J. Raptor Res. 21(1):3-7 ¸ 1987 The Raptor Research Foundation, Inc. PREY SELECTION OF COMMON BARN- ON ISLANDS AND MAINLAND SITES

DAVID W. JOHNSTONAND JAMESM. HILL

ABSTRACT.--Datafrom the literature and a recentcollection of CommonBarn- ( alba) pellets from Block Island, Rhode Island, were used to assessthe relative numbers of and non-avian vertebratestaken by this owl on islandsand mainland sites.Our analysissupports the hypothesisthat barn-owl diets include proportionatelymore birds (both speciesand individuals) on islands than at mainland sites.The percentof speciesand individual birds in the diet decreasesfrom the equator to 54øN. Possible causes for island vs. mainland diets and latitudinal trends are discussed.

The interaction between the Common Barn-Owl the percentof bird speciesamong all the vertebratespecies (Tyro alba) and its prey is well documented.Pub- captured, and 2) by consideringthe percent of all indi- vidualbirds vs. individuals of all non-avianvertebrate prey. lishedstudies deal with economicaspects (Bendire 1895; Errington 1932; Wallace 1950), population RESULTS AND DISCUSSION ecology(Davis 1959; Otteni et al. 1972; Herrera Prey Selectionon Islands.Most previousdietary and Jaksi• 1980), and range extensionsof mam- studies of barn-owls from mainland sites have shown malian prey (Kirkpatrick and Conway 1947; Stickel and Stickel 1948; Baker 1953; Parmalee 1954). This a preponderanceof mammalianprey. prey speciesfrom the 50 mainland sites examined here owl is believedto specializeon mammalian prey, constituteda mean of 92.4% (SD = 8.29) of the total butJohnston (1974: 172) reporteda high percentage vertebratediet. Despite this preponderanceof small of bird speciesin barn-owl pelletsfrom Grand Cay- mammal prey species,the mainland barn-owls took man Island, BWI. From that study and two other somesmall birds and, evenless frequently, reptiles, island reportsavailable then, he proposedthat "on ,and .By contrast,on 23 island some islands ... where small mammalian prey is sites mammal speciesconstituted a mean of only reducedin diversity and total numbers, the barn- 60.5% (SD = 25.47). owl becomesalternatively a significantpredator of birds and other non-mammalian vertebrates." We The number of bird speciesas a percent of the totalvertebrate prey species from islands (.• = 38.6) now test that hypothesisby using data from addi- is significantlygreater than values from mainland tional publishedaccounts of barn-owl diets on is- landsand mainland sites.While examiningthe data sites(-• = 19.9)(Table 1). Althoughthe number of from those locations,we developedan additional bird speciesper se doesnot differ significantlybe- tween islands and the mainland (Table 1), barn- hypothesisthat barn-owl prey on more northerly islands includes fewer birds than on islands closer owls took fewer mammal specieson islandsthan on the mainland, thus making the proportion of bird to the equator. speciestaken on islandshigher. We also examined MATERIALS AND METHODS published data on the total number of individual A searchof the literature from the northernhemisphere birds and non-avianvertebrates extracted from pel- yieldedquantitative analysesfrom 23 island sites lets. The number of individual birds identified as a occurringfrom the GalapagosIslands (0 ø) to SheppeyIsle (British Isles, 54øN) and from 50 mainland sites,mostly percentof all vertebratesis greateron islands(,• = in the United States,but alsofrom somelocalities in Spain, 10.5)than on the mainland(-• = 4.0). Thus,barn- Poland,and Italy. Unpublisheddata from severalislands owls on islandsprey proportionatelymore on birds in the British Isles were obtained from David E. Glue, as than other vertebrates(mainly ), than at were unpublisheddata from Martha's Vineyard, Mas- mainland sites. sachusetts(G. Jonesand K. Driscoll). In Appendix I are unpublisheddata from BlockIsland, RhodeIsland. Data Some publishedaccounts are of interest because extracted from these accounts are used in our statistical of the extremes(0-100%) of avian prey taken by analyses(Mann-Whitney U-Test) and in the regression barn-owls.For example,mainland areasfrom which analyses. no birds were reported include California (Foster To test the hypothesissuggested by Johnston(1974) that barn-owlstake proportionatelymore birds on islands 1927:6 of 11 sites;Hawbecker 1945; Fitch 1947), than on the mainland, pellet data from the literature were Massachusetts(Boyd and Shriner 1954), SouthCar- examinedand comparedin two ways: 1) by considering olina (Tedards1963: oneof four seasonalsamples), 4 ,JOHNSTONAND HILL VOL. 21, No. 1

Table 1. A comparisonof vertebrateprey of GommonBarn-Owls betweenmainland and island sites.

PREY MAINLAND a ISLANDSb

All vertebratespecies 10.7; SD = 4.84 ,,• = 8.6; SD = 5.40 (U=743; P=0.30) All bird species 2.6; SD =2.57 •=3.9; SD =4.39 (U=482; P=0.318) All mammal species 8.0; SD = 3.26 •'=4.4; SD = 1.82 (U = 941; P < 0.0005) Number of bird speciesas percentof all 19.9;SD = 16.52 •' = 38.6; SD = 24.63 vertebrate species (U=312; P< 0.01) Number of individual birds as percentof 4.0; SD = 6.51 ,,• = 10.5;SD = 14.34 all vertebrate individuals (U= 284; P=0.008) Data from 50 sites: references1, 2, 7, 10, 14, 16, 17, 18, 20, 22, 23, 25, 26, 27, 28, 29, 30, 33, 36, 37, 38, 39, 41, 42, 43, 44, 46, 47, 49, 50, 51, 52, 53, 54, 55; D. Glue (unpubl. data). Data from 23 sites:references 4, 5, 8, 11, 12, 13, 15, 21, 34, 56; D. Glue (unpubl.data); G. Jonesand K. Driscoll(unpubl. data), Appendix I. and Ireland (Fairley 1966: two of 10 seasonal By examiningonly mainlanddata for barn-owls samples). Island studiesreporting no birds were in Europe, Herrera (1974) proposeda latitudinal thosefrom Skomer(Brown and Twigg 1971), Bute effectand useda modificationof the now-question- (D. E. Glue, pers.comm.), and Martha's Vineyard able (Pielou 1977) Shannon-Wienerdiversity index, (Choate 1972). At the other extreme, on someislands namely"trophic diversity in relationto biomass"of where extensivecolonies of seabirdsoccur, barn-owls prey captured.Herrera's report, althoughnot strict- fed exclusivelyon birds (Bonnot 1928). The "out- ly comparableto the presentstudy which focuseson lier" or anomalouspoints in Figure I (60 units at avianprey selection,noted a significantnegative cor- 25øN) and Figure 2 (51 units at 25øN) came from relation betweentrophic diversityin relation to bio- the small and perhapsinadequate sample of Banks mass and north latitude. (1963) wherein the "remains of at least six Craveri Although the effectsof latitude were the samefor Murrelets [Brachyramphuscraveri] and at leastfour islands and mainland sites, the greater percentages wood rats ..., were identified." of bird speciestaken on islandscompared with main- Latitudinal Variation. We searched for relation- land sites at the same latitude merit comment. We shipsbetween latitude and bird prey in barn-owl believethat this differenceis due, at least in part, to diets using covarianceanalysis of the transformed the different relative numbers of available prey percentagesof vertebrateprey speciesthat were birds species,especially birds vs. small mammals.Such (Fig. 1). ANCOVA indicatedthat a simplelinear comparisonsare often impossibleto documentand modelis an adequatedescription for both the island quantifybecause of thelack of publishedinformation and mainland data (P > 0.05). The slopesof the on numbersof availablespecies. On Grand Cayman regressionlines were significantlydifferent from zero at 19øN where barn-owl diets included about 60% (P = 0.05), but there was no evidencethat the two bird species,only 5 smallmammal speciesincluding slopeswere different from each other even though 3 occur,whereas about 70 passerinebird species the y-axisintercepts were significantlydifferent (P < havebeen identified (Johnston 1974 and pers.obs.) 0.05). Consideringnumbers of speciesfound in pel- Regressionlines for percentagesof individualbirds lets, effects of latitude were, therefore, the same on in the prey items also show a negativecorrelation islandsand the mainland.Toward the equatorbird with latitude but the y-intercepts are not signifi- speciescomprised a significantlygreater percentage cantlydifferent from eachother (Fig. 2). The best of vertebrate prey speciesthan at higher northern estimate for a commonslope is a decreasein indi- latitudes,although no comparabledata were avail- vidual birds of approximately 7.3% for each 10ø able for mainland sites from 0ø-25øN. The best es- latitude northward. timate of a commonslope (islands and mainland) Our analysisdemonstrates that barn-owlsprey was that bird speciesin barn-owl diets decreaseby 1) on proportionatelymore avian speciesand in- approximately6.2% for each10 ø latitude northward. dividual birds on islands than on the mainland and SPRING 1987 COMMON BARN-OWL PREY SELECTION 5

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,'o' io' 3;' , o' 5o' 6'o' Figure 2. The relationshipsbetween degreesof north NORTH LATITUDE latitude and the percentof total birds among Figure 1. The relationshipbetween degrees of north lat- all vertebrateprey found in Common Barn- itude and the percentof bird speciesamong Owl pellets from islandsand mainland sites. all vertebratespecies of preyfound in Common Percentageshave bccn transformed(arc-sine Barn-Owl pelletsfrom islandsand mainland of •). Individualdata pointsarc from ref- sites.Percentages have been transformed (arc- erences cited in Table 1. sine of V•; see Sokal and Rohlf 1981). In- dividual data points are from referencescited in Table 1. reducedin diversityor abundanceon islands,barn- owls are believedto take alternativeprey to maxi- 2) on fewer birds with increasinglatitude north- mize their energy input. ward. These differencesraise questionson the causes ACKNOWLEDGMENTS of dietarypreferences. Is it becausemammalian fau- Unpublisheddata were suppliedby D. E. Glue, Gwi- nas on islandsare more depauperate?From 11 is- lym Jones, and Karen Driscoll. James W. Haefner and lands for which mammal data were available, the StephanR. Taub kindly offeredstatistical advice and ear- lier drafts were read by Brian Millsap, Dan Anderson, meannumber of mammalianprey specieswas eight, Peter Grant, Dennis Power, and Carl Marti. Their critical whereas the mean number from 13 mainland sites suggestionshelped to improve the manuscript.A grant was 10, suggestinga decreasein mammalianspecies from George Mason University supportedprey identifi- richnesson islands.Unfortunately, a fundamental cationsfrom Block Island. We are grateful to Susan R. Drennan who collectedthe pellets from Block Island. and perhapscrucial data setwas lackingin all these studies,namely population densities of all available LITERATURE CITED* prey species.We do not haveconvincing evidence to ALCORN,J. R. 1942. Food of the Bart• Owl at Soda know if barn-owlscapture prey in proportionto the Lake, Nevada. Condor44:128-129. (1) number of individualspresent in the foragingarea. ANDERSON,D. AND C. E. NELSON. 1960. Birds and Furthermore,we donot know the barn-owl'sfeeding mammalsin pelletsfrom near Laguna,Chi- efficiency.Is it, for example, more efficient for an huahua, Mexico. SouthwesternNat. 5:99-101. (2) owl to capture a small bird than a large rat, or BAKER,R. H. 1953. Mammals from owl pellets taken ? in Coahuila, Mexico. Trans. Kans. Acad. Sci. 56:253- 254. (3) Finally, for our analysesof prey capturedby barn- owls,it appearsthat this predator-preysystem is at least a qualitativeexample of optimal foragingthe- * Numbersin parenthesesfollowing each citation identify ory. When and if small mammal populationsare referencesused in the analyses. 6 JOHNSTONAND HILL VOL. 21, No. 1

BANKS,R. C. 1963. Birds of the BelvedereExpedition FOSTER,G. L. 1926. Contents of Barn Owl pellets. to the Gulf of California. Trans. San Diego Soc.Nat. Condor28:130. (26) Hist. 13:49-60. (4) 1927. A note on the dietary habitsof the Barn BANNERMAN,D.A. 1963. Birds of the Atlantic Islands. Owl. Condor29:246. (27) Vols. 1, 2, 4. Oliver and Boyd, Edinburgh. (5) FRANZREB,K. E. AND W. F. LAUDENSLAYER,JR. 1982. BENDIRE,C.E. 1895. The AmericanBarn Owl breeding Compositionand seasonalvariation of the Barn Owl in ,D.C., in winter. Auk 12:180-181. (6) (Tytoalba) diet in Arizona.Raptor Res. 16:36-39. (28) BETTS,E.S. 1936. Vertebratecontents of pelletscast by HALL, E. R. 1927. The Barn Owl in its relation to the Barn-Owls (Tyto alba alba) in N.E. Worchestershire. rodentpopulations at Berkeley,California. Condor29: Ibis (13th ser.) 6:598-600. (7) 274-275. (29) BLEM, C. R. AND J. F. PAGELS. 1973. Feeding habits HAWBECKER,A.C. 1945. Food habits of the Barn Owl. of an insular Barn Owl, Tyroalba. Va. J. Sci. 24:212- Condor47:161-166. (30) 214. (8) HERREe,•,C. M. 1974. Trophic diversityof the Barn BONNOT, P. 1928. An outlaw Barn Owl. Condor 30:320. Owl Tyto alba in continentalWestern Europe. Orms (9) Scand.5:181-191. (31) BOYD,E. M. ANDJ. SHRINER. 1954. Nesting and food --AND F. M. JAKSIC 1980. Feedingecology of the of the Barn Owl (Tyto alba) in Hampshire County, Barn Owl in central Chile and southernSpain: a com- Massachusetts.Auk 71:199-201. (10) parativestudy. Auk 97:760-767. (32) BROWN,J. C. ANDG. I. TWlGG. 1971. Mammalian prey HORNER,J., R. WALLACEAND D. W. JOHNSTON.1974. of the Barn Owl (Tyro alba) on SkomerIsland, Pem- Food of the Barn Owl at Gainesville, Florida. Fla. brokeshire. Proc. Zool. Soc.Lond. 165:527-530. (11) Field Nat. 2:28-31. (33) BRUCE,J. A. AND C. A. LONG. 1962. Nesting of the JOHNSTON,D.W. 1974. Foodof the Barn Owl on Grand Barn Owl, Tytoalba, on SanJuan Island,Washington. Cayman, B.W.I. Quart.J. Fla. Acad.Sci. 35:171-172. Murrelet 43:51. (12) (34) BUDEN,D.W. 1974. Prey remainsof Barn Owls in the KIRKPATRICK,G. M. AND G. H. CONWAY. 1947. The Southern Bahama Islands. Wilson Bull. 86:336-343. winter foods of some Indiana owls. Amer. Midl. Nat (13) 38:755-766. (35) CAHN, A. R. AND J. Y. KEMP. 1930. On the food of KNIGHT, R. L. ANDR. E. JACKMAN. 1984. Food-niche certain owls in east-central Illinois. Auk 47:323-328. relationshipsbetween Great Horned Owls and Com- (14) mon Barn-Owls in easternWashington. Auk 101:175- CHOATE,J. R. 1972. Notes on geographicdistribution 179. (36) and habitats of mammals eaten by owls in Southern MARTI, C. D. 1969. Somecomparisons of the feeding New England.Trans. Kans. Acad. Sci. 74:212-216. (15) ecologyof four owls in northcentralColorado. South- CLARK,J.P. ANDW. A. WISE. 1974. Analysisof Barn westernNat. 14:163-170. (37) Owl pelletsfrom Placer County, California. Murrelet MASER,G. ANDE. D. BRODIE,JR. 1966. A studyof owl 55:5-8. (16) pellet contentsfrom Linn, Bentonand Polk counties, COWAN, I.M. 1942. Food habits of the Barn Owl in Oregon.Murrelet 47:9-14. (38) British Columbia. Murrelet 23:48-53. (17) -- AND E. W. HAMMER. 1972. A note on the food CUNNINGHAM,J. D. 1960. Food habitsof the Horned habitsof Barn Owls in Klamath County, Oregon.Mur- and Barn owls. Condor 62:222. (18) relet 53:28. (39) DAVIS, D. H.S. 1959. The Barn Owl's contribution to OTTENI, L. C., E.G. BOLEN AND C. COTTAM. 1972. ecologyand palaeoecology. Ostrich (Suppl. 3): 144-153. Predator-preyrelationships and reproductionof the (19) Barn Owl in southern Texas. Wilson Bull. 84:434- DEBRUIJN,O. 1979. Voedseloecologievan de Kerkuil 448. (40) Tyroalba in Nederland.Limosa 52:91-154. (20) PARMALEE,P.W. 1954. Food of the DEGROOT,R.S. 1983. Origin, statusand ecologyof the and Barn Owl in east Texas. Auk 71:469-470. (41) owls in Galapagos.Ardea 71:167-182. (21) PEARSON,O. P. AND A. K. PEARSON.1947. Owl pre- ERRINGTON,P. L. 1932. Food habits of southern Wis- dation in Pennsylvania,with noteson the small mam- consinraptors. Part I. Owls. Condor34:176-186. (22) mals of Delaware County. J. Mammal. 28:137-147. EVANS,F. C. AND J. T. EMLEN, JR. 1947. Ecological (42) noteson the prey selectionby a Barn Owl. Condor49: PETRETTI, F. 1977. Seasonal food habits of the Barn 3-9. (23) Owl (Tyroalba) in an area of centralItaly. Le Gerfaut FAIRLEY,J. S. 1966. Analysesof Barn Owl pelletsfrom 67:225-233. (43) an Irish roost. Brit. Birds 59:338-340. (24) PHILLIPS,R. S. 1951. Food of the Barn Owl, Tyroalba FITCH, H. S. 1947. by owls in the Sierran pratincola,in HancockCounty, Ohio. Auk 68:239-240. Foothills of California. Condor49:137-151. (25) (44) SPRING 1987 COMMON BARN-OWL PREY SELECTION 7

PIELOU,E. C. 1977. Mathematical ecology.Wiley-In- TROST,12. H. ANDJ. H. HUTCHISON. 1963. Food of the terscience,New York. (45) Barn Owl in Florida. Quart.J. Fla. Acad.Sci. 26:382- RUPRECHT,A. L. 1979. Food of the Barn Owl, ryto 384. (53) albagutrata (C. L. Br.) from Kujawy. Acta Ornithol. VARGAS,J. M., E. MIGUELAND M. BLASCO.1980 (1982). 14:1-19. (46) Estudio estacionalcomparativo del regimen alimen- SMITH, D. G., C. R. WILSON AND H. H. FROST. 1972. tario de Tyto alba Scopolien Fuentepiedrade Malaga SeasonaI food habits of the Barn Owl in Utah. Great y el Padul de Granada (Espana). Misc. Zool. 6:95-102. Basin Nat. 32:228-234. (47) (54) SOKAL,R. R. AND F. J. ROHLr. 1981. Biometry. 2nd WALLACE,G.J. 1950. In support of the Barn Owl. Ed. Freeman and Co., New York. (48) Mich. Agric. Exper. Sta. Quart. Bull. 33:96-105. (55) STEARNS,E.I. 1950. PamperingBarn Owls. Aud. Mag. WETMORE, A. AND B. H. SWALES. 1931. The birds of 52:178-183. (49) Haiti and the Dominican Republic. U.S. Nat. Mus., STICKEL, W. H. AND L. F. STICKEL. 1948. Mammals Bull. 155:233-239. (56) of northwesternTexas found in Barn Owl pellets.]. Mammal. 29:291-293. (50) Department of Biology, George Mason University, TED•d•r)S,Mrs. R.C. 1963. A studyof Barn Owls and Fairfax, VA 22030. their food. Chat 17:1-3. (51) TOWNSEND,C. W. 1926. Findings in pellets of Barn Received30 September1986; Accepted20 January 1987 Owl. Auk 43:544. (52)

Appendix I. Prey remainsfrom Common Barn-Owl pellets,Block Island, RI (41ø11'N, 71ø34'W).

1981

4-5 OCTOBER1980 10-11 JANUARY 9-10 MAY Number of whole pellets 26 18 16 Micromspennsylvanicus 63 (51%) 42 (68%) 11 (38%) Rattusnorvegicus 12 (10%) 1 (2%) 6 (21%) Peromyscusleucopus 46 (37%) 18 (29%) 10 (34%) Hylocichlasp. a -- -- 1 (3.5%) Dumetellacarolinensis a 1 (0.5%) -- 1 (3.5%) Unidentified passerinebirds a 2 (1.5%) 1 (1%) -- Identificationsby PierceBrodkorb.

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