Typical Owls — Family Strigidae
Flammulated Owl Otus flammeolus
The Flammulated Owl is widespread through the montane pine forests of the western United States but is only a rare and sporadic visitor to San Diego County’s mountains. A few individuals have been seen and heard calling territorially, but there is no evidence any were mated and breeding, though some could have been. The Flammulated is California’s only owl that occurs as a summer visitor; it migrates to southern Mexico and Central America for the winter. There are three records in San Diego County of migrants near the coast.
Breeding distribution: Though the Flammulated Owl has never been found breeding in San Diego County, most records are from coniferous woodland in summer, where breeding is possible. In its core range, the Flammulated Owl inhabits open forests of ponderosa pine, habitat like that preferred by the Pygmy Nuthatch. In San Diego County, therefore, one might expect the species in the Laguna Mountains, dominated by the similar Jeffrey pine. However, the records are from Palomar, Hot Springs, and Cuyamaca mountains, though sometimes from open pine woodland. The Flammulated Owl has been found five times on Palomar Mountain, with two on 13 and 16 June 1971 (AB 25:907, 1971), two on 29 April 1972 (Winter 1974), one near the observatory (D15) 24 May–3 June 1981 (R. Higson), three along East Grade Road near Dyche Valley (F16) 14 May 1988 (K. L. Weaver, E. Littlefield), and another heard at 5100 feet elevation along Observatory Trail (D15) 19 July 2000 (K. L. Weaver). On Hot Springs Mountain, two were heard, one of which was seen, at 6000 feet elevation 1.8 miles southeast of the summit (E21) 8 June 1985 (P. Unitt et al., AB 39:962, 1985), one was reported in June 1987 (J. O'Brien, AB 41:1488, 1987), and one was heard at 6200 feet elevation 0.25 mile east-southeast of the summit (E20) 2–3 June and 17 June 2000 (K. L. Weaver, C. R. Mahrdt). In the Cuyamaca Mountains the only report is of one within Cuyamaca Rancho State Park 27 April 1972 (Winter 1974).
Migration: Though the Flammulated Owl is highly migratory, it is rarely seen in migration. In San Diego County, one was captured on a ship in San Diego Bay 10 October 1962 (Banks 1964); unfortunately, the specimen is no longer extant. A specimen (SDNHM 41184) from “San Diego area” bears the information “died 21 Oct. 1971” and “found locally, fallen out of tree, injured leg; brought in to S. D. Zoo by Cal. Fish and Game.” One was at photographed at Cabrillo National Monument, Point Loma (S7), 30–31 May 1991 (R. E. Webster, AB 45:496, 1991).
Western Screech-Owl Megascops kennicottii
The Western Screech-Owl is seldom seen without a special search but is a fairly common permanent resident in San Diego County’s oak and coniferous woodlands. Its ideal habitat is a grove of mature coast live oaks with an ample supply of rotted-out cavities. The screech-owl is active only at night so is normally found by call, a series of 7 to 15 soft hoots that accelerates over about 2 seconds.
Breeding distribution: Because of spotty nocturnal effort, our results for the Western Screech-Owl are less complete than for those of most birds. But they show that the owl’s distribution follows the pattern set by other birds of oak woodland. That is, the species ranges from the mountains west toward the coast but does not touch the coast, approaching it toward the north and retreating inland ever farther toward the south. In Camp Pendleton we found the screech-owl about 3 miles from the coast along the south fork of San Onofre Creek (D3; one on 25 June 1999, D. C. Seals), whereas along the Mexican border our most coastal locality was 20 miles inland in Marron Valley (V16; one on 31 May 2000, D. C. Seals). Numbers are greatest in foothill canyons, with up to nine per night along the San Luis Rey River near the Forest Service picnic area (G16) 3 July 1999, in Bandy Canyon (K13) 6 May 2001, and in Sloan Canyon (R15) 5 May 1999 (all W. E. Haas). In conifer-dominated woodland in the higher mountains the screech-owl is widespread but less common.
On the east slope of the mountains the Western Screech-Owl extends beyond the oaks along Banner and San Felipe creeks as far as the riparian woodland at Scissors Crossing (J22; two adults with fledglings 4–5 July 2001, T. Gallion). At lower elevations in the Anza–Borrego Desert it is rare, probably irregular, and has never been confirmed breeding. There are few nest sites, though the birds might use the skirts of California fan palms (G. L. Rogers in Cannings and Angell 2001). From 1997 to 2001 our only record in this region was of one calling territorially at Yaqui Well (I24) 18 March 1998 (P. K. Nelson), and the only previous records in the breeding season are of five (one family?) in Hellhole Canyon (G23) 9 June 1973 (M. C. Jorgensen), one at Palm Spring (N27) 2 April 1978 (P. Unitt), and one at Lower Willows (D23) 30 April 1995 (L. Clark, C. Sankpill).
Nesting: The Western Screech-Owl nests in tree cavities, either the result of decay or excavated by the Northern Flicker—San Diego County’s only woodpecker whose caliber equals the screech-owl’s. The nests are difficult to locate; only two certain nests in natural sites came to light during the atlas study, both in coast live oaks. The owls also take readily to nest boxes designed to their needs. Almost all our confirmations of screech-owl breeding were sightings of fledglings. Dates of these ranged from 15 April (1999 at Banner, K21, P. K. Nelson) to 15 July (1999, Palomar Mountain State Park, D14, P. D. Jorgensen), translating to egg laying from mid February to about early May. The breeding season we observed from 1997 to 2001 was thus somewhat earlier than that reported previously, on the basis of egg sets collected from 1897 to 1939 and extending from 11 March to 31 May (Sharp 1907, Unitt 1984).
Migration: The Western Screech-Owl is nonmigratory, but the young disperse short distances. Such dispersal may be responsible for about 12 records from the Anza–Borrego Desert extending from 12 August (1992, one along Pinyon Mountain Road, R. Thériault) to 12 February (1990, one in Cougar Canyon, D23, D. Minock, ABSDP database).
Winter: The differences between the distribution we recorded in winter and that in spring and summer are probably due to sampling error only. Because males call most actively in winter, as they advertise their territories in preparation for breeding, the screech-owl is noted in largest numbers at this season, up to 16 near De Luz (B6) 25 January 1998 and 11 in Corte Madera Valley (R21) 20 February 1999 (both W. E. Haas).
Conservation: No changes in the Western Screech-Owl’s abundance in San Diego County have been reported; if any, they have been slight. Egg sets collected in 1916 and 1923, respectively, in Rose Canyon (P8) and Oceanside (H5), outside the range where we found the species 1997–2002, suggest retreat from urbanization. Like other nocturnal birds, screech-owls are especially susceptible to being struck by moving vehicles. Human population growth brings increased road traffic to the foothills where screech-owls are concentrated, but the owls still survive in good numbers in the canyon of the upper San Luis Rey River along Highway 76, where traffic is heavy and growing. Extended droughts disfavor the screech-owl, like most other birds; Hardy et al. (1999) reported a 70% decline in southwestern Arizona over three years of drought.
Taxonomy: Marshall (1967) reported Western Screech-Owls from San Diego County to be closest to the finely barred subspecies of northern Baja California, M. k. cardonensis (Huey, 1926). But the barring on the underparts on all 12 skins of adults in the San Diego Natural History Museum is coarser than in cardonensis, matching instead M. k. bendirei (Brewster, 1882), which ranges throughout cismontane Alta California.
Great Horned Owl Bubo virginianus
The Great Horned Owl is San Diego County’s most widespread owl, a year-round resident in all parts of the county. It lives in all types of woodland and in any open scrub growing over rugged topography. It nests wherever there are old hawk, raven, or crow nests, be these in trees, on cliff ledges, or on buildings. In spite of being such a large bird, naturally living in well-dispersed territories, the Great Horned Owl appears to be maintaining its numbers and distribution in the face of urbanization.
Breeding distribution: Great Horned Owls are found nearly throughout San Diego County, from the coast to near the summit of Hot Springs Mountain (E20; pair 19 May 2001, K. L. Weaver, C. R. Mahrdt) to the floor of the Anza–Borrego Desert. A birder traveling on foot seldom encounters more than one pair or family per day, so the species is best termed uncommon, though it is relatively numerous for a large bird of prey. Regions where the Great Horned Owl appears especially plentiful are Los Peñasquitos Canyon Preserve (two active nests in square N8 on 19 April 1997, L. Ellis) and Miramar Air Station (eight in square O10 and another eight in O11 on 24 February 1999 (W. E. Haas). In the Anza–Borrego Desert the species is sparser than on the coastal slope but still confirmed nesting in both the Borrego Valley and in badlands and on steep rocky slopes elsewhere.
Nesting: The Great Horned Owl does not build its own nest but typically takes over old nests of other large birds. Starting to nest in midwinter, the owl gets a head start over other species that might want to reuse these nests. Rarely does the Great Horned Owl make any effort to refurbish its nest, and we noted this behavior just once, at the north end of Blair Valley (K24) 22 March 2001, where the owls were adding to a nest they had used the previous year (R. Thériault). Known builders of nests we observed Great Horned Owls using were the Common Raven and Red-tailed and Red-shouldered Hawks. The Great Horned Owl also lays its eggs on ledges of cliffs or buildings where there is no preexisting nest; one nest that fledged two young in Poway (M11) in 1998 was on the second-story window ledge of a medical center, situated so a person could see it only by standing on the desk of a psychiatrist’s office (E. J. McNeil).
Great Horned Owls often begin nesting as early as late January, occasionally even earlier. Our earliest occupied nest was 28 January; nestlings on Air Station Miramar on 24 February 1999 (W. E. Haas) must have hatched from eggs laid no later than the third week of January, and a fledgling in Lopez Canyon (N8) 8 March 1999 (B. C. Moore) implies a nest initiated in late December. The latest pairs lay in early April, and many young fledge during that month. The nesting activity we observed from 1997 to 2001 was thus consistent with the 28 January–13 April spread of 67 egg sets collected in San Diego County 1890–1943, and a female collected 6 January 1963 with a fully developed egg in her oviduct (A. M. Rea).
Winter: The Great Horned Owl is nonmigratory in southern California, and its breeding season overlaps much of the winter defined by our survey protocol. The species is probably resident in all the atlas squares where we noted in winter but not spring or summer, the difference being due to variations of our nocturnal effort rather than to movements of the owls themselves. Also, the birds call most consistently early in their breeding season, which means mid to late winter. Sites of notable concentrations recorded in winter were Pamo Valley (J15; up to eight on 3 January 1998, W. E. Haas) and Boden Canyon (J14; up to six on 2 January 1999, C. R. Mahrdt, R. L. Barber).
Conservation: There is no evidence for change in the Great Horned Owl’s abundance through San Diego County history. The species fares surprisingly well in cities, though it is less common there than in rural or natural areas. Buildings offer new nest sites, and some of the other birds that supply the owls with nests are thriving in the urban environment. Great Horned Owls, like other nocturnal birds, are more subject than diurnal species to being killed by moving cars, so increasing traffic threatens them. Offsetting this factor is society’s improved attitude toward birds of prey in general, so shooting of the owls is less frequent than in the past.
Taxonomy: San Diego County lies in the zone of intergradation between B. v. pacificus Cassin, 1854, and B. v. pallescens Stone, 1897. A few specimens are the darker pacificus, buffier and more heavily barred, especially on the feet. Others, even near the coast, are as pale (gray and white) and lightly barred as any pallescens. Most are intermediate between the two. The one specimen from the desert slope (San Felipe Valley, 22 December 1993, SDNHM 48720) is pallescens. For more details, see Rea (1983) and Unitt (1984).
Burrowing Owl Athene cunicularia
No bird in San Diego County is more endearing than the Burrowing Owl. And no bird is in more imminent danger of being extirpated from the county. Living mainly in grassland and open scrub, the Burrowing Owl was once common here but now reduced to a few scattered sites, some threatened by development. If the population crash continues at its current rate the Burrowing Owl will be the next species extirpated from coastal southern California. Intensive management such as provision of artificial burrows, habitat modification, and reintroduction of captive-bred birds may already be the owl’s only hope.
Breeding distribution: Breeding Burrowing Owls remain tenuously in only five areas of San Diego County. Perhaps the most viable site is North Island Naval Air Station (S8). Though there is no thorough census, there are several pairs scattered over various parts of the station, including the golf course, around runways, and near Zuñiga Jetty. The maximum single-day count there was of seven on 26 May 2000 (R. T. Patton). At the Imperial Beach Naval Auxiliary Landing Field (Ream Field), two pairs nested in 1999 (C. Winchell), and one pair fledged four young in 2001 (L. and M. Polinsky). The largest numbers occur probably on Otay Mesa. Again, there is no complete census; the maximum count, of 11 in square V14 and two in V13 on 15 April 2000 (S. D. Cameron, P. Unitt), did not include the site of four nests in V13 monitored in 1998 (and possibly eliminated by 2000). Two pairs of Burrowing Owls are on or near navy property not accessible to the public at the northwest corner of Brown Field (V12; J. L. Lincer). The greatest concentration on Otay Mesa is at the mesa’s extreme east end, at the southwest base of Otay Mountain, where the scrub is kept very open by frequent fires, started by children in Tijuana tossing burning objects over the international fence to taunt the Border Patrol.
A few Burrowing Owls may persist in Warner Valley. From 1997 to 2001, however, our only record during the breeding season was of one pair carrying food items to a burrow just northwest of the intersection of highways 79 and S2 (G18) 12 May 2001 (G. L. Rogers). In the Borrego Valley, two pairs nested northeast of the intersection of Palm Canyon Drive and Borrego Valley Road (F25) in 1998 (M. and P. D. Jorgensen), up to five were at burrows along Coyote Creek Wash (F25) 1–27 March 1999 (P. D. Ache), and a pair was at a burrow near the Borrego Air Ranch (H26) 26 March–27 April 1998 (M. L. Gabel). In the last two cases, the burrows were dug out by dogs or coyotes before any young fledged. Two individuals along Highway 78 between Ocotillo Wells and the Imperial County line (I29) 15 June 1999 (A. Lotz) may have dispersed from the Imperial Valley.
Nesting: Burrowing Owls take over the burrows of mammals, especially those of the California Ground Squirrel. In the Imperial Valley, and probably in the Borrego Valley, the Round-tailed Ground Squirrel provides the owls’ burrows (Patten et al. 2003). The owls maintain the burrows after appropriating them; if the owls are permanent residents (probably many of those breeding in San Diego County) they use their burrow year round. Burrowing Owls also use culverts and artificial burrows designed for them (Collins and Landry 1977).
Because of the species’ rarity its breeding schedule is not well represented by our atlas data. Sharp (1907) found eggs at Escondido from 23 March to 16 June. On the basis of an incubation period of 29 days and a nestling period of 44 days fledglings at North Island 27 May 1998 (T. Plunkett) hatched from eggs laid as early as 17 March.
Migration: The Burrowing Owl is migratory over much of its range and far from sedentary even in southern California. In the Imperial Valley it is considerably more numerous in summer than in winter, but in San Diego County, with the breeding population almost gone, it is more frequent in winter. Young still in juvenile plumage can disperse soon after fledging, as suggested by one at Los Peñasquitos Lagoon (N7) 6 August 2000 (P. A. Ginsburg). Observers covered this area regularly, not finding the species there earlier in the year. In the northeastern corner of the Miramar Air Station (N12), Burrowing Owls occurred regularly on ridge tops through the winter of 1996–97, with the last individuals being three on 18 March (W. E. Haas).
Winter: In winter the Burrowing Owl is seen both at the places where it breeds (up to seven at North Island 22 February 2000, R. T. Patton) and occasionally at other places (up to four at the Chula Vista Nature Center, U10, 20–24 January 1998 (B. C. Moore). We noted the Burrowing Owl in 20 atlas squares where it evidently no longer breeds. At some of these sites, such as the east end of Lake Hodges (K11; six winter sightings of single birds), the owls had bred fairly recently and the habitat was little changed, though ground squirrels were in short supply (J. L. Lincer). Another area where Burrowing Owls still winter fairly regularly is along the flood-control channel at the San Diego River mouth and the southeast corner of Mission Bay (R8; up to three on 27 February 2002, L. Hughes).
Conservation: The Burrowing Owl’s population collapse is well documented. Stephens (1919a) called the species “common resident in open ground from the seashore to the higher foothills.” Collections and observations in the early 20th century attest to the owls’ nesting at numerous locations such as Pauma Valley, Escondido, San Pasqual Valley, Poway, Rancho Santa Fe, Point Loma, and La Presa, where they do not remain today, although at some of these suitable habitat remains. As late as the 1970s and 1980s the birds remained in the lower San Luis Rey valley, at San Marcos, near Palomar Airport, Mission Bay, Sweetwater Reservoir, Lower Otay Lake, and the Tijuana River valley (Unitt 1984, J. L. Lincer), all locations where they are now gone. The Burrowing Owl was last recorded on both the Oceanside and Escondido Christmas bird counts, where it was formerly regular, in 1993. We evidently witnessed the extirpation of the Burrowing Owl from the region east of Chula Vista during the atlas period: the last individual observed during the breeding season was one near Upper Otay Lake (T13) 29 April 1998 (J. F. Walters). Burrowing Owls were resident on the nearby campus of Southwestern College in the early 1970s (J. W. Schlotte), but the region has now been blanketed by urban sprawl. Yet even the county’s most extensive grasslands, the Warner and Santa Maria (Ramona) valleys, now support few or no breeding Burrowing Owls.
Abbott (1930) provided the most interesting perspective on the history of the Burrowing Owl in San Diego: In 1921, “on El Cajon Boulevard, which was a well-traveled thoroughfare even in those days, Burrowing Owls could often be seen perched on the side-walk curb. They lived in the culvert drains under the intersecting streets. The paving of this boulevard has driven these birds away, … yet in spite of San Diego’s present 150,000 population Burrowing Owls still subsist wherever there is any extent of vacant land…. On Reynard Way, which is a short-cut between downtown and the Mission Hills residential district, these owls are common, because many of the sloping lots on each side have not yet been built upon. Even in broad daylight a ‘ground owl’ may often be seen standing upon some advertising sign, apparently unconcerned at the passing stream of automobiles. On the other hand, I have more than once seen the flattened body of one of these owls on the cement roadway…. Whereas such observations seem common-place and trivial, it may not be amiss to place them on record. At the speed with which some western cities are growing, remnants of primitive conditions are bound to disappear completely before long.”
Thus we can infer that the Burrowing Owl suffers from the factors that afflict other grassland birds: not only direct loss of habitat but high sensitivity to habitat fragmentation, proliferation of terrestrial predators, and high mortality from collisions with cars. The Burrowing Owl provides a clear warning that conservation of habitat that seems sufficient to conserve dozens or hundreds of pairs can be insufficient to counteract bad population dynamics over a large region. All remaining Burrowing Owl sites in coastal San Diego County are on either military or private property, largely or entirely already approved for development. Reintroduction may be necessary to establish the species on suitable public property, such as lands owned by the California Department of Fish and Game in Rancho Jamul or San Felipe Valley, or those owned by the city of San Diego in Pamo Valley, Marron Valley, Spring Canyon, or around Otay Lakes (J. L. Lincer). The Chula Vista Nature Center has raised the species successfully in captivity, and the Wildlife Research Institute has begun efforts at captive breeding as well.
The Burrowing Owl is on the decline over most of North America, suffering considerable contraction of its range. Yet in spite of much research on the species (Lincer and Steenhof 1997), the importance of the contributing factors remains unclear (Holroyd et al. 2001). In various parts of the range, low productivity, high mortality, adverse effects of pesticides, decreased food supply, and reduction of the mammals that supply the owl with burrows have all been documented (Haug et al. 1993, Wellicome and Holroyd 2001).
Taxonomy: All Burrowing Owls in western North America are A. c. hypugaea (Bonaparte, 1825).
Spotted Owl Strix occidentalis
William E. Haas
In San Diego County the Spotted Owl lives year round in shady woodlands of oaks and conifers on steep to moderate slopes. Ideal habitat is a stand of mature oaks with a closed canopy, a source of permanent water, and an ample supply of rotted-out cavities, abandoned raptor nests, or debris platforms. An abundance of the owl’s favored prey, the big-eared or dusky-footed woodrat, enhances the habitat as well. Although the Spotted Owl uses a wide range of forest types, in San Diego County it is limited by the paucity of forest: probably only 25–50 pairs currently reside here.
Breeding distribution: In San Diego County the Spotted Owl typically breeds at elevations above 2500 feet and is most frequent between 4000 and 6000 feet, where oak woodlands and dusky-footed woodrats are common. Recent known nest sites range from about 5800 feet elevation in the Laguna Mountains down to 2100 feet in Black Canyon (I16; nest in 1994, Cleveland National Forest data). They may be on moderate slopes or in steep ravines within conifer-dominated woodlands (e.g., in upper Agua Tibia Canyon, C13, Cleveland National Forest data). A nest near Espinosa Creek (R20) 24 May 1999 (W. E. Haas) is the southernmost known for the California subspecies of the Spotted Owl; the Spotted Owl’s occurrence in Baja California is based on only three sight records from the Sierra San Pedro Mártir.
On the basis of a survey in 1988, Gutiérrez and Pritchard (1990), estimated a population for all of Palomar Mountain to be 21 individuals, distributed among 13 locations.
Albert M. Ingersoll collected a set of two Spotted Owl eggs from a cliff ledge “near Oceanside” (= Ysidora Gorge, G5, along the Santa Margarita River?) 24 March 1894, and B. P. Cole collected another from a hole in a sycamore in San Onofre Canyon (C3/C4) 20 March 1908 (Willett 1912, WFVZ 21133, 69985). One recent sighting suggests a few Spotted Owls could persist in the Santa Margarita Mountains: one at 2350 feet elevation 1.3 miles south of Margarita Peak along a tributary to San Onofre Creek 14 August 1997 (J. M. Wells).
Nesting: In San Diego County, the Spotted Owl nests in abandoned raptor nests, in tree cavities, atop accumulations of debris trapped in the crotches of large oaks, and probably in broken tree trunks, if sufficiently high above the ground. Large trees (tall, and of large diameter) and a closed canopy are characteristic of all nest sites in San Diego County. At breeding sites throughout the species’ range, high canopy closure is common (Gutiérrez et al. 1992), and the presence of large trees is critical (Gutiérrez et al. 1995). Historically, however, Spotted Owls also nested on cliffs (Bent 1938, Gutierrez et al. 1995), as attested by the eggs collected near Oceanside.
The Spotted Owl frequently begins broadcasting its advertisement calls in December, when pair bonds are strengthened prior to the breeding season. It evidently lays eggs from mid March to April, though actual egg data from San Diego County are confined to the two collected sets mentioned above. Our dates for occupied nests range from 16 March to 24 May; our dates of fledglings range from 28 May to 25 July. At one nest in Cuyamaca Rancho State Park (N20) followed by Betty Siegel in 1999, the chicks were still all downy 16 May, then fledged with only the heads still downy 25 July. These records are within the Spotted Owl’s normal distribution of breeding activities rangewide (Gutiérrez et al. 1995).
Migration: The Spotted Owl is nonmigratory in San Diego County. Occurrences at lower elevations outside of the breeding season are few, just a couple of reports by Eleanor Beemer in the mid 20th century (Unitt 1984). Young Spotted Owls may disperse short distances from their natal territories, but climate-driven altitudinal migration as reported from the Sierra Nevada (Verner et al. 1992) is probably unnecessary in San Diego County, where there is no evidence for it. A report of one at the San Diego Sports Arena (R8) 19 November 1973 (Gould 1977) seems unlikely and lacks supporting details.
Winter: Winter records for the Spotted Owl are similar in location to those for the breeding season, as expected for a nonmigratory species. One at 2200 feet elevation in Agua Tibia Canyon 20 February 2002 (K. L. Weaver) was calling territorially. Some records from the upper San Luis Rey River valley (F15/F16), including one of a juvenile, may have been the result of displacement from breeding sites at higher elevations nearby that were burned in the fire of November 1999 (W. E. Haas).
Conservation: Data adequate to demonstrate a trend in the Spotted Owl population are not available for San Diego County, but elsewhere in southern California it is declining (LaHaye et al. 1994). Gutiérrez and Pritchard (1990) found the species’ density on Palomar Mountain in 1988 to be unusually high and suggested that a wildfire the previous year was responsible for driving birds away from former territories and concentrating them in the remaining habitat. Because of its low numbers and need for woodlands with a closed canopy, the Spotted Owl may be especially susceptible to habitat loss to fires. Many of the owl’s known sites burned between 1999 and 2003. High fidelity of adults to their territories, low survivorship of young, and lack of knowledge about dispersal of juveniles (see Gutiérrez et al. 1995) mean that the owl’s reoccupation of burned habitat is uncertain, even if the trees recover adequately.
Although the Spotted Owl is found in areas of relative isolation, the burgeoning of San Diego County’s human population into rural areas may affect this rare species. Many of the observations during the atlas period were on public lands, suggesting inherent protection. However, the health of the forests is related to availability of ground water during the dry season and extended droughts. Demands for ground water to provide for the needs of the human population expansion into the back country, in addition to drawdown of the water table for bottled drinking water, may lead to the extirpation of this species from all but the most remote natural areas. The owl’s ability to tolerate nearby development may be low; all currently known nest sites are secluded from human dwellings.
Taxonomy: The California Spotted Owl, S. o. occidentalis (Xantus, 1859) is the subspecies of Spotted Owl in San Diego County, as elsewhere in southern California. It is intermediate between the darker, more finely spotted Northern Spotted Owl, S. o. caurina (Merriam, 1898), of the Pacific Northwest and the paler, more coarsely spotted Mexican Spotted Owl, S. o. lucida (Nelson, 1903), of the southern Rocky Mountains. The subspecies differ in mitochondrial DNA sequences as well as in plumage (Gutiérrez et al. 1995).
Long-eared Owl Asio otus
William E. Haas
There is no sound in San Diego County’s woodlands more haunting than the caterwauling of the Long-eared Owl during the breeding season—a usually two-noted lingering moan given by the female when a trespasser approaches her active nest. In San Diego County the Long-eared Owl is a rare resident in shady oak woodlands and broad riparian forests. Ideal habitat includes a closed canopy, nearby open habitats for foraging, and a good supply of abandoned raptor and corvid nests or debris platforms for nesting. Another enhancement is an abundance of prey—the California vole, the big-eared or dusky-footed woodrat, or, in the desert, spiny pocket mice. Although widespread in San Diego County the Long-eared Owl is limited by the paucity of forest, reduction of adjacent grasslands, and human disturbance. Recent study suggests 50–200 pairs currently nesting within the county, far more than previously suspected. Atlas observers found nonbreeding birds surprisingly widespread in the Anza–Borrego Desert.
Breeding distribution: The Long-eared Owl occurs in all parts of San Diego County. Its breeding distribution suggests that away from the coast it favors oak woodlands. Near the coast, riparian forest is its habitat of choice, though few remain in the latter habitat. Few colonies remain of the often colonial species; perhaps the largest is that of five to eight pairs in Sycamore Canyon (N12/O12) 1997–2002 (W. E. Haas). Nests have been found most frequently in the foothills and inland valleys, but they range as near the coast as Guajome Lake (G7; two fledglings 13 April 1999, S. Grain) and the Tijuana River Valley near the west end of Sunset Road (V10; three active nests 22 April 2000, W. E. Haas). The Long-eared Owl probably persists at some of the sites in Camp Pendleton where Bloom (1994) reported it 1974–92, here all mapped in red. Atlas observers did not cover Camp Pendleton at night. The highest elevations at which we found the Long-eared Owl nesting were 4286 feet in Johnson Canyon (D19; two calling males 14 March 1998, W. E. Haas) and 4320 feet at the more eastern of Twin Lakes (C19; adult with fledgling 2 June 2001, P. D. Jorgensen).
During the atlas period 1997–2002 we did not confirm Long-eared Owl breeding in the Anza–Borrego Desert, perhaps because of the drought conditions prevailing over most of this interval. Previously, however, breeding was known at Tamarisk Grove (I24; nested frequently at least 1964–95, ABDSP database, Massey 1998) and Clark Dry Lake (D26; nested 1993–95, M. L. Gabel in Massey 1998). In 1998 a pair remained at Tamarisk Grove as late as 5 April but failed to nest (P. K. Nelson, P. D. Jorgensen). In 1999, three remained in the mesquite bosque of the Borrego Sink as late as 15 April; in 1993, six were there 8 May (R. Thériault).
Unexpected were four scattered sightings of single Long-eared Owls in sparsely vegetated desert 1–13 April 2000 (R. and S. L. Breisch, J. R. Barth, M. B. Mulrooney). These are mapped as presumed nonbreeding, though the birds might nest in crevices in eroded badlands, as does the Great Horned Owl. Also mapped as presumed nonbreeding are two additional sites of Long-eared Owl pellets found in late April and May (D. C. Seals).
Nesting: In San Diego County, the Long-eared Owl nests typically in abandoned raptor nests in willows and oaks and atop woodrat nests and accumulations of debris trapped in the crotches of large oaks. Of 69 egg sets collected 1889–1961 and summarized by Bloom (1994), 57 were in oak, willow, or cottonwood, but four were in eucalyptus and one was in an orange tree. At Tamarisk Grove the birds nested in the athel tamarisks. Nests may be a variety of heights, from the ground (rarely) to 40 feet up in trees. Higher nests are probably unsuitable because they lack sufficient cover or leave the young vulnerable to the wind; nests of the Long-eared Owl are rarely if ever improved, so the eggs and young typically develop in a shallow depression or on a platform of sticks or debris rather than in a more protected cuplike nest (Marks et al. 1994). The increase in San Diego County of the American Crow, Cooper’s Hawk, and Red-shouldered Hawk has augmented the supply of nest sites; the owl uses all of these, as well as old nests of the Red-tailed Hawk and, formerly, Swainson’s Hawk (Bloom 1994, W. E. Haas).
Strengthening the pair bond before the breeding season, the male Long-eared Owl frequently begins broadcasting its advertisement calls in December or January—in San Diego County earlier than recorded farther north, as in Idaho and Montana (Marks et al. 1994). Egg-laying occurs from February to May. Dates of 66 egg sets collected 1889–1961 range from 7 February to 4 May; Sharp (1907) reported eggs on 10 May. Our dates for occupied nests range from 24 February (1998) to 26 May (also 1998—the breeding season was most extended in the wettest year of the atlas period). Our dates of fledglings range from 28 April to 10 June. These records are within the Long-eared Owl’s normal breeding season in North America (Marks et al. 1994).
Migration: In San Diego County, the Long-eared Owl is at least partially migratory. Some pairs remain year round near nest sites; others move short distances. For example, owls banded in Bandy Canyon (K13) 16 May 2001 were found in winter in the nearby San Pasqual Valley (J12/K12), and some from the oaks of Sycamore Canyon were found in winter in the riparian forest near Kumeyaay Lake (P11; W. E. Haas). Young may migrate much greater distances: a bird banded in Escondido 22 April 1934 was found at Corbeil, Ontario, Canada, 9 October of the same year (Lincoln 1936a). At many breeding sites Long-eared Owls cannot be found once fledglings become independent. This may be the result of migration and dispersal but also because the birds go nearly silent once the breeding season has ended. The migration schedule, if any, is obscured by the several sightings of apparently nonbreeding birds through the breeding season.
Winter: Winter records for the Long-eared Owl range from single individuals to relatively large communal roosts. Roost sites and the number of owls using them change from year to year. Communal roosting is common in this nomadic species; in Europe but not in North America it has been linked to fluctuations in the abundance of prey (Hagen 1965, Korpimäki and Nordahl 1991). Long-eared Owls roosted in Sycamore Canyon continuously from November 1997 through the following breeding season, corresponding to heavy rain and a population explosion of the California vole, which far outweighed all other prey items taken by the owls during that period.
Communal winter roosts are known mainly from the Anza–Borrego Desert, especially in rows of athel tamarisk near Clark Dry Lake (D26; 12 on 3 January 2002, S. Bell) and the Borrego Sink (G25; 25 on 8 February 1999, R. Thériault; 30 on 22 December 1991, G. L. Rogers). In natural desert habitats aggregations are smaller, up to five in Wonderstone Wash (D29) 10 January 2002 (P. D. Jorgensen), three in an isolated palo verde at the north end of Clark Valley (C25) 1 December 2001 (H. E. Stone). On the coastal slope a former roost in Rancho Otay (U12) had up to 12 on 15 December 1979 (B. Cord). But in this area wintering Long-eared Owls are more frequently solitary, roosting in a wide assortment of tree species including willows, oaks, eucalyptus, and tamarisk.
Conservation: The Long-eared Owl has experienced a steep decline in southern California during the 20th century, usually attributed to the loss of riparian and grassland habitats (Marti and Marks 1989, Bloom 1994). Historically, the Long-eared Owl was common in riparian forests along the coast (Cooper 1870, Sharp 1907). By 1944 their declining numbers had been noted, “in the main probably as a result of clearing of bottomlands” (Grinnell and Miller 1944). Garrett and Dunn (1981) considered the species “rare coastally, and virtually eliminated there as a breeder.” In 20 years of monitoring birds of prey in Camp Pendleton, Bloom (1994) located only seven territories, three of which had been abandoned by the early 1990s. It should be noted that Bloom’s data on the Long-eared Owl were incidental to his other work with raptors; he undertook no focused surveys for the species in San Diego County.
Although the original data presented here suggest that the Long-eared Owl has made a comeback of sorts since the early 1990s, none of the recent records even faintly echoes the size of historic breeding colonies. Most breeding locations are sites of single pairs only, rarely clusters of three to eight territories. The Long-eared Owl’s ability to tolerate nearby development and other types of disturbance is low—all currently known nest sites are secluded from human dwellings. Bloom (1994) suggested that this species rarely tolerates disturbance within one kilometer of a breeding territory. Most of our records during the atlas period support this hypothesis. Where the hypothesis did not hold true, a mitigating factor could be found; for example, the nest in Mission Trails Regional Park (P11) was isolated from nearby development by a steep canyon—and abandoned after 1998.
Continued loss of and encroachment near riparian woodlands will surely reverse what gains the species has made during the past decade. Conversely, maintenance and enhancement of existing riparian corridors and oak groves and preservation or restoration of adjacent lands to grassland will be needed to provide breeding habitat sufficient to ensure a reasonably stable breeding population within the county. Bloom (1994) emphasized that in ever decreasing patches of natural habitat the Long-eared Owl loses out in competition for territories and nesting sites with other large birds, suffering predation by hawks, crows, and ravens.
Taxonomy: In North America, the Long-eared Owl has been divided into two subspecies, A. o. wilsonianus (Lesson, 1830) in the east and the paler A. o. tuftsi Godfrey, 1947, in the west. Kenneth C. Parkes (in Rea 1983) and Marks et al. (1994), however, questioned the validity of this distinction, which seems unlikely in view of the species’ highly nomadic tendencies.
Short-eared Owl Asio flammeus
Most owls live in woodland and forest, but the Short-eared Owl lives in marshes and grassland. It is principally a winter visitor to San Diego County, regular in small numbers around south San Diego Bay and in the Tijuana River estuary but seldom seen elsewhere. The Short-eared Owl is rare and declining in California, recognized as a species of special concern by the California Department of Fish and Game. Thus the finding in 1998 and 2000 of at least three individuals from mid April to mid July, including an apparent pair, was most unexpected.
Winter: In San Diego County, the Short-eared Owl occurs regularly in small numbers only around south San Diego Bay and in the Tijuana River estuary. From 1997 to 2002, the highest counts here were of at least two between the Chula Vista Nature Center and the salt works (U10) 18 December 1999 (B. C. Moore et al.) and two in the Tijuana estuary (V10) the same day (R. B. Riggan). The Short-eared Owl has also been seen with some frequency at Fiesta Island, Mission Bay (Q8/R8), with up to six on 30 January 1976 (J. L. Dunn) and one found dead 18 December 1996 (SDNHM 49048).
At least three are known from the San Pasqual Valley, with one seen near the east end of Lake Hodges (K11) 16 November 1985 (K. L. Weaver), one found long dead there in spring 1997 (SDNHM 50409), and one seen near Ysabel Creek Road (J12) 1 December 1998 (W. E. Haas) and 2 January 1999 (C. G. Edwards). The few other records are scattered over the coastal wetlands like San Elijo Lagoon and grasslands in the coastal lowland like Lopez Canyon and Otay Ranch (the latter now developed). During the atlas period there were three winter records away from the San Pasqual Valley, San Diego Bay, and the Tijuana estuary, of one in Dameron Valley (C15) 3 February 2001 (K. L. Weaver), four near Swan Lake between Lake Henshaw and Warner Springs (F18) 10 December 2000 (J. R. Barth, M. G. Mathos), and one in Sycamore Canyon (O12) 27 February 1998 (G. L. Rogers).
In the Anza–Borrego Desert the only identifications of the Short-eared Owl probably correct are of one in Clark Valley (D25) 15 October 1977 and one in Collins Valley (D23) 21 December 1983 (A. G. Morley).
Migration: Dates for the Short-eared Owl away from coastal wetlands in San Diego County range from 30 September (1980, one at Point Loma, S7, AB 35:226, 1981) to 11 April (1975, one in the Santa Margarita River valley at Basilone Road, E6, A. Fries).
Breeding distribution: The only record of the Short-eared Owl’s breeding in San Diego County is Willett’s (1933) report of E. E. Sechrist collecting two sets of eggs at National City (T10) 10 April 1906. J. B. Dixon (in Willett 1933) observed the species in summer at San Diego Bay and the Santa Margarita River mouth. Alice Fries noted one at the latter locality 23 May and 12 June 1972. By the time the atlas study began, the species was long inferred as absent from coastal southern California in summer (e.g., Garrett and Dunn 1981, Small 1994). Therefore its showing up at the Tijuana River estuary in 1998 was a great surprise. Brian Bonesteel trapped and photographed one in the Least Tern nesting colony there 28 May, then photographed two on the fence separating the refuge from the Imperial Beach naval auxiliary landing field (Ream Field) 16 June. In 2000, R. T. Patton and S. M. Wolf noted one at the Chula Vista Wildlife Reserve in south San Diego Bay on 12 and 19 April. Thus it is possible that the occasional pair of Short-eared Owls still attempts to nest in San Diego County.
Nesting: Short-eared Owls nest on the ground among marsh vegetation or grasses. No details on the nests in San Diego County beyond that mentioned above are available.
Conservation: Stephens (1919a) called the Short-eared Owl “rather rare” in San Diego County, but Willett (1933) said it was “common” in coastal southern California in general. Grinnell and Miller (1944) called attention to a decrease, attributing it to shooting by duck hunters. In San Diego County, there seems to have been a decrease since the 1960s; the San Diego Christmas bird count averaged 4.7 per year from 1966 to 1972 but only 0.85 from 1989 to 2001. A factor contributing to the decline is undoubtedly the loss and degradation of coastal wetlands and native grasslands, both of which now cover only a small fraction of their original extent. Increased predation and human disturbance threaten all ground-nesting birds along the coast, including high-level predators like the Short-eared Owl.
Taxonomy: As a species breeding on four continents and numerous islands the Short-eared Owl not surprisingly consists of several subspecies. But only nominate A. f. flammeus (Pontoppidan, 1763) occurs on the mainland of North America.
Northern Saw-whet Owl Aegolius acadicus
In San Diego County the Northern Saw-whet Owl occurs in coniferous woodland almost exclusively, spilling over into pure oak woodland around Palomar Mountain. A year-round resident, it is largely nocturnal and difficult to see, generally detected only by its monotonous hooting. One surprise coming from the atlas study was that the owl often calls in the evening before sunset and rarely even at midday. Until 1994, when the species was first reported in Baja California, San Diego County was thought to represent the southern tip of its range along the Pacific coast.
Breeding distribution: The Saw-whet Owl is found in all of San Diego County’s mountains with coniferous woodland but is more common on Palomar and Hot Springs mountains than farther south. Indeed, in the coniferous forest on these mountains, it is the most numerous owl. Nightly counts within a single atlas square in the breeding season range up to six calling territorially near the Palomar Observatory (D15) 13 May 1999 (K. L. Weaver) and eight near the summit of Hot Springs Mountain (E20) 18 May 2001 (K. L.Weaver, C. R. Mahrdt). The owl is particularly widespread on Palomar Mountain, extending into oak woodland with few or no conifers, on the north slope down to at least 3400 feet elevation in Cutca Valley (C14; up to four on 24 June 2000, J. M. and B. Hargrove), on the south slope down to about 3200 feet along South Grade Road (F14; one on 20 March 1999, K. L. Weaver, R. Wissa) and to 2550 feet along the San Luis Rey River near Wigham Creek (male calling sporadically but apparently unmated, W. E. Haas).
Elsewhere the Saw-whet Owl is uncommon but occurs on Bucksnort Mountain (C20; one on 26 June 1999, L. J. Hargrove), in the Cuyamaca Mountains (one at Pine Hills Fire Station, L19, 22 May 1999, R. Breisch et al.; one on North Peak, L20, 31 March 2001, G. L. Rogers), and in the Laguna Mountains (up to two near Morris Ranch, P23, 27 March 2001, E. C. Hall, J. O. Zimmer). The species is undoubtedly resident as well on Volcan Mountain, where we found it in winter but missed it in the breeding season.
Nesting: Very little information is available on Saw-whet Owl nesting in San Diego County. From 1997 to 2001, our only breeding confirmations were of a family of two adults and two fledglings near the summit of Hot Springs Mountain 15 August 2000 (W. E. Haas) and a fledgling just south of Filaree Flat, Laguna Mountains (N22), 7 July 2001 (G. L. Rogers). The only previous records are of young seen in Cuyamaca Rancho State Park 4 July 1939 (Grinnell and Miller 1944), on Middle Peak, Cuyamaca Mountains (M20), 19 July 1987 (R. E. Webster, AB 41:1488, 1987), and on Hot Springs Mountain 21 June 1986 (C. G. Edwards, AB 40:1256, 1986). The species nests in tree cavities and uses nest boxes; one calling from a nest box along the San Luis Rey River at Wigham Creek was apparently advertising an available nest site (W. E. Haas).
Winter: San Diego County’s Saw-whet Owls are sedentary; the sites where we noted the species in winter but not spring or summer do not represent any dispersal. Late February, the time of many of our winter records, begins the season of the species’ peak in calling. Winter counts range up to 10 near the summit of Hot Springs Mountain 15–16 February 2002, when the birds called most of the night (K. L. Weaver, C. R. Mahrdt). Numbers on Volcan Mountain (H20/I20) in winter, where we missed the species during the breeding season, ranged up to three on 17 December 2001 (R. T. Patton).
Migration: The Saw-whet Owl is partly migratory in the northern part of its range and a casual winter visitor to the deserts of southeastern California. There are no records of migrants, however, from San Diego County.
Conservation: During the atlas period we found the Saw-whet Owl more often than expected on the basis of the meager previously published records, but this apparent increase probably reflects only better nocturnal coverage of the species’ habitat. Roger Higson observed the species regularly around the Palomar Observatory through the early 1980s.
Taxonomy: The Northern Saw-whet Owl consists of two subspecies, the dark A. a. brooksi (Fleming, 1916), endemic to the Queen Charlotte Islands, and A. a. acadicus (Gmelin, 1789), extending over the remainder of the species’ transcontinental range.