Mockingbirds and Thrashers — Family Mimidae
Gray Catbird Dumetella carolinensis
Though the Gray Catbird breeds west almost to the coast of British Columbia, it is only a rare vagrant to California—the bulk of the population migrates east of the Rocky Mountains. But the species is on the increase: of 107 reports accepted by the California Bird Records Committee 1884–1999, one third were in just the last four years of this interval. Similarly, of the 20 records of the Gray Catbird in San Diego County, 10 have come since initiation of the field work for this atlas in 1997.
Migration: Half of San Diego County’s known catbirds have been fall migrants, occurring as early as 24 September (1976, one at Point Loma, S7, K. van Vuren, Luther et al. 1979). Besides eight fall records from Point Loma, there is one from the Tijuana River valley 7–8 November 1964 (the only specimen, SDNHM 35095), one from a boat 15 miles off Oceanside 26 October 1983 (M. W. Guest, Bevier 1990), and two from Paso Picacho Campground (M20) 29 October 1988 (D. W. Aguillard, Pyle and McCaskie 1992) and 17 November 2002 (T. McGrath, M. San Miguel, NAB 57:118, 2003). The four spring records are of single birds in San Clemente Canyon (P8), 27 April 1999 (M. B. Stowe, Rogers and Jaramillo 2002), at Southwest Grove, Mountain Palm Springs (P27), 31 May 2000 (D. G. Seay; NAB 54:327, 2000), and at Point Loma 28 May–2 June 2001 (R. E. Webster, M. U. Evans, NAB 55: 357, 583, 2001) and 27–28 May 2002 (R. E. Webster, NAB 56:358, 2002). One in riparian woodland along the Santa Margarita River near the Camp Pendleton airport (E5) 28 June 2000 (P. A. Ginsburg, NAB 54:424, 2000) may have been summering, and one in molt at Cabrillo National Monument, Point Loma 11–17 July 1988 (B. and I. Mazin, Pyle and McCaskie 1992) certainly was.
Winter: Three wintering Gray Catbirds have been reported from San Diego County, from Balboa Park (R9) 16 December 1972 (P. Unitt) and from Point Loma 7 November 1983–13 March 1984 (V. P. Johnson, Roberson 1986) and 31 October 1999–21 January 2000 (D. Aklufi, NAB 54:222, 2000).
Conservation: The Gray Catbird’s increase in California most likely reflects the westward expansion of the breeding range; the species exemplifies changes in migration route lagging behind changes in breeding distribution.
Taxonomy: The validity of the division of the Gray Catbird into eastern and western subspecies is dubious. The difference of a paler crissum in western catbirds, upheld by Phillips (1986), is not evident in specimens in the San Diego Natural History Museum.
Northern Mockingbird Mimus polyglottos
The Northern Mockingbird has become such a fixture in southern California’s domesticated landscape that it now seems out of place in natural habitats. From inner-city neighborhoods to rural ranches it is common year round. Nevertheless, mockingbirds still occur also in desert washes and in open sage scrub with scattered large shrubs or cacti. Field work for this atlas revealed that in the Anza–Borrego Desert the mockingbird nests widely in wetter years, retreating to towns and oases in dry ones.
Breeding distribution: The mockingbird is widespread in San Diego County, but its breeding distribution is neatly divided into two blocks. It is common in agricultural and urbanized areas throughout the coastal lowland and lower foothills and lacking only from the north side of the Santa Margarita Mountains, where development so far is almost nil. It is generally absent from unbroken chaparral. Its upper elevational limit is about 4000 feet, where it is rare (one near Julian, J20, 24 June 2001, A. P. and T. E. Keenan; one in Pine Valley, P21, 29 June 1997, J. K. Wilson).
The mockingbird’s primitive habitat along the coast was probably open sage scrub with scattered large shrubs and cactus thickets. The birds are still common in this now rare habitat wherever it remains, as in the San Diego Wild Animal Park (J12/J13) and along the western and northern slopes of Otay Mesa (V12/V13).
The mockingbird also occurs throughout the Anza–Borrego Desert, at least in wet years. It is common around houses and mesquite thickets, competing with the Phainopepla for mistletoe berries. From the desert, it ranges west in small numbers onto the coastal slope in Warner Valley and on the Campo Plateau. The narrow gap in the Mockingbird’s range is due to the zone of thick chaparral as well as to the forests of the higher mountains.
The most interesting thing we observed about the mockingbird from 1997 to 2002 was its response to variation of rainfall in the Anza–Borrego Desert. In this area, away from the town of Borrego Springs, the number recorded per hour in the wet year 1998 was 67% higher than in the year with the next highest number, 2001. And the hourly count in 2001 was over twice that in 1997, 1999, or 2000. The jump from 1997 to 1998 was by a factor of over 8. The pattern parallels that of the Horned Lark and some wintering sparrows. It recalls the scenario observed by Gale Monson in southwestern Arizona in 1952 and 1958, when, after wet winters, “Mockingbirds, Western Meadowlarks, and other species appeared ‘from nowhere’ and nested abundantly” (Phillips et al. 1964).
Nesting: Mockingbirds build their nests in trees or shrubs with dense screening foliage or protective thorns. As expected of a bird so well adapted to the urban landscape, it uses ornamental plants freely. Garden plants with both dense foliage and thorns, like pyracantha and bougainvillea, are particularly attractive nest sites. Prickly pear cacti are favored sites as well. In the desert, use of mesquite is common; mesquite often hosts mistletoe, which offers the mockingbird a staple food as well as an ideal nest site. In more sparsely vegetated desert, we noted nests in ocotillo, placed in the “cage” formed by the multiple spreading thorny branches at the plant’s base.
The Northern Mockingbird enjoys an unusually long breeding season, commonly raising two broods per year, rarely three (Bent 1948). Observations of adults carrying food to a nest as early as 10 March imply egg laying by 26 February; we found nests with eggs as late as 16 July. Our earliest observations of nesting activity were from Borrego Springs, but some along the coast followed just a few days later. Interestingly, our latest records of eggs were also from Borrego Springs, in mid July when high temperatures were reaching 116° F. Collected egg sets extend the nesting season we observed even further; Bent (1948) reported California egg dates ranging from 16 February to 2 September.
Migration: Though the mockingbird is a permanent resident in San Diego County, whether or not individual birds are sedentary probably varies by habitat. Birds living in urban areas may have no need to move. At my home in the Hillcrest area of San Diego (R9), one pair of mockingbirds maintained the same territory year round for four years; the female was recognizable as the same individual by an injury to one foot. The species’ irregularity in the Anza–Borrego Desert, however, suggests that many mockingbirds move opportunistically as conditions demand.
Winter: The mockingbird’s distribution in San Diego County in winter shows interesting differences from that in spring and summer. On the coastal side, the species is still widespread and clearly concentrated in the agricultural areas of the north county and the urbanized area of metropolitan San Diego. In the Anza–Borrego Desert, however, the mockingbird is much more restricted in winter than in spring to developed areas and at oases. Even in the wet winter of 1997–98 we did not find it over much of the sparsely vegetated desert, so the winter distribution shows large gaps where the breeding distribution appears nearly continuous. In the upper foothills and lower mountains we found the Mockingbird in winter rarely but, surprisingly, more frequently and more widely than in summer. Winter numbers in these areas were generally small, three or fewer per day, but up to five were at Santa Ysabel (J18) 17 December 2001 and seven were in La Jolla Indian Reservation (F15) 21 January 2000 (W. E. Haas et al.). Winter records also are all from about 4000 feet elevation and lower, except for one about 4900 feet elevation near San Ignacio, Los Coyotes Indian Reservation (E21) 19 December 1998 (K. L. Weaver, C. R. Mahrdt).
Conservation: The mockingbird adapted to the urbanization of southern California as soon as it began. Grinnell (1911) noted the suitability of orange groves as nesting habitat and estimated that suburban development plus the planting of orange groves allowed the Mockingbird population of Los Angeles County to increase by a factor of five. More recent data, such as Christmas bird counts, show no consistent trend, though the species’ numbers probably continue to increase in tandem with increase in the extent of development. Because of lack of early data, the mockingbird’s history in the Anza–Borrego Desert is unclear; there has been no obvious change since the 1960s. The opportunistic breeding we observed suggests that the Mockingbird has always been an irregular species in this area. Yet the species was absent from the nearby Salton Sink at the beginning of the 20th century, colonizing only after the area was turned to farmland (Arnold 1935, 1980).
Taxonomy: An eastern and a western subspecies of the Northern Mockingbird are commonly recognized, but Phillips et al. (1964) ascribed the color difference to soot and fading; the figures in Ridgway (1907) suggest the measurement differences are insufficient to diagnose the subspecies either. So all Northern Mockingbirds of mainland North America are best called M. l. leucopterus (Linnaeus, 1758).
Sage Thrasher Oreoscoptes montanus
The Sage Thrasher is a migrant and winter visitor in San Diego County, rare from September to mid January, uncommon from late January through March. It occurs mainly in and near the Anza–Borrego Desert, frequenting scrub like saltbush and mesquite, as well as stands of the big sagebrush at higher elevations that resemble its breeding habitat in the Great Basin. Near the coast the Sage Thrasher is rare—the sage scrub that it once visited more numerously now being largely converted to cities.
Migration: Sage Thrashers may arrive in San Diego County as early as 18 September (1974, one in the Tijuana River valley, G. McCaskie) but remain rare through mid January. By the end of January their numbers increase gradually, to reach a peak in late February and March. Patten et al. (2003) reported spring migration in the Salton Sink beginning as early as 17 January, but in San Diego County establishing an exact date for spring arrival is impossible. The highest count ever reported in the Anza–Borrego Desert, of 20 in Little Blair Valley (L24) 2 February 1991 (B. Cord), was presumably of migrants. The highest count during the atlas period was of seven east of the Borrego Sink (G26) 17 February 2000 (M. B. Mulrooney). The species is rare by early April, and the latest dates are 16 April (1999; one in the Borrego Valley east of Peg Leg Road, F26, M. B. Stowe; 1999, three near In-Ko-Pah, T29, D. C. Seals), 25 April (1999, four in the Borrego Valley, F25, P. D. Ache), “May” (1881, two specimens collected at San Diego, Belding 1890), and 4 June (1970, one at Agua Caliente Springs, M26, A. Fries, AFN 24:718, 1970). G. Holterhoff (in Belding 1890) reported the Sage Thrasher as “common” near National City in the summer of 1883 but apparently collected no specimen and the record has not been repeated since.
Winter: Because of the Sage Thrasher’s migration schedule, the map of its winter distribution shows records for December and January only. The species is widely but thinly scattered through the Anza–Borrego Desert, all records before February being of just one or two birds. A few apparently wintering Sage Thrashers turned up during the atlas field work in high, dry valleys in north-central San Diego County: three in Oak Grove Valley (C17) 23 January 1999, one at Puerta La Cruz (E18) 23 January 1999 (R. and S. L. Breisch), one about 3700 feet elevation along Agua Caliente Creek at the east base of Hot Springs Mountain 26 January 2002 (K. L. Weaver), and three in big sagebrush at Ranchita (H21/H22) 14–15 January 1999 (P. Unitt).
Lower on the coastal slope we encountered the Sage Thrasher only eight times from 1997 to 2002, generally in sage scrub or cactus thickets, as in Pamo Valley (one on 2 and 30 January 2000, W. E. Haas, L. E. Taylor). The Sage Thrasher formerly occurred with some regularity in halophytic scrub along the coast, but our only sightings from this habitat, of which hardly any remains, were of one at the south end of Ysidora Basin (G5) 26 December 1998 (P. Unitt) and another at the south end of San Diego Bay (U10) 16 December 2000 (P. R. Pryde).
The abundance of the Sage Thrasher varied greatly over the atlas period. Figures for winter and the following spring combined, the number reported varied from 7 in 1997–98 to 90 in 1998–99 to 55 in 1999–2000 to 20 in 2000–01. Possibly the birds enjoyed an exceptionally productive breeding season following the wet winter of 1998, leading to a population spike, then the numbers declined steadily over the following dry years.
Conservation: Though the Sage Thrasher was never common in coastal San Diego County, it was once considerably more frequent than it is now, though still irregular. Heermann (1859) “remarked it on several occasions in the environs of San Diego,” and Baird (1858) listed two specimens collected there. Huey (1924) listed several specimens collected around San Diego; on 15 March 1923, 5 miles east of National City, “Sage Thrashers were abundant on the mesa, and a great many could have been collected” (four actually were—SDNHM 8545–48). Single birds were noted on five San Diego Christmas bird counts from 1966 to 1974 but on only one from 1975 through 2002. Apparently a bird whose winter habitat around San Diego is the same as that of the San Diego Cactus Wren and California Gnatcatcher declined with the loss of that habitat. The lack of early data from the Anza–Borrego Desert precludes assessment of any trend there.
Brown Thrasher Toxostoma rufum
The only thrasher breeding in the eastern United States is a rare vagrant to California. Though there are only 18 records for San Diego County, they are scattered through every month of the year. Nevertheless, the Brown Thrasher occurs primarily as a migrant and winter visitor between 20 September and 15 May, with only two known in summer.
Migration: There are six records of the Brown Thrasher in fall migration, four from Point Loma (S7), one from La Jolla (P7; 17 October 2001, F. A. Belinsky, NAB 56:107, 2002), and one from the Borrego Springs sewage pond (H25; 1 November 1992; A. G. Morley, AB 47:150, 1993). In spring, there are five records of birds not known to have wintered, two from Point Loma (5 April 1973, AB 27:821, 1973; 15 May 1979, AB 33:806, 1979), one from Pio Pico Campground (T15; 10 April 1981, D. Povey), one from Oceanside (H6; 24 April 1973, AB 27:821, 1973), and one from Borrego Springs (G24; 27 April 1998, P. D. Ache). One at Tamarisk Grove Campground 19–21 June 1988 (L. Walton, R. Thériault, AB 42:482, 1341, 1988) was also likely a late spring vagrant. However, one at Point Loma 24 July–10 September 2001 was in molt, as expected of the species in its summer breeding range (V. Conway, L. M. Dorman, NAB 55:483, 2001).
Winter: The Brown Thrasher has been recorded in San Diego County five times in winter; four of the birds were known to have remained at least 11 days, on dates ranging from 15 October to 3 May. Four of the records are from parks and gardens in San Diego (Unitt 1984); one is from chaparral on the south side of San Elijo Lagoon 13 January–24 February 1996 (P. A. Ginsburg, NASFN 50:224, 1996).
Taxonomy: No specimen of the Brown Thrasher has yet been collected in San Diego County, but only the expected paler western subspecies T. r. longicauda is known from elsewhere in California and Arizona.
Bendire’s Thrasher Toxostoma bendirei
In California, Bendire’s Thrasher is confined largely to the Mojave Desert. Even there it is so rare and irregular it is regarded as a species of special concern by the California Department of Fish and Game. San Diego County is off the species’ normal migration route, so it occurs here less than annually, as a very rare migrant and winter visitor. Nevertheless, a single pair nested successfully near Ocotillo Wells in 1993.
Breeding distribution: I found the single known nest of Bendire’s Thrasher in San Diego County 2.5 miles southwest of Ocotillo Wells (J28) on 26 April 1993. On that date the nest contained two chicks, one of which later fledged successfully (AB 47:1151, 1993). The winter of 1992–1993 was unusually wet; presumably the lush growth and better food supply of the following spring allowed the species to spread exceptionally far to the southwest of its normal range. On 19 May 1998—also following a wet winter—Lori Hargrove found a single Bendire’s Thrasher in ornamental trees around a house 3 miles southeast of Ocotillo Wells (J29)—San Diego County’s only other record of the species during the breeding season (FN 52:504, 1998).
Nesting: The nest near Ocotillo Wells was in a large, dense desert lavender, about 5 feet off the ground. It was thus typical for the species in height and selection of a rather spiny shrub as a site.
Migration: Of San Diego County’s 28 records of Bendire’s Thrasher, 13 are of fall migrants, on dates as early as 18 August (1991, Point Loma, R. E. Webster, AB 46:151, 1991) and 27 August (1964, Solana Beach, McCaskie et al. 1967b). Of the four spring records of birds not known to have wintered, two are from near the coast (one at Sweetwater County Park, T12, 6 March 1999, S. L. Breisch; one in the Tijuana River Valley 4 April 1970, AFN 24:645, 1970), two from the Anza–Borrego Desert (two in Smoke Tree Wash, E28, 9 March 1997, P. D. Jorgensen, NASFN 51:928, 1997; one at Palm Spring, N27, 19 March 1983, E. A. Cardiff, AB 37:913, 1983).
Winter: Winter records of Bendire’s Thrasher in San Diego County are nine, four from the Tijuana River valley (Unitt 1984), one from Coronado (28 January–3 March 1985, J. L. Coatsworth, AB 39:211, 1985), one from Otay Mesa (16 February–16 March 1985, M. Orell, AB 39:211, 1985), one from Escondido (2–22 January 1994, J. L. Coatsworth, NASFN 48:249, 1994), one from San Felipe Valley 1.1 miles northwest of Paroli Spring (H21; 21 December 1998, P. Unitt, NAB 53:210, 1999), and one from Agua Caliente Springs (M26; 21 January 1999, E. C. Hall, NAB 53:210, 1999). Several of these records are of birds remaining for extended periods, yielding a range of 8 November–16 March for known wintering Bendire’s Thrashers.
Conservation: The reasons for Bendire’s Thrasher’s rarity in California are not clear, as much of its high-desert habitat is little disturbed. Nevertheless, habitat fragmentation and disturbance resulting from development, often spreading over large areas at low density, is a concern (J. Sterling unpubl. data). Sterling’s survey in 2001 yielded only two or three Bendire’s Thrashers in areas where England and Laudenslayer (1989) found 41 and 23 in 1986 and 1987, respectively. The difference may have been due to population cycles following rainfall variation, however, than to long-term decline.
Curve-billed Thrasher Toxostoma curvirostre
The Curve-billed Thrasher is common in southern Arizona but known in California from only 14 records from the Colorado River and Imperial Valley. When I saw one on Otay Mesa, I questioned whether it might be an escapee from captivity, as it was less than one half mile from the Mexican border. Nevertheless, the California Bird Records Committee accepted the bird as a natural vagrant, noting the lack of Curve-billed Thrashers reported in captivity by Hamilton (2001) and the species’ history of vagrancy as far north as Canada (Cole and McCaskie 2004).
Migration: The Curve-billed Thrasher on Otay Mesa (V13) 28 April 2002 was singing and calling with the species’ characteristic “whit-wheet!”
California Thrasher Toxostoma redivivum
A sickle-shaped bill, long tail, and creative song give the California Thrasher the character its plain brown plumage does not. A sedentary resident, the thrasher is one of the characteristic birds of chaparral, though it occurs in lower density in sage scrub, oak and riparian woodland, desert-edge scrub, and mesquite thickets as well. It is still common over much of San Diego County but does not adapt to urban development. Within the city of San Diego it survives in large canyons like Florida and Tecolote but disappears from small canyons surrounded by housing tracts.
Breeding distribution: The California Thrasher is widespread over the coastal slope of San Diego County, lacking only in heavily urbanized areas. It is common wherever there is extensive chaparral and considerably more common in dense chaparral than in coastal sage scrub or other habitats (Cody 1998). So it is no surprise that the greatest concentrations are where chaparral is most extensive: along the north side of the Santa Margarita Mountains east to Fallbrook, along the north side of Palomar Mountain east to Bucksnort Mountain and Indian Flats, in central San Diego County from Miramar east to El Cajon Mountain and Alpine, and all across southern San Diego County from Otay Mountain to the Jacumba Mountains. The species ranges from sea level to the summit of Hot Springs Mountain (E20; up to three on 9 June 2001; K. L. Weaver).
The California Thrasher extends down the east slope of the mountains to their bases, being uncommon in open desert-edge scrub. Small numbers are isolated in the pinyon/juniper zone of the Vallecito Mountains (up to three in Pinyon Mountain Valley, K25, 26 May 2000, D. C. Seals). In the Santa Rosa Mountains the California Thrasher ranges barely into San Diego County on the south flank of Rabbit Peak (C27; two singing on 3 May 2000, P. Unitt). At low elevations in the northern Anza–Borrego Desert the species ranges east to Lower Willows (D23; up to five on 12 May 2001, B. Peterson) and Tamarisk Grove (I24; up to six on 17 March 1998, P. K. Nelson), except for irregular occurrences in the mesquite thicket at the west end of Clark Dry Lake [E25; seen repeatedly in 1993, M. L. Gabel in Massey (1998); one on 17 March 2000, K. L. Weaver]. In the southern Anza–Borrego Desert the California Thrasher occurs at all the oases along the bases of the mountains, being especially numerous in the extensive mesquites at Vallecito (M25; up to 12 on 27 April 1998, M. C. Jorgensen). Along Carrizo Creek it ranges east to Carrizo Marsh on the Imperial County line (O29; up to three, including an agitated pair, on 6 May 1998, P. D. Jorgensen).
Nesting: California Thrashers build a bulky cup nest of sticks, usually placing it in the upper half of a dense shrub where it is screened from above by foliage. Like most sedentary chaparral birds, the California Thrasher begins laying typically in the third week of March, but occasionally it begins earlier. Reports of feeding young as early as 18 March and fledglings as early as 28 March imply incubation begun about 1 March. California Thrasher eggs have been collected at San Diego as early as 9 February, however, and at Pasadena as early as 15 December (Grinnell 1900, Bent 1948). November nests have been reported from Pasadena by Sargent (1940) and from Los Angeles by Davis (1952). In San Diego County a barely fledged young was picked up in Valley Center (G11) 16 October 1997 (SDNHM 49967). Michael A. Patten noted young one to two weeks out of the nest near Spring Valley (R13) 11 March 2002. Evidently a few California Thrashers will nest in fall or winter if stimulated by conditions such as early rain.
Winter: As expected for a sedentary species, the California Thrasher’s pattern of abundance in winter is the same as in the breeding season. Our maximum daily count was 45 around Oriflamme Mountain (M22) 22 February 2000 (J. R. Barth).
The species’ dispersal outside its breeding range is minimal. In winter, we noted it in the Anza–Borrego Desert in 12 atlas squares where we did not record it during the breeding season, but all these were adjacent to squares where we did find it in spring or summer. Winter records include two for the mesquite thicket at the west end of Clark Dry Lake (D25; up to two on 20 December 1998, E. Post).
Conservation: The California Thrasher is moderately sensitive to habitat fragmentation. Among San Diego’s canyons isolated by urbanization, Crooks et al. (2001) found thrashers consistently in tracts of scrub of 30 or more hectares only; they found them inconsistently in fragments of 8 to 30 hectares and not in fragments smaller than 8 hectares. By 1997 they had disappeared from 4 of 11 canyons where they occurred in 1987 and colonized only one additional canyon (Crooks et al. 2001). On the scale of our atlas grid, however, the California Thrasher appears absent from only the most heavily urbanized areas.
The California Thrasher prefers a habitat that is subject to fire, but it is among the slower species to recolonize recovering burned chaparral. Surveys near Pine Valley found the California Thrasher to be one of three chaparral birds still significantly less abundant in areas averaging 6 years since a fire than in areas averaging 30 years since a fire (Cleveland National Forest unpublished data).
Taxonomy: The California Thrasher has been divided into two or three subspecies, but the differences (in color) are slight. Whether they suffice to support a taxonomic distinction needs reevaluation on the basis of unfoxed, freshly molted specimens.
Crissal Thrasher Toxostoma crissale
Here at the western limit of its range, the Crissal Thrasher has one precarious toehold on San Diego County: the thicket or bosque of mesquite in the floor of the Borrego Valley. Though year-round residents, the birds are rare, shy, and difficult to see. The poor health of the bosque, likely the result of a water table lowered by groundwater pumping, bodes ill for the thrasher’s long-term survival here. The Crissal Thrasher is scarce throughout its California range and is regarded as a species of special concern by the California Department of Fish and Game.
Breeding distribution: The Crissal Thrasher’s habitat in San Diego County covers about 3 square miles, almost entirely within atlas square G25 between the Borrego Valley airport and Borrego Sink, east of Borrego Valley Road. The population’s size is unknown but is unlikely to be more than 10 pairs. The highest daily counts during the breeding season consisted of just one family group of five, on 4 and 11 June 1998 (R. Thériault). The only sighting outside of square G25 was of two just to the east, on the northeast side of the Borrego Sink in G26 18 April 1998 (P. Unitt, F. L. Unmack).
M. L. Gabel (in Massey 1998) noted the Crissal Thrasher in the mesquite thicket at the west end of Clark Dry Lake (D25/E25) on 4 June 1993 and 27 June 1994 but found the California Thrasher there more consistently. From 1997 to 2002 our only sighting here was of one in winter. Lying 7 miles north of the Borrego Valley mesquite bosque, this is the only place in California where both the California and Crissal Thrashers have occurred. Reports of the Crissal elsewhere in the Anza–Borrego Desert are based on misidentified California Thrashers.
Nesting: The Crissal Thrasher usually builds its nest in dense thorny shrubs, using mesquite primarily if it is available (Gilman 1909, 1915, Hanna 1933). The only information recorded on Crissal Thrasher nesting in San Diego County is Robert Thériault’s observations in 1998: fledglings following or being fed by their parents on 4, 11, and 15 June. On the basis of an incubation period of 14 days and a nestling period of 11–16 days, these young hatched from eggs laid in early May. Most likely they were the result of a second or replacement clutch, as the mean date of eggs collected in the Coachella Valley, Riverside County, is 27 March (Cody 1999). Fifty-six egg sets from throughout California range from 10 February to 10 June (Bent 1948).
Winter: Winter records of the Crissal Thrasher are from the same area in the Borrego Valley as those in the breeding season, except for a few sightings near the Borrego Springs sewage ponds, on the south side of the Borrego Sink (H25; P. D. Jorgensen, Massey 1998), and one at the northwest corner of Clark Dry Lake (D25) 4 February 1999 (B. Scheible et al.). All winter sightings during the atlas period were of single individuals only. The species is found on most Borrego Springs Christmas bird counts, always as a result of a targeted search; the maximum recorded on the count was four on 28 December 1986.
Conservation: The Crissal Thrasher has been noted continuously in the Borrego Valley’s mesquite bosque since Stott and Sams (1959) first reported four on 12 December 1958. Data are too skimpy to suggest any trend. But the long-term outlook is not good: the mesquite bosque suffers from a falling water table and illegal wood cutting (R. Thériault in Massey 1998). The area is divided among multiple privately owned parcels, but public access is uncontrolled. Overpumping of groundwater, putting the water table out of reach of mesquite roots, is probably the most serious threat to this habitat, unique in San Diego County. Habitat loss to agriculture, urbanization, and the spread of saltcedar threaten the Crissal Thrasher throughout its California range (Laudenslayer et al. 1992, S. D. Fitton unpubl. data).
Taxonomy: The Crissal Thrashers of California belong to the pale subspecies T. c. coloradense van Rossem, 1946. The species has not yet been collected in San Diego County.
Le Conte’s Thrasher Toxostoma lecontei
No North American bird is more adapted to extreme desert conditions than Le Conte’s Thrasher. Year round it lives on the floor of the Anza–Borrego Desert, in washes and sandy areas with only scattered shrubs. Its population density is very low, so the county’s total population may be no more than a few dozen pairs. Its need for sandy terrain makes it especially vulnerable to disturbance and habitat loss from off-road vehicles.
Breeding distribution: Le Conte’s Thrasher occurs in two disjunct blocks of the Anza–Borrego Desert, corresponding to areas of sandy soil. The vegetation can be halophytic scrub dominated by saltbushes or open creosote bush scrub, as long as there are a few cacti or shrubs capable of protecting or concealing a nest. In the northern part of the desert, the thrasher ranges from Box Canyon (D23) and the north end of Clark Valley (C25) southeast to the Ocotillo Wells off-road vehicle area. A few range west along San Felipe Wash to Mescal Bajada (J25); at Yaqui Meadows (H24), south of Borrego Springs, P. K. Nelson noted one on 10 and 25 April 1999. The birds appear to be most numerous around Clark Dry Lake; our maximum daily count was of 11 birds and four nests on the north side of the lake bed (D26) 11 May 2000 (P. K. Nelson). They avoid any rugged or rocky terrain, the mesquite bosque, and the developed area around Borrego Springs, being absent even where houses are scattered sparsely. Several pairs persist, however, in the state off-road vehicle area (up to seven, in two family groups, northwest of Squaw Peak, H29, 13 May 2001, J. R. Barth).
The southern block lies south of the Vallecito Mountains and Split Mountain. In this region Le Conte’s Thrasher ranges northwest from Dos Cabezas Spring (S29) and South Mesa (N29) to Vallecito (M25; one on 27 April and 24 May 1998, M. C. Jorgensen, C. G. Edwards) and The Potrero (N24; up to three on 7 February 2002, J. R. Barth). The highest count in this area is of six near Palm Spring (N27) 18 April 1999 (R. and S. L. Breisch).
Nesting: Le Conte’s Thrashers commonly build their nests in the densest thorny shrub in their territory. The site maximizes protection from predators and often from the sun. Plants in which atlas observers described nests were in desert thorn (3), saltbush (2), pencil cholla, mesquite, ocotillo, smoketree, mistletoe, and athel tamarisk.
The species’ nesting season probably varies with the vagaries of the rains, though our data are too skimpy to establish this clearly. Our observations establish egg laying at least from late February to about 1 May, much the same as the 22 February–25 April of six San Diego County egg sets. At Palm Springs in the Coachella Valley, however, Le Conte’s Thrasher eggs have been collected as early as 22 January (Sheppard 1996).
Winter: Le Conte’s Thrasher is a permanent resident. The nine atlas squares in which we saw the species December–February but not March–July were most likely areas in which it occurs year round in low numbers. In Collins Valley (C22/D22) there are multiple breeding-season observations before 1997 (Anza–Borrego Desert State Park database). “Winter” records from Grapevine Canyon (I22; one on 24 February 2002, A. P. and T. E. Keenan) and Inner Pasture (N25; two on 18 February 2000, M. B. Mulrooney) represent, at 2200–2400 feet, the highest elevations at which we recorded Le Conte’s Thrasher (late February already being within the species’ breeding season).
Conservation: The range of Le Conte’s Thrasher has retreated somewhat from Borrego Springs. From 1997 to 2002 we did not find the species in or near the town, including Borrego Palm Canyon (F23) and Hellhole Bajada, where it was noted at least to 1973 (Anza–Borrego State Park database). Frank Stephens found it in the San Felipe Valley in late August 1911, from which there have been no subsequent records.
Urban development, conversion of desert to agriculture, fire, and livestock grazing have all contributed to the contraction of Le Conte’s Thrasher’s range and the reduction of its numbers in various parts of its range (Laudenslayer et al. 1992, Patten et al. 2003, S. D. Fitton unpubl. data). In San Diego County, the greatest threat to Le Conte’s Thrasher is habitat degradation by off-road vehicles. The sandy terrain the thrasher requires attracts the drivers of these vehicles. In the state park, vehicles are confined to existing trails, but many of these follow washes, which the thrasher favors. In the Ocotillo Wells State Vehicular Recreation Area motorists have free range. In this extremely arid area, where shrubs grow very slowly, crushed plants do not have the opportunity to regenerate themselves. Much of the thrasher’s best habitat in the Borrego and Clark valleys lies outside the state park and is vulnerable to off-road vehicles as well.
Taxonomy: The subspecies of Le Conte’s Thrasher in San Diego County is the pale, long-tailed T. l. lecontei Lawrence, 1851.