et al. 1993). ACKNOWLEDGMENTS Our resultsindicated that small prey such as the 47-g We thank Enrique Silvafor doing laboratorywork, and leaf-earedmouse does not allow segregationby size be- Juan Carlos Torres-Mura and Jose Yanez for allowing us tween these two owls, likely because of its limited size access to the mammal collection of the Museo Nacional range (Castro and Jaksic 1995). However, the two owls de Historia Natural (Santiago, Chile). Funding was pro- did showsegregation by sizewhen preyingon larger prey vided by FONDECYI7 grant 196-0319 and a Presidential Chair in Science. Fred Gehlbach and Carl Marti made such as the 182-g chinchilla rat, likely becauseof the cogent criticismsthat helped us improve this paper. greater opportunity afforded by its ample size range. These observationssupport Wilson's (1975) assertion LITERATURE CITED that prey size matters to predatorschiefly at the upper tail of the frequencydistribution. BI•EM,C.R., L.B. BI•F.M,J.H. FF.I•IXAND D.W. HOIST.1993. Estimationof body massof volesfrom crania in Short- RESUMEN.--En un estudio previo en Chile central eared Owl pellets.Am. Midl. Nat. 129:282-287. (Auc6), se detect6 que las lechuzas Tyto alba (300 g) y CASTRO,S.A. AND F.M. JaKSIC.1995. Great Horned and Bubovirginianus (1200 g) consumianindividuos del roe- Barn Owls prey differentially according to the age/ 172 SHORT COMMUNICATIONS VOL. 33, NO. 2

size of a rodent in northcentral Chile. J. Raptor Res. The eftkcts of morphology and body size on rates of 29:245-249. owl predation on desert rodents. Oikos53:145-152. DICKMAN,C.R., M. PREVADECAND AJ. LYNAM.1991. Dif- LONGLAND,W.S. ANDS.H. JENI•NS. 1987. Sex and age af- ferential predation of size and sex classesof mice by tbct vulnerability of desert rodents to oM predation the Barn Owl, Tytoalba. Oikos62:67-76. J. Mammal.68:746-754. FULK,G.W. 1976. Owl predation and rodent mortality:a MARTI,G.D. ANDJ.C. Hoc;uE.1979. Selectionof prey by casestudy. Mammalia 40:423-427ß size in screech owls. Auk 96:319-327. GREEN,A. ANDD.L. JAMESON.1975. An evaluationof the --, E. KORPI• AND EM. JAKSm.1993. Trophic zygomaticarch for separatingjuvenile from adult cot- structure of raptor communities: a three-continent ton rats ( Sigmodonhispidus). J. Mammal.56:534-535. comparisonand synthesis.Pages 47-137 in D.M. Pow- JAKSIC,EM. 1989. What do carnivorouspredators cue in er [ED.], Current ornithology. Vol. 10. Plenum Pub- on: size or abundance of mammalian prey? A crucial lishing Corporation, New York, NY U.S.A. test in California, Chile, and Spain. Rev.Chil. Hist. Nat. SOKAL,R.R. ANDF.J. ROHLF.1981. Biometry.W.H. Free- man and Go., San Francisco, CA U.S.A. 62:237-249. V)mc;AS,J.M., L.J. PALOMOAND P. PALMQUIST.1988. Pre- ß 1997. Ecologlade los vertebradosde Chile. Edi- daci6n y selecci6n intraespecfficade la lechuza co- cionesUniversidad Cat61ica de Chile, Santiago,Chile. mfin (Tyt0 alba) sobre el rat6n moruno (Mus spretus) , J.E. JIMENEZ,S.A. CASTROAND P. FEINSINGER. Ardeola 35: 109-123. 1992. Numerical and functional responseof predators WILSON,D.S. 1975.The adequacyof bodysize as a niche to a long-term decline in mammalian prey at a semi- difference. Am. Nat. 109:769-784. arid Neotropical site. Oecolo•a89:90-101. ZAMORANO,E., L.J. PALOMO,A. ANTUNEZAND J.M. VAR- KOTLER,B.P. 1985. Owl predation on desert rodents G^S. 1986. Criterios de predaci0n selectivade Bubo which differ in morphology and behavior.J. Mammal. buboy Tytoalba sobre Rattus.Ardeola 33:3-9. 66:824-828. , J.S. BROWN,RJ. SMITH AND W.O. WIRTZ. 1988. Received17 May 1998; accepted7 March 1999.

j. RaptorRes. 33(2):172-175 ¸ 1999 The Raptor ResearchFoundation, Inc.

SPATIAl. AND TEMPORAL VARIATIONS IN THE DIET OF THE COMMON KESTREL (FALCOTINNUNCULUS) IN URBAN ROME, ITALY

EMANUELE PlATTELLA Dipartimentodi BiologiaAnitaale e dell'Uomo(Zoologia), Univers'ita di Roma "La Sapienza" Viale dell'Universita32, 1-00185 Rome,Italy

LuCA SALVATI PiazzaE Morosini12, 1-00136 Rome,Italy

ALBERTO MANGANARO Via di Donna Olimpia152, 1-00152Rome, Italy

SIMONE FATTORINI Dipartimentodi Biolo•a Anitaalee dell'Uomo(Zoolq•a), Universitadi Roma "La Sapienza" Viale dell'Universita32, 1-00185 Rome,Italy

KEY WORDS: CommonKest'rel; Falco tinnunculus; diet;,avi- quently occurring in urban areasin higher densitiesthan an prey;urban area;Rome, Italy. in farmland areas (Village 1990, Shrubb 1993). Never- theless,few studieshave described detailsof the feeding Several studies have described the ecology of raptors ecologyof kestrelsin theseurban areas(Quere 1990, Ro- urban areas (e.g., Galeotti 1994). Common Kestrels manowski1996). Therefore, the aim of our studywas to (Falco tinnunculus)breed in many European towns, fre- describethe composition of the kestrel diet and any sea-