Selection and Use of Nest Sites by Barn Owls in Norfolk, England

j Raptor Res. 28(3):149-153 ¸ 1994 The Raptor ResearchFoundation, Inc.

SELECTION AND USE OF NEST SITES BY BARN OWLS IN NORFOLK, ENGLAND

PAUL N. JOHNSON The Durrell Instituteof Conservationand Ecology,The University,Canterbury, Kent CT2 7NX United Kingdom

AI•STR•CT.--Between1989 and 1993 reproductionwas monitoredat 96 barn owl (Tyro alba) breeding sitesin England. Nestswere locatedin tree cavities,buildings, and nest boxesin farm buildingsand maturetrees. Many of the sitesother than nestboxes used by owls were of human origin, but the number of thosedeclined during the courseof the study due to deteriorationof human-madestructures and competitionfrom otherspecies. Nest boxesincreased the populationdensity from 15 breedingpairs/100 km2 to 27 pairs.Over the studyperiod, pairs usingnest boxes produced significantly larger clutchesthan at other sites,but the numberof fledglingswas not significantlydifferent amongtypes of nest sites. KEY WORDS: barn owl; breedingecology; England; human-altered habitats; nest boxes; Tyro alba.

Selecci6ny usode sitiosde nidificaci6npor Tyroalba en Norfolk, Inglaterra RESUMEN.--Entre1989 y 1993 semonitore6 la reproducci6nen 96 sitiosreproductivos de Tyroalba. Los nidosse localizaron en cavidadesde firboles,edificios, cajas anideras en construccionesagricolas y firboles madufos.Mucho de estossitios, aparte de las cajasanideras, usados por T. alba fueron de origen humano, peroel nfimerode ellosdeclin6 durante el cursodel estudiodebido al deteriorode las estructurasartificiales y a la competenciacon otras especies.Las cajasanideras incrementaron la densidadpoblacional de 15 parejasreproductivas/100 km 2 a 27 parejas.En el periodode estudio,las parejasque usaronlas cajas aniderasprodujeron nidadas significativamentemils grandesqueen otros sitios, pero el nfimero de volantonesno fue significativamentediferente entre los distintostipos de sitios. [Traducci6n de Ivan Lazo]

The barn owl (Tyro alba) is currentlyclassified box studieshas been criticized by Moller (1989, as vulnerable in northwest Norfolk, England by 1992). Shawyer(1987) who recordeda 66% declineto just The primary objectivesof this studywere to mon- 82 breedingpairs and a densityof 2.9 pairs per 100 itor and compare reproductive levels between nat- km2 from the 240 pairs and 8.4 pairs per km2 re- ural and nest-boxbreeding sites, and to promotea cordedby Blaker (1933). A surveyof this area found sustainableexpansion of breedingbarn owls by ex- that 60% of the barn owl population used natural tendingthe range and numberof nestingsite types. tree cavitiesfor nesting (Johnson 1991). Regional METHODS trendsin the type of nestingsite selectedby a barn owl population have been reported in Bunn et al. The study area (10 x 15 km) was intensivelyfarmed (1982). in individual units ranging from 300-15 000 ha. Principle cropswere winter and springcereals, sugarbeets, and rough Artificial nestingsites have beenreadily usedby grassland. owlsand havebeen widely usedin long-termstudies Nest siteswere locatedduring 1989-90 by methodically of owls (Southern1959, 1970, KorpimSki and Sul- searchingall possiblestructures, interviewing landowners, kava 1987, Saurola1989). By providingnest boxes, and by following birds to their nest sites.The locationsof all potentialnatural nestingsites were recorded,together the breeding population densitiesof owls were in- with details of their dimensions. creasedabove the levels previouslythought to be Nest boxes placed in buildings (N -- 43) were con- limited by the availability of natural sites.Lenton structedfrom 8 mm exterior grade plywood and located (1978) demonstratedthat the breedingdensity of in suitably safe and quiet siteson every farm within the study area. Tree-mountednesting boxes (N = 17) were previouslyrare barn owls in Malaysia couldbe in- made from 19 mm exterior grade plywood for increased creasedwith the provisionof nestboxes. On the other thermal insulation becausethis speciesis known to be hand, the naturalness of results obtained from nest- sensitiveto extreme cold (Johnson1974). Tree-mounted

149 150 PAULN. JOHNSON VOL. 28, NO. 3

Table 1. Reproductionby barnowls at four nestsite types in Norfolk, England.Tabular valuesare percentagesor mean _+ 1 SD. Numbers of nest typesare in parentheses.

REPRODUCTIVE NESTSITE TYPES PARAMETERS TREE Box (8) BUILDINGBOX (43) TREE CAVITY(28) BUILDING(14) Percent of site use 29.4 25.6 57.1 10.6 Clutch size 5.4 _+ 1.8 5.0 _+ 1.4 4.3 _+ 1.3 3.9 _+ 1.1 Brood size 4.1 _+ 1.7 3.7 _+ 1.2 3.3 _+ 1.2 2.9 _+ 1.4 Fledglings 4.1 _+1.7 3.1 _ 1.1 2.9 _ 1.2 2.6 _ 1.3 Fledglings/eggs 75.0 _ 9.2 65.8 _+21.3 66.3 _+18.5 64.9 _ 25.1

boxesreplicated the darknessfound in deepernatural tree were determinedat all sites. The number of young of cavitiesby incorporatinga baffle to shield the entrance fledgingage was recordedduring visitsto band youngat hole from the nestingchamber and increasethe protection 5-6 wk of age. of birdsfrom weatherand predators(Johnson 1990). Tree Data were analyzedfor differencesin selectionof site nestboxes were erectedin areaslacking suitable buildings typesand productivitybetween natural nestingsites and and were mountedon old tree stumpsand telegraphpoles nestingboxes using Chi-square analysis. Because the data 3 m abovethe ground. All boxeswere lined with wood for the reproductiveefforts were not normally distributed, chips. dataon clutch,brood, number of fledglings,and fledgling/ All siteswere monitored at 8-wk intervals throughout egg amongthe four site typeswere comparedusing the the year. The frequencyof visitswas increasedto every 3 Kruskal-Wallis test. Comparisonsof clutch, brood, and wk during the breedingseason from March to September fledglings/broodproduced in natural sitesand in all types at occupiedsites. Egg clutch size and initial brood size of boxeswere done using the Mann-Whitney U-test.

12'• INBUILDING [• TREE CAVITY • BOX(BUILDING) • BOXTREE):

10-

UJ 6-

Z 4-

1989 1990 1991 1992 1993

YEAR Figure 1. Nest sitesavailable in the studyarea. SEPTEMBER 1994 BARN OWL NEST SITES IN ENGLAND 151

700 •

600 i No.EGGS HATCHEDLAID No.FLEDGED 500 400 300,

200 100

O' TREE CAVITIES. NATURALSITES. BOX(BUILDING) BOX(TREE)

SITE TYPES

Figure 2. Nest site use by barn owls, 1989-93.

RESULTS AND DISCUSSION number of eggslaid) in this study was 66.6% (SD = 20.3), which comparesfavorably with an average Site useand reproductivedata are shownin Table of 62.5% (SD = 14.8) for 11 other speciesof cavity- 1. Tree cavitieswere usedat a higher rate than non- nestingowls (F.R. Gelbach pers. comm.). box sitesin buildings(P = 0.06). The combineddata My early surveyresults (Johnson 1991) showed for nest boxesversus all other types showedno sig- a 58% use of large tree cavitiesand only 31% use of nificant differencein selectionrelative to availability buildings by breeding owls in comparisonto a na- (X2 = 0.3, NS). Tree boxesproduced more eggs,but tional trend of 64.6% nesting in buildings and 24% not significantlymore fledglingsthan other sitetypes in trees (Shawyer 1987). Within the studyarea, the (Kruskal-Wallis, H -- 9.6, P = 0.02 for clutch; H three pairs nestingin buildingsused lofts in derelict = 6.3, P > 0.08 for brood and fledglings). housesor abandonedmilitary buildings. These find- Significantdifferences were found in the clutch ingsare in line with other studiesin the East Anglian and broodsizes and the numbersof young produced region.Cayford (1992) alsofound a high proportion betweennest boxesand other sites,but productivity of nest sitesin tree cavities;only buildingswith lofts of fledglingsin relation to clutch sizeswas compa- or nest boxeswere usedas breedingsites. Such fea- rable between natural and artificial sites, (Mann- tures and natural sites were scarcein the area prior Whitney U = 802, P < 0.04 for the first three, but to this study (Fig. 1). Tree cavitieswere usedin ash U = 1005, NS for productivity).Therefore, box sites (Fraxinusexcelsior; N = 5), oak (Quercusrobur; N apparentlydo producea greaterreproductive effort, = 5), elm (Ulmusprocera;N = 3), and beech(Fagus but not necessarilya greater reward. Overall, pro- sylvatica;N = 1). The low annual rainfall in the ductivityof fledgedyoung (number of youngfledged/ region (• = 65 cm comparedto 100-300 cm nation- 152 PAULN JOHNSON VOL. 28, No. 3

50' • % BREEDPOE •- 45- [T• % MEANOUTPUT o 40- z :,•',<'*,sq•'::,'g•:*'/,½1111111It111111 ca 35- ,';/;¾/#, =: 30- z 25-

,,, 20- z •5-

::3 5- O a. 0 TREE CAVITY NATURAL SITES o BOX(BUILDING) BOX(TREE)

TYPES OF SITE

Figure 3. Reproductiveperformance by barn owls in four typesof nestsites.

ally) is thoughtto be a critical factorallowing such the greatestimpact on the densityof suitablenatural open structuresto be used successfully(Shawyer breedingsites during this study. The observedtrend 1987). The lossof elms to diseaseacross the region for nesting barn owls was an increaseduse of nest has significantlyreduced the availability of tree cav- boxesand a declinein the availability of natural tree ities. Osbourne and Krebs (1981) estimated a loss cavities(Fig. 2). Eleven (25%) of the 43 nest boxes of over 11 million trees nationally, which has in- erected in buildings were used, together with five creasedcompetition among other cavity-nestingspe- (41%) of the 17 tree boxes,increasing the breeding cies. populationdensity within the studyarea from 23 to The tawny owl (Strix aluco),often viewed as a 41 pairs, an increaseof 178% during the 5 yr of the nest-sitecompetitor with the barn owl (Shawyer study. Much like the situationdescribed by Lenton 1987), onlyused one box during this study.Whereas, (1978), the increasedbarn owl population recorded thejackdaw (Corvusmonedula) proved to be a major in my studyis attributableto the wider availability competitorfor tree cavitiesand tree nest boxesre- of goodquality nest sites.Nesting siteswere appar- quiring the annualremoval of jackdaw nest material ently a limiting factoron the populationand without which would otherwiserestrict accessby barn owls. the provisionof nest boxesthe deterioration of nat- Predationby a stoat(Mustela erminea) caused the ural sitesavailable would have reducedthe density permanentdesertion of one nestbox in a building, of breeding pairs. and deliberatehuman disturbance in buildingscaused Data for all site typesclearly showa reductionin the lossof clutchesduring incubation. broodsize from the numberof eggslaid (Fig. 3 and The widespreadfelling of decayingtrees has had Table 1). This was attributable to the common oc- SEPTEMBER 1994 BARN OWL NEST SITES IN ENGLAND 153 currenceof infertile eggsin clutches.A further re- LITERATURE CITED ductionbetween hatching and fledgingwas recorded BLAKER,G.B. 1933. The barn owl in England--results for all sitesexcept tree boxes. Partial broodmortality of the census.Bird Notes News 15:69-172; 207-211. is oftena symptomof poorfood supply to the brood BURN, D.S., A.B. W•mBURTONAND R.D.S. WILSON. 1982 (Southern1959) but was attributableto predation The barn owl. T. and A.D. Poyser, Calton, U.K. in three present cases. CAYFORD,J. 1992. Barn owl ecologyon East Anghan Taylor and Massheder(1992) using data from farmland. Royal Society for the Protection of Birds Conservation Review 6. southwestScotland, modeled a populationthat re- JOHNSON,W.D. 1974. The bioenergeticsof the barn owl quired a mean reproductiveoutput of 3.2 young per (Tyto alba). M.S. thesis,California State Univ., Long pair to maintain a population and 3.5 young per Beach, CA U.S.A. pair to producea sustainedpopulation growth. If JOHNSON,P.N. 1990. Experimenting with nestboxes, applicableto this study area, only the pairs using Norfolk barn owl boxes. Br. Trust Ornithol. News No. nest boxeswere achievingthis level. 169:5. To safeguardthis threatenedspecies, nest boxes 1991. Barn owls in North Norfolk. Norfolk Bzrd needto be correctlylocated and maintained in order Mammal Rep. 1990:91-95. to increasethe potentialbreeding density determined KORPIM,•KI, E. AND S. SULKAVA. 1987. Diet and breed- by foragingarea and prey density.For the mid- to ing performanceof Ural owls under fluctuating food conditions. Ornis Fenn. 64:57-66. long-term future in this area, conservationistsmust protectand enhancethe remaining habitat and re- LENTON, G.M. 1978. Owls as rat controllers--a pre- liminary report. Planter, Kuala Lurepar 54:72-83. stricttree felling acrossthe region.A revival of his- MOLLER,A.P. 1989. Parasites,predators and nestboxes torictree management, particularly pollarding, would facts and artefacts in nest box studies of birds? Oikos stimulate the growth of trees with widened trunk 56:421-423. diameters and provide spaciousnatural cavities in 1992. Nest boxes and the scientific rigour of time. Within this intensivelyhuman-manipulated experimental studies. Oikos 63:309-311. environment,a comparisonbetween "natural" and OSBOURNE,L. AND J. KREBS. 1981. Replanting after man-made structuresis not valid. Many natural Dutch elm disease. New Scientzst 90:212-215. structuresare in unnatural locations,only the qual- S^UROL^,P. 1989. Breeding strategy of the Ural owl ity of the structureand surroundingenvironment (Strix uralensis).Pages 235-240 in B.-U. Meyburg and varies and perhaps directly influencesthe nesting R.D. Chancellor[EDS.], Raptors in the modernworld, World Working Group on Birds of Prey and Owls successand density of the breedingbarn owl pop- Berlin, Germany. ulation. Only speciesthat build their own nestscan SHAWYER,c.j. 1987. The barn owl in the British Isles, locatein optimum foragingconditions. By selecting its past,present and future. Hawk Trust,London, U.K nestboxes in treesor buildingsbarn owls are adapt- SOUTHERN,H.N. 1959. Populationecology, mortality ing to an adequate substitutefor lossesof natural and population control. Ibis 101:429-436. sitesand showreproductive output equivalent to that 1970. The natural control of a population of of other sites. tawny owls (Strix aluco). J. Zool. (Lond.). 162:197- 285. T^¾LOR,I.R. ^ND J. MASSHEDER. 1992. The dynamics ACKNOWLEDGMENTS of depletedand introducedfarmland barn owl (Tyro I would like to acknowledgethe considerablesupport alba) populations, a modelling approach. Pages 105- and assistance I have received from Fred Gehlbach in the 110 zn C.A. Galbraith, I.R. Taylor and S. Percival analysisof my data and for his guidancein preparingthis lEDs.I, The ecologyand conservationof Europeanowls. manuscript,and the supportof Colin Shawyerthroughout the yearsof field work. The Hawk and Owl Trust grate- Joint Nature ConservationCommittee, Peterborough, fully acknowledgesthe Goldsmiths Trust, the Mac- U.K. Taggatt Third Fund and Hilleshog U.K. Ltd., for their support of this conservationproject. Received3 December1993; accepted25 May 1994