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

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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 number of 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
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