SHORT-EARED OWL (Asio Flammeus) Don Roberson
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II SPECIES ACCOUNTS Andy Birch PDF of Short-eared Owl account from: Shuford, W. D., and Gardali, T., editors. 2008. California Bird Species of Special Concern: A ranked assessment of species, subspecies, and distinct populations of birds of immediate conservation concern in California. Studies of Western Birds 1. Western Field Ornithologists, Camarillo, California, and California Department of Fish and Game, Sacramento. Studies of Western Birds No. 1 SHORT-EARED OWL (Asio flammeus) Don Roberson Criteria Scores Population Trend 10 Range Trend 5 Population Size 7.5 Range Size 5 Endemism 0 Population Concentration 0 Threats 10 + ? + ? ? Current Breeding Range Historic Breeding Range + + Recent Extralimital Breeding ? Status Uncertain County Boundaries Water Bodies Kilometers 100 50 0 100 Current and historic (ca. 1944) breeding range of the Short-eared Owl in California. Breeding is most regular in northeastern California and in Suisun Marsh, and mainly irregular or extralimital elsewhere, particularly on the immediate southern coast. Breeding numbers have declined at least moderately, but sparse historical information on the species’ distributional limits, and cyclic or episodic breeding in many areas, make it difficult to assess changes in the overall breeding range. Although fluctuating greatly annually, numbers are generally greatest during migration and winter, when birds occur more widely in lowland areas of the state. 242 Studies of Western Birds 1:242–248, 2008 Species Accounts California Bird Species of Special Concern SPECIAL CONCERN PRIORITY coast is a 2 July 1920 record for the “Estero” in Santa Barbara, Santa Barbara County (Lehman Currently considered a Bird Species of Special 1994), and observations by J. B. Dixon (in Willet Concern (breeding), priority 3. Included on both 1933) of birds at San Diego Bay and at Santa prior special concern lists (Remsen 1978, 2nd Margarita, San Diego County, “during summer priority; CDFG 1992). months.” By contrast, Grinnell and Miller (1944) described winter visitants as “common and widely REEDING IRD URVEY TATISTICS B B S S distributed.” They considered the species formerly FOR CALIFORNIA “abundant in winter,” and attributed a notable Data inadequate for trend assessment (Sauer et reduction in “late years” to shooting by duck al. 2005). hunters. GENERAL RANGE AND ABUNDANCE RECENT RANGE AND ABUNDANCE IN ALIFORNIA Breeds over much of northern North America; C additional populations occur in Eurasia and South The cyclical nature of range expansion and retrac- America and on many oceanic islands (Holt and tion can make it difficult to distinguish between Leasure 1993). Many northern populations are areas of regular versus irregular breeding. While migratory; North American breeders winter south small resident populations of Short-eared Owls to northern Mexico and Florida. Numbers fluctu- remain in the Great Basin region and locally in ate dramatically in response to periodic “bust or the Sacramento–San Joaquin River Delta (see boom” cycles of the owls’ primary prey; the breed- map), most recent breeding from coastal cen- ing range dramatically expands and contracts fol- tral California and the San Joaquin Valley has lowing these prey cycles, making it difficult to give been episodic. Breeding in mainland southern any general geographic statement about range or California is now exceptional and limited to years abundance. North American owls are attributed to of unusual incursions. Recent incursions occurred A. f. flammeus; other named subspecies are from from 1983 through 1984 and from 1987 to 1992 oceanic islands. after El Niño winter rains produced bumper crops of herbaceous cover that coincided with peak SEASONAL STATUS IN CALIFORNIA cycles of vole productivity. The breeding range Year-round resident in certain areas within of the Short-eared Owl retracts dramatically in California; the breeding season stretches from drought conditions and during prey reductions. March through July (Dixon 1934, Gill 1977, Both the large fluctuations in owl numbers Collins and Jones in press). Influxes of birds from and the nature of the fragmentary and anecdotal the north, which increase the number of owls data make it very difficult to compile any reason- within the state tenfold or more during some able population estimates for this cyclical species. winters, are highly variable but generally occur In poor years with few microtine prey and when between late October and early March (Fisler marsh habitat is reduced by drought, very few 1960, Garrett and Dunn 1981). breeding owls are left in resident areas, and num- bers likely total fewer than 50 pairs statewide. In HISTORIC RANGE AND ABUNDANCE wet years that bring substantial cover and coincide with peaks of prey cycles, local nest density may IN CALIFORNIA exceed 7 nests per 40 hectares of appropriate habi- Grinnell and Miller (1944) described Short-eared tat (Larsen 1987), and the statewide owl popula- Owls as breeding interruptedly the entire length tion may exceed 500 pairs. of the state west of the southern deserts “in very Below, key information is described by subre- small numbers.” They knew of nesting at Lava gions of the state for nesting and for the state as a Beds National Monument, Siskiyou County; June whole for winter. Lake and McGee Creek, Mono County; Redwood Northeastern California. This region, includ- City, San Mateo County; New Hope, Fresno ing mainly the Klamath Basin, Modoc Plateau, County; Newport, Orange County; and National and Great Basin of California, contains the larg- City, San Diego County. Additional historical nest est populations of nesting Short-eared Owls. In records are from Wasco, Kern County (WFVZ Siskiyou, Modoc, and Lassen counties, Short- egg set data), and Laws, Inyo County (J. Dixon eared Owls breed at major refuges such as Lower field notes, MVZ). Also notable for the south Klamath NWR, Tule Lake NWR, Modoc NWR, Short-eared Owl 243 Studies of Western Birds No. 1 Honey Lake WA, and Ash Creek WA, but no San Joaquin Valley and adjacent Coast Range reliable population estimates have been made valleys. Nesting by Short-eared Owls in this region (Larsen 1987, R. Ekstrom, F. Hall, T. Rickman, is generally episodic, particularly after wet win- J. Sterling in litt.). In good years, dozens of pairs ters. A nesting pair was observed on Santa Fe likely nest at Lower Klamath NWR, Modoc Grade, Merced County, sometime in the 1980s (D. NWR, and Honey Lake WA (P. Bloom, W. D. Shearwater in litt.). Following the El Niño rains of Shuford in litt.). Spring road kills found in the 1998, a vole population explosion in the Panoche Surprise Valley, Modoc County, suggest nesting Hills of Fresno County was apparently responsible there. In some years, Short-eared Owls nest in for several Short-eared Owl broods where none are Fall River Valley, Shasta County (B. Yutzy in litt.), usually found (S. Fitton in litt.). About a dozen and Sierra Valley, Sierra County (MPCR files, W. owls have been resident near Mendota WA, Fresno D. Shuford in litt.). Birds suspected of nesting in County, since 2000, with three nests found in 2002, Mono County were pairs in Bridgeport Valley in and individuals were resident at another restoration May 1984 (Gaines 1992) and along the east shore site, near Alpaugh, Tulare County (K. Kreitinger of Mono Lake in June 1996 (T. Beedy in litt.), in litt.). The species may also nest regularly in and an adult at Fish Slough on 4 June 2006 (W. alfalfa and grain fields in the Tulare Basin of the D. Shuford in litt.). A 20 June 1978 record from southern San Joaquin Valley (R. Hansen in litt.). about 10 mi southeast of Bishop near the Warm Ten nests were in alfalfa fields during the summer Springs Rd. (T. Heindel in litt.) suggests occa- of 1983 near Wasco, Kern County (R. Hansen in sional nesting south to Inyo County. litt., MPCR files). On the Carrizo Plain, San Luis Sacramento Valley and Sierra foothills. In some Obispo County, rodent numbers rebounded fol- years, these owls nest at Table Mountain in west- lowing rains in March 1991, and by the spring of ern Butte County (T. Beedy in litt.). Pairs occa- 1992 nesting Short-eared Owls were observed there sionally nest in the Sacramento Valley in irregu- (S. Fitton in litt.). A Breeding Bird Survey route in larly grazed wetlands west of the Sutter Buttes, the Carrizo Plain recorded 17 and 15 birds in 1992 Sutter County (R. Hasey in litt.), although none and 1993, respectively, but none in any other year are known to breed in the federal and state wildlife from 1981 to 2001 (Sauer et al. 2005). refuges in the Sacramento Valley (B. E. Deuel in Coastal California. Following the winter of litt.) despite a few summer records there (MPCR 1989–90, when up to 24 owls remained into April files). One nest was found southwest of Lincoln, in ungrazed pastures with high rodent popula- Placer County, in 1998 (J. Ranlett, B. Williams tions, at least one owl pair was feeding young in in litt.). There is a record of nesting near Davis in early June 1990 near what is now the Mad River 1976 (Remsen 1978) and recent possible breeding Slough WA in the Arcata bottoms, Humboldt nearby at the Yolo Bypass wetlands, Yolo County County (Harris 2005, Hunter et al. 2005). The (S. Hampton in litt.). compilers of the Humboldt County breeding Suisun Marsh and Sacramento–San Joaquin bird atlas project, 1995–1999, found it difficult River Delta. Grizzly Island WA, Solano County, in to distinguish between wintering birds, migrants, Suisun Marsh supports the only resident popula- and prospective summering birds in early spring tion of owls in this region. In the spring 1987, 39 (Hunter et al. 2005). Their records of possible nests and 100 fledglings were documented (Larsen breeding included sightings at Mad River Slough 1987), apparently in response to upland manage- WA of three owls, 7–19 April 1998; two, 13 April ment that led to major increases in microtine prey.