Review of Technical Knowledge: Boreal Owls

Chapter 9 Review of Technical Knowledge: Boreal Owls Gregory D. Hayward, Rocky Mountain Forest and Range Experiment Station, Laramie, WY 82070 INTRODUCTION are strongly related to local forest conditions and prey populations. The boreal owl (Aegolius funereus), known as In contrast with studies in Europe, habitat use has Tengmalm's owl in Eurasia, occurs throughout the been emphasized in the few investigations in North holarctic in boreal climatic zones. This medium-size America. Studies on the two continents have gener- owl (100-170 g) occupies boreal and subalpine for- ated few data with which to contrast the biology of ests in an almost continuous circumboreal distribu- the species between continents. Therefore, the basis tion that extends from Scandinavia eastward across for inferring North American biology and ecology the northern forests of Siberia and from Alaska across based on European results is unclear. The variabil- Canada to the Atlantic (Dement'ev and Gladkov ity witnessed in Europe suggests caution. However, 1954). On each continent, disjunct populations oc- to the degree that variation in Europe follows geo- cur in mountains south of the broad transcontinen- graphic, climatic, or habitat gradients, a more sound tal boreal forest populations (Cramp 1977, Voous basis upon which to build inferences for North 1988). Boreal owls in the mountain regions of Eu- America is possible. rope and Asia have long been recognized as isolated The paucity of scientific knowledge from North resident breeding populations, whereas in North America necessitates reliance on the extensive America, breeding status was only recently docu- knowledge accumulated in Europe for portions of mented in the mountains of the western United the assessment. Ignoring that knowledge would be States (Hayward and Garton 1983, Palmer and Ryder careless. However, we cannot directly infer ecologi- 1984, Hayward et al. 1987a, Whelton 1989). cal patterns in North America based on the Euro- In-depth study of boreal owl biology and ecology pean knowledge. Therefore, I have been careful to in North America is limited to four, short-term in- point out the geographical source of knowledge, and vestigations (Bondrup-Nielsen 1978, Meehan 1980, where appropriate, describe ecological patterns for Palmer 1986, and Hayward et al. 1993). As an ex- Europe that have been related to environmental gra- ample of the lack of attention paid this species, prior dients. By doing so, I seek to describe patterns rec- to 1979 the USDI Fish and Wildlife Service had no ognized in Europe that may relate to populations in records for banded boreal owls west of the Missis- North America. sippi (W. Martin, pers. comm.). Knowledge of the Note: Throughout this paper, measures of varia- species' biology and ecology comes mostly from tion are 95% bounds on estimates unless otherwise Fennoscandia where Aegolius funereus may be the indicated. most studied owl. Many investigations in Europe are long-term efforts. Franz et al. (1984), Sonerud SYSTEMATICS (1989), Schelper (1989), and Korpimaki (1992) each report studies lasting over 15 years. Korpimaki, who Ford (1967) associated the genus Aegolius with initiated investigations in 1966, continues work on Surnia and Ninox (northern and southern hawk owl the same sites today. Ecolog~stsin Fennoscandia and genera) based on osteology of 75 owl species. Aside eastern Europe have emphasized study of breeding from the boreal owl, the genus Aegolius includes biology, productivity, movements, food habits, and three species: the northern saw-whet owl (A. relationship with prey populations. These studies acadicus), unspotted saw-whet owl (A. ridgwayi), and stem largely from examination of populations that buff-fronted owl (A.harrisii), which all occur only in breed almost exclusively in nest boxes. Results from the New World. The largest species of the genus, A. studies in the Old World indicate that the biology funereus occurs north of the others and is more and ecology of boreal owls vary geographically and widely distributed. Norberg (1987) speculates that the genus originated in the New World and only the boreal owl expanded its range beyond the Ameri- cas. The more northern distribution and larger size of A. funereus likely facilitated range expansion via the Bering Strait. Boreal owls in North America represent a homog- enous taxonomic group and are recognized as a single subspecies, A. funereus richardsoni. Six subspecies are recognized in Eurasia. Abrupt distinctions are apparent in only A. f. beickianus and caucasicus, which are southern, more isolated populations. Oth- erwise, A. f.funereus -north and central Europe; A. f. sibiricus - north and central Asia; A. f. magnus - northeast Siberia; and A. f. pallens - west and central Siberia vary as a cline across Eurasia (Dement'ev and Gladkov 1954). Generally the largest and light- est forms are found in northeast Siberia, with a size reduction and darkening westward and southward (Dement'ev and Gladkov 1954). A. f. richardsoni is among the darkest forms. DISTRIBUTION Species Range Boreal owls occupy boreal forests throughout the Figure 1.-Example of the patchy nature of boreal owl northern hemisphere forming an almost continuous distribution in the western United States based on the species' band across North America and Eurasia. In Europe, estimated distribution in Idaho. Owl distribution inferred from scattered populations extend south of the distribution of forest vegetation types. Potential habitat is defined as forested sites in the subalpine-fir zone throughout circumboreal range in the Pyrenees, Alps, and the state and Douglas-fir woodland in southeastern Idaho. Other Caucasus mountains and in Asia along Tarbagatai, montane forests are not considered potential habitat. Tien Shan, and Zervshan ranges (see maps in Data taken from Idaho gap analysis project (adapted from Hayward et a/. 1993). Dement'ev and Gladkov 1954:436 and Cramp 1977:607, 608 for worldwide distribution). Similar Maine (Catling 1972). East of the Rocky Mountains, southern populations occur in North America as breeding has been confirmed only in Minnesota. In described below. western North America the species' range extends Recently the species' documented range has ex- southward beyond 38" N latitude (Map 2). South of panded in Europe like in North America. Most new the continuous transcontinental band, populations records are from mountainous locales (see Cramp are restricted to subalpine forests in the Rocky Moun- 1977:607 and Hayward et al. 1987a). Rather than a tains, Blue Mountains, and Cascade Ranges (Palmer recent range expansion, these records likely repre- and Ryder 1984, Hayward et al. 1987a, Whelton sent increased interest in owls and increased human 1989). The southernmost records occur in mountains recreation in mountain areas during winter. of northwestern New Mexico (Stahlecker and Rawinski 1990). North America Due to the species' association with high eleva- tion forests in the western United States (discussed Within North America, boreal owls occur in a con- in-depth under Habitat Use), populations may oc- tinuous band concurrent with the boreal forests of cur as geographic isolates dispersed throughout the Alaska and Canada (see Johnsgard 1988 for conti- western mountains (for an example see figure 1).As nental distribution). The breeding range extends a result of the naturally fragmented nature of boreal from northern treeline southward in forested regons owl habitat in the western mountains, the species is of Canada to the extreme northern United States in distributed in North America in two contrasting Minnesota (Eckert and Savaloja 1979, Lane 1988) and patterns. In the north, populations of interacting in- likely Wisconsin (Erdman 1979), Michigan, and dividuals may extend for hundreds of miles, while Some surveys were conducted by inexperienced in the south, numerous breeding populations occur persons and the level of training and experience of as islands of habitat linked only through long-dis- personnel conducting surveys varied. Because the tance dispersal through extensive areas without majority of survey personnel received some train- breeding habitat. ing and discussed their observations with owl ex- Although boreal owls are thought to breed in much perts, however, I believe the estimated distribution of the forested portion of Alaska, surveys have been to be reliable. conducted in few portions of the state (see Gabrielson and Lincoln 1959, Armstrong 1980). Lit- Species Status and Trend erature documentation for boreal owls in Alaska extends from the Brooks Range (Campbell 1969), to Direct measures of population status or trend are the Pribilof Islands (Evermann 1913), and to the not available for populations in North America. In north Gulf Coast (Isleib and Kessel 1973). Recent contrast with Europe, investigations of boreal owls surveys document singng boreal owls in southeast in North America have been short term and have Alaska on the mainland and a number of islands not emphasized study of productivity or (draft agency report, Suring 1993; see Map 2 in sleeve demography. Due to the paucity of historical infor- of this book). mation, direct estimates of status and trend will be The recognized distribution of boreal owls has difficult in the near future. Currently, I am aware of changed yearly since 1979 as interest in the owl de- only one effort, begun in 1988, to intensively moni- veloped and efforts to locate breeding populations tor population trend in North America (Hayward et

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