j RaptorRes. 30(2):90-92 ¸ 1996 The Raptor ResearchFoundation, Inc.

INTRA-YEAR REUSE OF GREAT HORNED NEST SITES BY BARN IN EAST-CENTRAL COLORADO

DAVID E. ANDERSEN 1 Departmentof WildlifeEcology, University of ,Madison, WI 53706 U.S.A. and ColoradoFish and WildlifeAssistance Office, U.S. Fish and WildlifeService, 730 SimmsSt., No. 292, Golden, CO 80401 U.S.A.

A•STP,•CT.--Barnowls ( Tytoalba) sequentiallyreused nest sitesof great horned owls (Bubovirginianus) within the samebreeding season on two occasionsin east-centralColorado during 1982 and 1983.Two of 22 cliff nest sitesused by great horned owlsduring the 2-yearperiod were subsequentlyreused by barn owls,while no red-tailedhawk (Buteojamaicensis,N = 8) or common (Corvuscorax, N = 20) cliff nestswere sequentiallyused by barn owls.In temperatelatitudes, only rarely are sympatricraptor speciesexpected to exhibit breedingbehavior that could accommodateintra-year sequential nesting at the same site.

KEYWORDS: breeding;,Bubo virginianus; Colorado; bar• owl;; nest site;, Tyto alba.

Re-usointra-anual de nidos de Bubovirginianus por Tytoalba en el centro-estede Colorado RESUMEN.--Tytoalba re-us6 secuencialmente nidos de Bubovb•inianus en la mismaestaci0n reproductiva yen dos ocasiones(1982 y 1983), en el centro-estede Colorado.Dos de los 22 nidos usadospor B. virginianus,durante el periodo de dos aftos,fueron subsecuenternentere-usados por T. alba.En latitudes templadas,estas especies simp/ttricas, raramente se esperaque exhibanuna conductareproductiva que incluya nidificaci6n secuencialintra-anual en el mismo sitio. [Traducci6n de Ivan Lazo]

Barn owls (Tyto alba) are sympatricwith great open ,breeding densityof many raptorsis horned owls (Bubovirginianus) throughout much thought to be limited by the availabilityof suitable of their breeding range in North America (Johns- nest sites (Olendorff and Stoddart 1974, Newton gard 1988), and these two 'nest-site char- 1979, Andersen 1991). In these habitats, different acteristics are often similar (Knight and Smith raptor speciesoften nest in closer proximity to one 1982). Great horned owlsare also potential pred- another than would be expected if nest siteswere ators of barn owls (e.g., Wayne 1924, Rudolph placed randomly or regularly (Schmutzet al. 1980, 1978, Knight andJackman 1984, Millsap and Mill- Restani 1991), and this pattern appearsto corre- sap 1987) and havereportedly killed barn owlsat spond to the distribution and density of suitable their nestsites (Millsap and Millsap1987). Presum- nest sites.Where nest sites are not limiting, inter- ably, barn owls selectnest sitesthat provide pro- specificnest dispersionmay become more regular tection from by great horned owlsif oth- (Rothfelsand Lein 1983). Perhapsdue to low avail- er factors that influence nest site selection (e.g., ability of suitablenest sites,many raptorsappro- thermal protection,Millsap and Millsap 1987) are priate nestsfrom other speciesfrom one year to met. the next (e.g., Newton 1979, Smith and Murphy Several authors (e.g., Smith and Marti 1976, 1982), suggestingthe occurrenceof interspecific Marti et al. 1979, Bunn et al. 1982) havesuggested competition for nest sites.However, intra-yearuse that barn owl populationsmay be limited by the of the samenest site by two raptor speciesis prob- availabilityof suitablenest sites.In predominantly ably rare. During a 2-yrstudy of raptorsin east-centralCol- 1Present address: Minnesota Cooperative Fish and Wild- orado I observedtwo instancesof within-yearuse life ResearchUnit, National BiologicalService, Depart- of great horned owl nest sitesby barn owls.These ment of Fisheriesand Wildlife, Universityof Minnesota, observations are consistent with the suggestion St. Paul, MN 55108 U.S.A. that nest site availabilitymay influence distribution

90 JUNE 1996 REUSEOF GRFATHORNED OWL NESTSBY BARN OWLS 91

Table 1. Cliff nests of large located on the Fort halfway through the nestling period at both nests Carson Military Reservation, Colorado from 1982 (basedon photographsin Bunn et al. [1982]) and through 1983 and the occurrenceof intra-yearreuse of thus had a high probabilityof fledging. those nestsby barn owls. Barn owl nesting only infrequently occurred at vacated nest sites of great horned owls (two of 22 No. OF cliff nest sitesof great horned owls over a 2-yr pe- NESTING riod) and was not observed in old nests of other AT- No. OF TEMPTS NEST large cliff-nestingbirds (red-tailed [Buteoja- maicensis] or common [ corax]) on No. OF THAT SITES SPECIES YEAR NESTS FA•LF•D REUSED the FCMR (Table 1). Both nest sitesused by barn owls were on cliffs; one in an old stick nest that Red-tailed 1982 4 1 0 appeared to have been constructedby common 1983 4 0 0 ravensand the other in a large natural cavityin a Great Horned Owl 1982 9 0 0 1983 13 3 2 sandstone cliff. The only other barn owl nesting Common Raven 1982 10 3 0 attempt located on the FCMR during the 2-yr study 1983 10 -- -- was in a natural cavityin a cliff (Andersen 1988). On the FCMR, great horned owls laid eggs in early March and young fledged in early to mid- of raptorsnesting in predominantlyopen habitats, May. In north-central Colorado, barn owlsfledged and provide an example of the conditionsunder young from July to early September (Millsap and which interspecific,intra-year breeding in the same Millsap 1987); egg laying occurred from April nest site might occur. To my knowledge, similar through early July (Pickwell 1948, Smith et al. observationshave not previouslybeen reported in 1974, Colvin 1985, Marshall et al. 1986). The pe- the literature. riod of nest initiation on the FCMR is not known, but if similar to north-central Colorado, then barn METHODS owlsmay be able to use both successfuland unsuc- During 1982 and 1983, I monitored nestingraptors on cessfulgreat horned owl nest sites as potential the Fort Carson Military Reservation (FCMR) in east-cen- nests. tral Colorado (see Andersen et al. [1985] for a descrip- tion of the FCMR). Nestswere located each spring from Within-yearuse of the samenest site by two rap- the ground (on foot or from a vehicle) by searching po- tor speciesis likely to be rare. In temperate areas, tential nesting . In March and/or April of each most raptors of medium to large body size begin year, potential nesting areas (primarily cliff lines, promi- nent trees and canyons)were also surveyedfrom a heli- nestingin late winter or earlyspring, and generally copter, and all nest sitesoccupied in 1982 were recheck- attempt nestingonly once during a singlebreeding ed in 1983. Each nesting attempt wasmonitored approx- season(Newton 1979,Johnsgard1988). Sequential imatelyweekly until youngfledged or the nestingattempt use of a nest by different specieswithin a single failed. Follow-up visits were made to nests where no breeding seasonrequires one speciesto relinquish young fledged in order to determine causeof failure, and nest siteswere visitedduring Augustin the year they the site early in the season,and the other to adopt were monitored to quantifytheir physicalcharacteristics. the site relativelylate in the season.Reuse of nest sites may also be more likely when suitable nesl RESULTS AND DIS(,USSION sites are limited and when neither species con- During follow-upvisits to nestsin 1983, I located structs its own nest, further limiting potential nest two barn owl nestsat siteswhere great horned owls sites.These conditions are probably only met in a had nestcd earlier in the year (Table 1). One nest- fbw speciespairs, with barn owls being one of the ing attempt, located on 6 August, contained three few specieslikely to initiate nesting late in the sea- barn owl young, and was situated where a great son (Stewart 1952, Henny 1969). horned owl nestingattempt had failed on approx- ACKNOWLEDGMENTS imately 13 April 1983. The other nest contained five barn owl young and wasfBund on 7 Augustat Support for this studywas provided by the U.S. Army, a site where great horned owlshad fledged on ap- Directorate of Environmental Compliance and Manage- ment, Fort Carson, Colorado, through the U.S. Fish and proximately 16 May 1983. Neither barn owl nest Wildlife Service (Colorado Fish and Wildlife Assistance was subsequentlyvisited to determine nesting suc- Office and the Wisconsin CooperativeWildlife Research cess,although I estimatedthat young were at least Unit). Support was also provided by the College of Ag- 92 ANDV,RSV, N VOL. 30, NO. 2 ricultural and Life Sciences, the Graduate School and the diction, and incubation period. RaptorRes. 20:108- Department of Wildlife Ecologyat the Universityof Wis- 112. consin-Madison. I thank W.R. Mytton, T.S. Prior, S.R. MA•TI, C.D., P.W. W^GNER AND K_W. DENNE. 1979. Nest Emroohs and B.D. Rosenlurid,who helped coordinate boxes for the management of barn owls. Wildl. Soc the project on military property and OJ. Rongstadfor Bull. 7:145-148. invaluable guidance and assistance.Field assistancewas MILLSAP,B.A. ANDP.A. MILLSAP. 1987. Burrownesting by provided by T. Aydelott and G.M. Hughes. Previousdrafts common barn-owls in north central Colorado. Condor of this manuscriptwere reviewedby C. Bandy,T.L. War- 89:668-670. ren, R. Bunn, B.D. Rosenlund, K-M. Canestorp, M.G. NEWTON,I. 1979. Populationecology of raptors.Buteo Henry, L.L. Kinkel, B.A. Millsap,J. Marks,C.S. Houston, Books, Vermillion, SD U.S.A. D.C. Smith and an anonymousreviewer. OLENDORFF,R.R. ANDJ.W. STODDART,JR. 1974. The po- LITERATURE CITED tential for managementof raptor populationsin west- ern .Pages 47-87 in F.N. Hamerstrom.Jr., ANDERSEN,D.E. 1988. Common barn owl killed by a prai- B.E. Harrell and R.R. Olendorff [EDS.],Management rie falcon. Southwest. Nat. 33:377-378. of raptors.Raptor ResearchFound., Raptor Res.Rep. 1991. Management of North American grass- No. 2. lands for raptors. Pages203-210 in B. Giron Pendle- PICKWELL,G. 1948. Barn owl growth and behaviorisms. ton, D.L. Krahe, M.N. LeFranc,Jr., K_Titus, J.C. Bed- Auk 65:359-373. narz, D.E. Andersen and B.A. Millsap [EDs.], Proc. RESTANI,M. 1991. Resource partitioning among three Midwest Raptor Management Symposiumand Work- Buteospecies in the Centennial Valley, Montana. Con- dor 93:1007-1010. shop. Nat. Wildl. Fed. Sci. Tech. Ser. No. 15. ROTHEELS,M. ANDM.R. LEIN. 1983. Territoriality in sym- --, OJ. RONGSTADAND W.R. MYTTON. 1985. 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North American owls:biology and tor communities on the eastern . natural history. Smithsonian Institution Press,Wash- Great Basin Nat. 42:395-404. ington, DG U.S.A. , G.R. WII.SONAND H.H. FROST.1974. History and KNIGHT, R.L. AND R.E. JACKMAN.1984. Food-niche rela- ecologyof a colony of barn owlsin Utah. Condor76: 131-136. tionships between great horned owls and common barn owlsin easternWashington. Auk 101:175-179. STF•WAgT,P.A. 1952. Dispersal, breeding behavior, and longevity of banded barn owls in North America. Auk --AND D.C. SMITH. 1982. Summer raptor popula- 69:227-245. tions of a coulee. NorthwestSci. 56:303- WAYNE,A.T. 1924. A death trap to the American barn 309. owl ( Tytopratincola). Auk 41:342. MARSHALL,J.D., G.H. H^C,ER AND G. MCKEE. 1986. The barn owl egg:weight losscharacters, fresh weight pre- Received26 September1995; accepted15 February 1996