THE JOURNAL OF RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.

VOL. 33 SEPTEMBER 1999 No. 3 j. RaptorRes. 33(3):181-189 ¸ 1999 The Raptor ResearchFoundation, Inc.

NATAL AND BREEDING DISPERSAL IN BARN

CARL D. MARTI 1 Departmentof Zoology,Weber State University, Ogden, UT 84408-2505 U.S.A.

ABSTRACT.-•Istudied dispersal of the Barn (Tyt0alba) in northern Utah from 1977-96. Basedon 144 recoveriesof 2085 banded nestlings,the averagedispersal distance was 102.9 km (median = 60 km, range = 0-1267 km), occurredin most compassdirections from natal sites,but wasnot random with mountains,deserts, and the Great Salt Lake altering dispersalroutes. Dispersaldistance was not corre- lated with severityof winter weather nor population density.Among owls banded as nesdingsand re- capturedas breeders,females (N = 48) moved significantlyfarther (i = 61.4 km, median = 57.5 km, range = 0-160 km) than males (N = 34, i = 35.7 km, median = 14.7 km, range = 0.8-120 km, P = 0.015). Turnover of breeders at nest sites resulted mostly from individuals dispersinginto the study area. Only 19 (of at least 500) breedersmoved from one breeding site to another. The mean distance moved between breeding sitesof 2.3 km (median = 2.25 km) was not significantlydifferent between males and females (P -- 0.9), but more females (16) than males (3) made these moves.Eight of the adults that shifted breeding sitesdid so in the same year either after a failed first attempt (2) or to produce a secondbrood (6). The remainder changed nest sitesin subsequentyears. K•Y WOADS: ; alba; breedingdispersa• long-term study; natal dispersal;Utah.

Dispersi6nnatal y reproductivade Tytoalba PmSUMEN.--Estudi6la dispersi6nde Tytoalba en el norte de Utah desde 1977-96. Con base en 144 recapturasde 2085 pichonesanillados, encontr6 que la distanciade dispersi6nfue de 102.9 km (media = 60 km, rango = 0-1267 km), ocurridas en todas las direccionesdesde el sitio de nacimiento. Esta situaci6nno ocurri6 al azar en montafias,desiertos y el Great Salt Lake los cualesalteraron las rutas de dispersi6n.La distanciade dispersi6nno estuvocorrelacionada con la severidaddel clima invernal, ni con la densidad poblacional. Entre las lechuzasanilladas como pichonesy recapturadascomo re- productores,las hembras (N = 48) se movilizaronsignificativamente mas lejos (i = 61.4 km, media = 57.5 km, rango = 0-160 km) que los machos(N = 34, i = 35.7 km, media 14.7 km, rango = 0.8-120 km, P = 0.015). E1 regreso de los reproductoresa los sitios de los nidos, fue el resultado de individuos dispersadosdentro del area de estudio. S61o 19 (de por lo menos 500) reproductores se movilizaron de un sitio de reproducci6n a otro. La distanciapromedio recorrida entre los sitiosde reproducci6n fu• de 2.3 km (media = 2.25 km). Esta distanciano fue significativamentediferente entre machosy hembras (P = 0.9). Mas hembras (16) que machos (3) hicieron estosmovimientos. Ocho de los adultos que cambiaron sussitios de reproducci6n lo hicieron en el mismo afio despuesde fracasaren un primer intento (2) o para producir una segunda nidada (6). E1 resto cambi6 el sitio del nido en los aftos subsecuentes. [Traducci6n de C6sar M/trquez]

The Barn Owl (Tyto alba) is among the most pects of its biology closely resemble other owls widespreadof land , and although some as- (e.g., trophic biology), other attributesare striking- ly different. Among the important disparitiesare • Presentaddress: Boise State University, Raptor Research aspectsof the Barn Owl's reproductivebiology and Center, Boise, ID 83725 U.S.A. life-history (Marti 1997). Here, I show that dis-

181 182 MARTI VOL. 33, No. 3 persal in Barn Owls conformswith the species'r- selected life-history strategy (reproduction at an early age, short reproductive life, high reproduc- tive output, and an ability to find new resources-- sometimesat great distances--throughnatal dis- persal), but alsofits somepatterns of dispersalthat are widespreadin other birds. Dispersalis a very important but poorly under- stood element of population biology (Begon et al. 1990). Dispersalmay be either natal--the one-way \1 salt movement by an individual from its birthplace to a breeding (or potential breeding) site, or breed- ing--the movement by adults between breeding sites.Natal dispersalusually covers greater distanc- Figure 1. Location and topographic features of es than breeding dispersal (Greenwood and Har- OM study area in northern Utah. vey 1982). Advantagesattributed to natal dispersal include reducing the chance of inbreeding, reduc- ing competition,and extending the range (Green- persal occurred and look for supportthat dispersal wood 1983, Swingland1983). In many species, reduces inbreeding. dispersalpatterns differ between adults and juve- niles and between males and females (Greenwood STUDY AREA AND METHODS 1983, Greenwood and Harvey 1982). The studyarea was a narrow(12-25 km wide,500 km2) Relativelyfew studiesof dispersalhave been con- valley lying between the Wasatch Mountains and the ducted on raptors. See for example, Newton Great Salt Lake in Box Elder, Weber, and Davis counties of northcentral Utah (Fig. 1) that is close to the Barn (1979) and referenceswithin, Newton (1986) and Owl's northern range limit in the Intermountain Region Ferrer (1993) for European diurnal raptors, and (Marti 1992). The area was shrubsteppedesert but that Korpimfiki et al. (1987), Korpimfiki (1988), Kor- communityhas been entirely supplantedby irrigated ag- pimfiki and Lagerstr6m(1988), and Colesand Pet- riculture and urban development.Hot dry summersand cold winters characterizethe region; mean temperatures ty (1997) for Europeanowls. In North America,see for July andJanuary are 23.9øcand -3.5øC, respectively. Jacobs (1995), Woodbridgeet a1.(1995), Steenhof Barn Owl nestinghabitat is limited and disjunctin this et al. 1984, and Miller and Smallwood (1997) for area; most Barn Owls nest in lower elevation valleys diurnal raptors,and VanCampand Henny (1975), where irrigated agriculture occurs. Rugged mountains and high elevationvalleys immediately east of the study Adamcik and Keith (1978), Marks (1985), Bull et area were unsuitable Barn Owl habitat, and, likewise, the al. (1988), Belthoff and Ritchison (1989), Ganeyet Great Salt Lake and alkali desertsto the westof the study al. (1998), Gehlbach (1994), and Arsenault et al. area offered little habitat for Barn Owls. See Marti (1994) (1997) for owls. for more details on the studyarea and owl nest sites. Most of the Barn Owls on my studyarea nestedin nest Dispersal in Barn Owls has been studied in boxes (Marti et al. 1979). From 1977-96, I visited these North America (Stewart 1952), Europe (Frylestam nest boxesyear-round at leastonce per month. I made 1972, Sch6nfeld 1974, Glutz von Blotzheim 1979, additional visits as needed to band and color mark nest- Bairlein 1985, Baudvin 1986, Chanson et al. 1988, lings and adults with a standardUSGS aluminum band Taylor 1994, Martinez and L6pez 1995), and to a and a combination of colored plastic bands unique to each bird (two bands per leg) permitting identification very minor extent in Australia (Purchase1972). of individualswithout havingto recapturethem. Few oth- Only Taylor (1994) presenteddata on both the dis- er suitable nest sitesexisted on the studyarea, but owls persal of nestlings to breeding sites and move- occasionallynested in buildings and hay stacks.These ments of adults between nest sites. were often reported to me by farmers or by ownersof buildings havingvarious problems caused by the nesting Previously,I documented the reproductive pat- owls.Thus, I wasable to document nestingin these sites tern (Marti 1994) and lifetime reproductive suc- as well. cess(Marti 1997) in a Barn Owl population breed- I attempted to capture all breeding owlseach year to determine their identity, age, and movements.Most fe- ing close to the northern limit of its range. Here, malesand somemales were caughtby hand in nestboxes I present dispersalpatterns in the same popula- but, because males were less often found in nest boxes, tion, test whether sex and age differences in dis- I sometimes used nest-box traps to capture them (Sau- SEPTEMBER 1999 BARN OWL DISPERSAL 183

60 2o! Males

• 50 E • 40

'0 30 • lO

{= 2o Z 10

0 0.40 81-120 161-200 241-260 0.40 81-120 161-200

41-80 121-160 201-240 261+ 41-80 121-160 Dispersal distance, km Dispersal distance, km Figure 2. Dispersaldistances in Barn Owls banded as Figure 3. Comparison of dispersal distancesbetween nestlingsin northern Utah. breeding male and female Barn Owlsbanded asnestlings in northern Utah. rola 1987). For breeding owls not banded as nestlings, age wasdetermined by wing-moltpattern. Barn Owlsdo not molt any primaries until 13 months of age (P. Bloom recovered (either found dead or identified alive) pers. comm., Lenton 1984, Taylor 1993). Thus, in the at an average of 102.9 --- 162.03 (-SD) km from spring,breeding owlswith one generationof primaries their natal sites(median = 60 km, range = 0-1267 are in their first year of life, and thosewith two genera- km; Fig. 2). Among owls banded as nestlingsand tions of primary feathersare at least 2-yr old. I also in- cluded data from some nestlingowls that I banded on a recaptured as breeders,females (N = 48) moved site similar to my study area located in Cache County, significantlyfarther (i = 61.4 +--52.04 km, median Utah. Similarly,I useddata from severalBarn Owlsband- = 57.5 km, range = 0-160 km) than males (N = ed asnestlings by the Utah Divisionof Wildlife Resources 34, / = 35.7 +--36.61 km, median = 14.7 km, range m Utah County, Utah and recaptured in my studyarea. = 0.8-120 km; t = 2.48, df = 80, P = 0.015, power Barn Owls were nonmigratory in northern Utah as they appear to be in most if not all other parts of the species' = 0.66; Fig. 3). One female nestedin her natal site range (Schneider1937, Cramp 1985, Taylor1994). and two siblings that dispersedonly 8 km from Statisticalanalyses (t-tests and linear correlation)were their natal site paired and raised young. performed using the StatisticalAnalysis System (SAS Inst. Sixty-twoowls banded as nestlings were found 1988). Rayleigh'stest was used to checkfor uniformityin direction of owl dispersalafter the data were transformed dead off the studyarea at distancesof 7-1267 km into unimodal data (Zar 1984). Alphas for all testswere (i = 171.98 +__223.63 km, median = 109 km) from 0.05 and all tests were two-tailed. their natal sites.Sex was determined for only 17 of these and dispersaldistances were not significantly RESULTS different between sexesin this small sample (fe- Natal Dispersal.I banded 2085 nestlings(locals male, N = 8, i = 93.5 --- 63.5 km, median = 110.3, in USGS Bird Banding Laboratory terminology), range = 7-167 km; male, N = 9, / = 94.2 --- 90.44 384 breeding adults (adults) and 161 fledglings km, median = 52, range = 7-221 km; t = 0.02, df (hatch year) from 451 nestingattempts by at least = 15, P = 0.98, power = 0.98). individual Barn Owls. To exclude birds that may Owls dispersedin all compassdirections from have died before completing their dispersal,only their natal sites (Fig. 4), but the pattern of dis- those that were recovered >6 mo after fledging or persal direction was not random (Rayleigh'sz = after they beganbreeding were includedin the fol- 38.43, P < 0.0005, N = 82). The local topography lowing analyses. (Fig. 1) causedmany owls to move either to the Of thosebanded as nestlings,144 (6.9%) were north, northwest or to the south, southeast. Those 184 M•RTI VOL. 33, NO. 3

2 km 202kmkm

,... ß gnUeeasne:ligni:tiannaCenonit:alpo;ua;onn Owls

populationswere 100 km to the south and •100 km to the north and northwest.

Figure 4. Direction and distance of natal dispersalin Breeding Dispersal. Nineteen of at least 500 Barn Owls in northern Utah. breeders dispersed from one breeding site to an- other. The mean distance moved between breed- ing sites,2.28 --- 1.77 km (median= 2.25, range = that moved beyond the local topographicfeatures 0.1-6.2 km), did not differ significantlybetween dispersedin all directions(Fig. 5). No relationship males and females, but •5 times as many females was found between the year of fledging and the made those moves (female, N = 16, i = 2.3 + 1.63 distanceof dispersal(r = -0.08, P = 0.37, N = km, median = 2.3, range = 0.1-6.2 km; male, N = 135, power = 0.54). Likewise,the severityof a win- 3, i = 2.17 +- 2.87 km, median = 0.5, range = 0.5- ter (basedon ambient temperature and depth and 5.5 km; t = 0.12, df = 17, P = 0.90). Eight of the persistenceof snowcover) was not significantlycor- adultsshifted breeding sitesin the sameyear either relatedwith dispersaldistance (r = 0.07, P = 0.42, after a failed first attempt (N = 2) or to produce N = 137, power = 0.47). Populationdensity on the a secondbrood following a successfulfirst one (N study area did not appear to be a factor either; = 6). The others changed nest sitesin subsequent even though numbers of fledglings varied greatly years. among years (Marti 1994), the number fledged in DISCUSSION a year was not correlated with the distanceof dis- persal (r = -0.01, P = 0.89, N= 18, power = The natal dispersalthat I observedfollowed a 0.89). The distance moved from natal site to breed- pattern similar to that seen in other Barn Owl pop- ing site was not significantlycorrelated with life- ulations (Taylor 1994) with young dispersingsoon time breeding successin a 19-yr interval (success after fledging and making one-waymovements in = number of young fledged in lifetimes [Marti any direction from the natal site subject to geo- 1997]; r = 0.11, P = 0.32, N = 82, power = 0.70). graphic constraints.Distances were usually about Unbanded birds that became breeders on my 60 km but the longestexceeded 1000 km. Adults, studyarea provideda measureof dispersalinto the in contrast,tended to be sedentary,rarely moving area. On average,turnover of breeders at nest sites far from their breeding sites. was 48.1% (range = 21.4-75.0%/yr), mostlyindi- Stewart (1952) analyzed all banded Barn Owls vidualsdispersing into the studyarea. Only 23.3% recovered to 1950 in the U.S. Nestlingsbanded of first-time breedershad been banded as nestlings south of 35øN were all recovered within 144 km of on the studyarea (range = 0-93.8%/yr). The re- the banding site. Those banded north of 35øN maining 76.7% (range = 6.2-100%/yr) were un- moved farther: 61% moved •80 km, 28% •320 km banded, apparently having been raised outsidethe and 1% •1600 km. Dispersal,even in the north, study area (Fig. 6). The nearest known breeding was in all directions. Other Barn Owls have been SEPTEMBER 1999 BARN OWL DISPERSAL 185

• Total annual turnover of breeders I I New breeders banded as nestlings F22P222-• New breeders unbanded 80

70 so E so

E 40 P 30

20

lO

1978 1981 1984 1987 199( 1993 1996 Year Figure 6. Annual turnover rates of breeding Barn Owls in northern Utah. recovered in the U.S. after the 1950s over •1000 similarities. Newton (1979) noted that numerous km from their banding sites(Broun 1954, Mueller diurnal raptors in Europe rarely dispersed•50 km and Berger 1959, Bolen 1978, Soucy1985). and that femalesdispersed farther than males.The In the United Kingdom, natal dispersalwas rel- most comprehensivestudy of dispersalin a diurnal ativelyshort; only one individual banded as a nest- raptor wasNewton's (1986) studyon the Sparrow- ling in Scotlandmoved •20 km to a breeding site (Accipiternisus). Female Sparrowhawksdis- (Taylor 1994) and the longestdispersals were •200 persed significantlyfarther from their natal areas km (Bunn et al. 1982). In continentalEurope, dis- than did males and both sexes moved in all direc- persals•1000 km were reported from Barn Owl tions. Most of the natal dispersaloccurred in late populationsin France (Baudvin1986) and Switzer- summer,and population densitydid not seem to land (Glutz von Blotzheim 1979). Over 50% of affect dispersal.Dispersal distanceswere shorter nestlingsbanded in Germany bred at distances than in Barn Owls (•1-265 km) and 75% settled •50 km from their site of birth, but 24% were re- within 20 km of their natal site. Newton did not covered at distances •100 km (Bairlein 1985); record any inbreeding in Sparrowhawks.Breeding movementswere shorter in high yearsthan in dispersalby Cooper'sHawks (Accipitercooperii) also low vole years (Sch6nfeld 1974). In Spain, natal resembledthe pattern I found in Barn Owls.Male dispersal covered significantly greater distances Cooper'sHawks did not changebreeding sites,but than did breeding dispersal(Martinez and L6pez a few females moved short distances to new sites 1995). Natal dispersaloccurred in all compassdi- (Rosenfield and Bielefeldt 1996). rectionsin Europe, even in Scandinavia(Frylestam American kestrels (Falco sparverius)in Florida 1972) which, like Utah, is at the northern edge of dispersedout of their natal territoriesbut distances the Barn Owl's range. In Australia, two nestlings were short (71% were •8 km) and the sexesdid were recovered 250 and 840 km from their nests not differ significantly in distance (Miller and (Purchase 1972). Smallwood1997). In Wisconsin,natal dispersalby Dispersalstudies of other raptors reveal many kestrelswas much greater and males dispersedfar- 186 M_ARTI VOI•. 33, NO. 3 ther than females (Jacobs 1995). Natal dispersal lings that moved only 5.4 km from their natal site, was not sex-biased in Lesser Kestrels (Falco nau- and another between a mother and son. Incest and manni) and 57% settled to breed in their natal col- closeinbreeding have been reported only rarely in onies. Those that dispersedmoved on averageonly other raptors (VanCamp and Henny 1975, Bow- 18.5 km (Negro et al. 1997). Swainson'sHawks man et al. 1987, James et al. 1987, Postupalsky (Buteoswainsoni) moved on averagejust 8.2 km (0- 1989, Millsap and Bear 1990, Rosenfieldand Bie- 18.1 km) between natal and breeding sitesand dis- lefeldt 1992, Taylor 1994, Gutitrrez et al. 1995, and tances were not significant between the sexes Carlson et al. 1998). It is not clear whether this low (Woodbridge et al. 1995). Natal dispersalin a small level of reported inbreeding is due to the difficulty sample of Ospreys(Pandion haliaetus) averaged 441 of detecting it or to a truly low level of occurrence. km (Johnson and Melquist 1991). Natal dispersalmay aid in range expansionand Dispersalin owlshas not been well documented, repopulation of areas where extinction has oc- but most other speciesappear to move shorter dis- curred. The Barn Owls' ability for long-distance tancesin natal dispersalthan do Barn Owls. Me- dispersalcoupled with their versatilityin nest-site dian distance moved by radio-tagged Eastern and foraging habitat have permitted them to ex- Screech-Owls(Otus asio) from natal siteswas only pand their range particularly in responseto hu- 4.4 km (0.4-16.9) (Belthoff and Ritchison 1989). man-causedhabitat changes (Brown 1971, Reese Also in Eastern Screech-Owls,Gehlbach (1994) re- 1972, Stewart1980, Lenton 1985, McLarty 1995). corded a mean natal dispersal of 3.2 kin, but Barn Owls probablydid not nest on my studyarea VanCamp and Henny (1975) gave 32 km as the until humans provided nesting places (buildings) mean natal dispersaldistance. However, about half and increased food availability through irrigated of their birds dispersed<16 kin. Mean dispersalby agriculture. Great Gray Owls (Strix nebulosa)was 18.5 km (7.5- Even though I document long-distancemove- 32; Bull et al. 1988), but Tengmalm's Owls (Aego- ments by Barn Owls out of my studyarea, I do not liusfunereus) in Finland dispersed on average 55- have any data on the reproductivesuccess of those 70 km (0-320 kin) dependingon the stageof the individuals. However, several individuals that were vole population cycle (KorpimSkiand Lagerstrtm banded as nestlings made long-distance move- 1988). (Bubovirginianus) fledg- ments out of my study area and were recovered lings from northern populationsmoved up to 1305 km from their nests but 53% were recovered within severalyears later (one 12-yr old), making it likely 25 km (Adamcik and Keith 1978). A few male that they did reproduce. Martinez and Ltpez (1995) considered long-distancedispersal by Barn Long-eared Owls (Asio otus) were known to nest Owls in Spain to be a disadvantage,and Newton within 2.0 km of their natal nest, but females ap- and Marquiss(1983) showedthat reproductivesuc- parently dispersedfarther than males before nest- ing (Marks et al. 1994). Dispersalin Spotted Owls cessfor Sparrowhawksthat dispersedfurthest was less than those that moved shorter distances. I have (Strix occidentalis),despite recent intense study of the species'biology, is poorly known. Arsenaultet considerabledata on reproduction by individuals al. (1997) and Ganey et al. (1998) radiotracked dispersinginto my area, but no knowledgeof the fledgling Mexican Spotted Owls to distances of origin of most of them. 2.1-73.5 kin, but only one individual was tracked Dispersal may also speed the flow of genes to a breeding territory at 5.8 km from its natal site. among breeding populations, but almost nothing Dispersingjuvenile Northern Spotted Owls were is known about this in Barn Owls or other raptor tracked from 20-98 km, but none were traced to a species.McLarty (1995) compared genetic similar- breeding territory (Gutitrrez et al. 1985). ity among three Barn Owl populations in British Distance and direction of the natal dispersalI Columbia, northern Utah and southern California, found in northern Utah were effective in reducing and found sufficient genetic differencesto suggest inbreeding. The only known inbreeding in my that little gene flow occursbetween these popula- population occurred when dispersaldistance was tions. Utah and California populationswere more short and siblingsfrom the samebrood paired and similar to each other than the British Columbia raisedyoung. Another female bred in her natal site population was to either suggestingmore east-west but her mate was not identified. Shaw and Dowell than south-north movement by dispersing owls. (1989) found one instanceof pairing between sib- The British Columbia population is at the north- SEPTEMBER 1999 BA• OWL DISPERSAL 187 ern limit of the species'range and is of relatively are they related? Pages 145-150 in D.M Bird and R. recent origin. Bowman [EDS.], The ancestral kestrel. Raptor Res. My results offer little evidence that natal dis- Rep. No. 6, Raptor Research Foundation, Hastings, MN U.S.A. persal relieves competition becauseI have repro- ductive data only on birds that moved relatively BROUN,M. 1954. Long-distancerecovery of Barn Owl. Bird-Banding25:149. short (for Barn Owls) distances between natal and BROWN,L. 1971.African birdsof prey.Houghton Mifflin, breeding sites.I was able to showthat lifetime re- Boston, MA U.S.A. productive successwas not related to distanceof BULL, E.L., M.G. HENJUM AND R.S. ROHWEDER. 1988. dispersalup to the dispersaldistances I wasable to Home range and dispersal of Great Gray Owls in track, and that dispersaldistance was not related northeastern Oregon. J. RaptorRes. 22:101-106. to population density. BUNN, D.S., A.B. WARBURTON AND R.D.S. W1LSON. 1982 Dispersalin Barn Owls in northern Utah con- The Barn Owl. T. & A.D. Poyser,Calton, U.K. formed to the patterns seen in many birds with CARLSON, P.D., W.S. LAHAYE AND A.B. FRANKLIN. 1998. In- natal dispersal covering much greater distances cestuousbehavior in Spotted Owls. WilsonBull. 110: than breeding dispersal,and females dispersing 562-564. farther than males. Natal dispersalapparently was CHANSON,J.M., P. COURBET,P. GIRAUDOUX,G. MICHAUD effective in reducing inbreeding, becausethe dis- ANDO. MICHELAT.1988. •tude surla reproductionet tance and randomness of the direction of natal dis- les dCplacementsde la Chouette effraie (Tyroalba) en Franche-Comte: r•flexions m•thodologiques. Alauda persal made pairings by closerelatives unlikely. 56:197-225.

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