j. RaptorRes. 34(4):299-304 ¸ 2000 The Raptor ResearchFoundation, Inc. ARE NORTHERN SAW-WHET OWLS NOMADIC? JEFFREYS. MARKS MontanaCooperative Wildlife Research Unit, Universityof Montana,Missoula, MT 59812 U.S.A. JOHN H. DOREMUS U.S.Bureau of Land Management, Boise District, 3948 Development Avenue, Boise, ID 83705U.S.A. ABSTRACT.---Thefirst knownnesting of a Northern Saw-whetOwl (Aegoliusacadicus) in the SnakeRiver Birdsof PreyNational Conservation Area occurredin a nestbox in 1986,4 yr after nestboxes were constructedin the studyarea. Occupancy of nestboxes by Northern Saw-whetOwls varied substantially overthe next 13 yr (0-8 nestsper yr). The numberof mice countedon nocturnalsurveys fluctuated widelyduring this same period, and the annualnumber of NorthernSaw-whet Owl nestsin the boxes waspositively correlated with an indexof mouseabundance. Only one of the 52 breedingadults that we bandedwas recaptured in a subsequentyear, and none of the 139 nestlingsproduced in the boxes wasreencountered. A male that we bandedat a nestin April 1990 wasfound dead in BritishColumbia in January1993, more than 900 km NNW of our studyarea. Data from the Bird BandingLaboratory were insufficientto evaluatebreeding-site fidelity because few researchershave banded adult Northern Saw-whet Owls at nests. Northern Saw-whet Owls seem to exhibit some of the characteristics associated with nomadismin birds (e.g., high fecundityand low survival),but they differ from typicalnomadic speciesbecause they do not specializeon cyclicprey. We suggestthat Northern Saw-whetOwls are nomadicin someparts of their range,settling to breed in areasof high food abundancethat they encounter during the nonbreedingseason. KEYWOP, DS: NorthernSaw-whet Owl; Aegolius acadicus; nomadism; breeding biology; Idaho. Es Aegoliusacadicus una especienomada? RES0MEN.----E1primer registro de un Aegoliusacadicus en anidacitnen el/treade conservaciondel Snake Riverocurri6 en una cajade anidacitnen 1986,4 aftosdespu•s que lascajas fueron construidasen el firea de estudio.La ocupacitnde las cajaspor los buhosvari6 substancialmenteen los prtximos 13 aftos.(0-8 nidospor afio). E1nfimero de ratonescontabilizados de nochefluctfio ampliamente durante estemismo periodo y el nfimeroanual de nidosde buhosen cajasfue positivamentecorrelacionado con el indice de abundanciade ratones. Solo uno de los 52 adultos en reproduccitn anillado, fue recapturadoy ninguno de los139 pichones producidos en lascajas de anidacitnfue reencontrado.Un machoque fue anilladoen el nidoen abrilde 1990fue encontradomuerto en ColumbiaBriffmica en enero de 1993, a mas de 900 km NNW, del area de estudio. Los datos del laboratorio de anillacion fueroninsuficientes para evaluar la fidelidadal territoriode reproduccitndebido a que pocosinvesti- gadoreshah anilladoadultos de Aegoliusacadicus en losnidos. Aegolius acadicus parece mostrar algunas de las caracteristicasasociadas con el nomadismoen las aves(e.g., alta fecundidady baja sobrevivencia), pero difierende las especiesntmadas tlpicas debido a que no se especializanen una presaciclica. Sugerimosque los buhos son ntmadas en partede surango, estableci•ndose para reproducirse en fireas de alta abundanciade cornidaque encuentrandurante la estacitn no reproductiva. [Traduccitn de C•sar Mgrquez] The raptorsnesting in the SnakeRiver Birdsof Burrowing Owl (Athenecunicularia), Long-eared Prey National ConservationArea (NCA) in south- Owl (Asiootus), and Short-eared Owl (A. flammeus). western Idaho have been under intensive study The first three speciesare permanent residentsin since the mid-1970s (USDI 1979). Through 1985, the NCA, the Burrowing Owl is a typical migrant, six speciesof owlswere knownto nestin the area: and the two speciesof Asioare year-roundresidents Barn Owl (Tyto alba), Western Screech-Owl(Otus in someyears and migrantsin others (J.S. Marks kennicottii), Great Horned Owl ( Bubovirginianus) , and J.H. Doremus pers. obs.). 299 300 MARItS AND DOREMUS VOL. 34, NO. 4 The first known nest of a Northern Saw-whet suggestthat Northern Saw-whetOwls are nomadic Owl (Aegoliusacadicus) in the NCA occurred in a in southwestern Idaho. If we are correct, then our nest box in 1986, 4 yr after boxeswere constructed study birds would constitute the only known ex- in the study area. In marked contrast to the six ample of nomadism in a strigid that does not spe- speciesof owlsthat occurregularly in the NCA, the cialize on cyclicprey. presence of nesting Northern Saw-whetOwls var- STUDY AR•A AND METHODS ied substantiallyover the next 13 yr (0-8 nestsper yr). This variation in numbers, combined with a We studied breeding Northern Saw-whetOwls in the NCA in southwestern Idaho from 1986-99. The NCA is nearly complete lack of recapturesof adults,led us a shrubsteppedesert dominated by big sagebrush(Arte- to speculate that Northern Saw-whetOwls are no- misia tridentata).Compared with typical breeding habitat madic in the NCA. tbr Northern Saw-whet Owls (i.e., coniferous forest), Many speciesof birds exhibit strong breeding- trees are scarce and are confined to riparian areas and site fidelity, remaining in or returning to the same human settlements. All of the owls that we studied bred in nest boxesplaced in willows (Salix spp.), Russianolives breeding placesyear after year (Andersson1980). (Elaeagnusa%mstifolia), and black locusts(Robinia pseu- Site fidelity may be adaptivebecause it allowsin- doacacia).We set out boxes in pairs (1-40 m apart) be- dividualsto learn the best placesto feed, nest, and ginning in 1981. Since 1986, when Northern Saw-whet avoid predators, which in turn may enhance the Owls first nested in one of our boxes, two or more boxes have been availableat 95-47 siteseach year along 95 lin- ability of territory holders to attract mates (Hinde ear km of the Snake River and its tributaries. 1956, Greenwood and Harvey 1976, Greenwood To assessthe availability of small mammals, we con- 1980). Nomadism (i.e., lack of breeding-site fidel- ducted nocturnal surveysin the NCA each spring from ity) in birds is much lesscommon than site fidelity 1984-94 (see Marks et al. 1989). With the aid of a spot- and tends to be restricted to speciesthat feed on light, observersin a slowly moving vehicle counted all "mice" seen along 547-709 km of secondary roads. cyclic prey or for which environmentalvariation Basedon thesesurveys, we calculatedan index of mouse (e.g., periodic rains) resultsin large fluctuationsin numbersby dividing the total number of mice seen by the availability of suitable breeding habitat (An- the total length of roads surveyedeach year.No surveys dersson 1980, 1981). were conducted after 1994. The classicexamples of nomadismin birds come We captured breeding female owls in nest boxesdur- from boreal seedeaters such as finches and cross- ing the brood-rearingperiod and caughtmales at night in mist nets set in front of the boxes. We determined the bills that move large distances in response to sex of adults by the presence (females) or absence changing availability of beechmast and conifer (males) of an incubation patch. Adults were weighed, seeds (Newton 1972). In owls, nomadism is best measured, and banded at first capture, and nestlings were banded about a week before they left the nest. known in vole specialistssuch as Boreal Owls (Ae- We used partial correlation analysisto assessthe rela- goliusfunereus), Long-eared Owls, and Short-eared tionship between the mouse index (log transformed) Owls (Wallin and Andersson1981, Village 1987, and the number of owl nests in the boxes and to see Korpimfiki and Norrdahl 1991), although docu- whether the number of Northern Saw-whet Owl nests was correlated with the number of Western Screech-Owl mentation of the sameindividuals breeding in vast- nests.All testswere one-tailed becausewe predicted that ly different locationsis rare. Severalfemale Boreal numbers of nesting owls of both specieswould be posi- Owls have been captured at nestsmore than 500 tivelycorrelated with the mouse index. Similarly,we pre- km apart in different years (Wallin and Andersson dicted that the number of nesting Northern Saw-whet 1981, Korpimiiki et al. 1987), but the extent of no- Owls would be negativelycorrelated with the number of nestingWestern Screech-Owls in the boxesbecause West- madism in this speciesvaries widely among popu- ern Screech-Owlsare permanent residentsin the NCA, lations, and individuals may remain on the same and their body massis two to three times that of North- home ranges for two or more years in succession ern Saw-whetOwls (J.S. Marks andJ.H. Doremus unpubl. (Korpimfiki et al. 1987, Haywardet al. 1993). In data). short, comparedwith "classic"nomadism exhibit- RESULTS ed by boreal seedeaters, the owl species noted above seem to be "periodically" nomadic. The number of Northern Saw-whet Owl nests in The Northern Saw-whet Owl is common in for- our boxes varied considerablyamong years,rang- ested habitats across the northern United States ing from zero to eight (• = 2.7 ñ 3.09 [ñSD]) and southern Canada. Despite its abundance, rel- (Table 1). In contrast, Western Screech-Owls nest- atively little is known about its breeding biology ed in the boxes every year (range 4-13), and the (Cannings 1993). In this paper, we present data to number of nests per year was larger (• = 8.8 ñ DECEMBER 2000 NOMADISM IN NOR•rHEaN SAW-WHET OWLS 301 Table 1. Number of Northern Saw-whet Owl and West- 10 ern Screech-Owl nests in boxes in the Snake River Birds of Prey National ConservationArea, 1984-99. Pairs of • 8 nest boxes were availableat 25-47 siteseach year. The number of adult Northern Saw-whetOwls captured at nests each year is in parentheses. NORTHERN WESTERN SAW-WHET SCREECH- OWL OWL 1984 0 7 ß ß 1985 0 5 ß ß ß ß ß ß 1986 1 5 1987 7 (11) 8 2 4 6 8 10 12 14 1988 0 4 NO. of screech-owl nests 1989 0 6 1990 8 (11) 9 Figure 1. Relationship between the number of North- 1991 6 (10) 11 ern Saw-whet Owl nests in boxes and the number of West- 1992 7 (5) 13 ern Screech-Owlnests in boxes,Snake River Birds of Prey 1993 1 13 National ConservationArea, 1984-99. Only 15 points are 1994 0 8 shown because two were identical.