California Gnatcatcher (Polioptila californica)
Photo by Peter Knapp
Prepared by: Patrick J. Mock (Patrick_Mock@URSCorp.com)
URS Corporation, San Diego
Mock, P. 2004. California Gnatcatcher (Polioptila californica). In The Coastal Scrub and Chaparral Bird Conservation Plan: a strategy for protecting and managing coastal scrub and chaparral habitats and associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/scrub.html
action plan summary
Three subspecies recognized based on subtle morphological differences (californica, pontilis, margaritae; Atwood and Bontrager 2001). Mellnick and Rea (1994) restricted the californica subspecies to north of Ensenada and described P. c. atwoodi for the region between Ensenada and 30°N. Analysis of mtDNA shows little geographic structure or differentiation, suggesting no support for recognition of evolutionary-based subspecies (Zink et al. 2000). This species account will focus on the californica subspecies.
The californica subspecies (coastal California Gnatcatcher) has been listed as a Species of Special Concern in California and was listed as Threatened by the U.S. Fish and Wildlife Service in 1993 (USFWS 1993). Critical Habitat designated in 2000; but the economic effects of this designation are under court-ordered review; California Gnatcatcher is a focal species under California's Natural Communities Conservation Planning (NCCP) program. Several subregional coastal sage scrub focused conservation plans are approved or in the late planning stages throughout southern California.
Species restricted to Baja California, Mexico and coastal areas of southern California, USA. California populations described as "rather scarce and irregularly distributed (Grinnell 1898, Dawson 1923, Woods 1949). Habitat reduction noted by Grinnell and Miller (1944). Current loss of potential habitat in U.S. estimated at 70 to 90 percent (Westman 1981, MBA 1991).
CURRENT BREEDING DISTRIBUTION:
P. c. californica subspecies occurs north of 30°N in nw Baja California,
Mexico to Ventura County. Limited to lower elevations (<500m) south and west
of Transverse and Peninsular Ranges (Bolsinger and Atwood 1992). Highest densities
occur in coastal areas of Orange and San Diego counties (Atwood 1993, Mock 1993,
Preston et al. 1998a, Weaver 1998 USFWS 2002). Lower densities occur in western
Riverside and southwestern San Bernardino counties and inland San Diego County
(Davis et al. 1998, PSBS 1995, Mock 1998). Small, now disjunct populations documented
for Ventura and Los Angeles counties (Atwood et al. 1998a, Atwood and Bontrager
"Core" population areas supporting 30 or more pairs of California Gnatcatcher include Palos Verdes Peninsula, Montebello, Coyote Hills near Fullerton, Puente/Chino Hills, El Toro Air Station, coastal (Upper Newport Bay to Dana Point Headlands east to Interstate-5), north-central and southern Orange County (Interstate-5 to base of Santa Ana Mountains, from El Toro to southern county Border), Camp Pendleton/Fallbrook, Oceanside, North Carlsbad, southeast Carlsbad, Southwest San Marcos, Rainbow/Pala, Olivenhain/Lake Hodges/San Pasqual, Poway, Upper San Diego River/El Capitan Reservoir, Mission Trails/Miramar, Lakeside/Dehesa, Sweetwater River Reservoir, Jamul Mountains, Otay Lakes/Mesa, Tijuana River mouth, Lake Elsinore Lake Skinner, and Temecula ( Atwood et al. 1998a, Bontrager 1997, Mock 1993, PSBS 1995?, Dudek 2002).
Extensive areas of potentially suitable habitat lack California Gnatcatcher (Mock 1998). The distribution and abundance of gnatcatchers appears to be constrained by local winter weather patterns and sage scrub plant species composition (Mock et al. 1990, Bontrager 1991, Bontrager et al. 1995, Atwood et al. 1998a, Mock 1998, Weaver 1998, USFWS 2002).
AVERAGE TERRITORY SIZE:
Highly variable correlated with distance from the coast, ranging from less than 1 ha to over 9 ha (Braden 1997, Preston et al. 1998a, Atwood et al. 1998b). Non-breeding season home range size about 80% larger than breeding season home range (Preston et al. 1998a, Bontrager 1991).
TIME OF OCCURRENCE AND SEASONAL MOVEMENTS:
California Gnatcatcher is non-migratory. Post-breeding dispersal by fledglings occurs during late summer and fall. Natal dispersal distance typically documented at less than 3 km. Longest documented dispersal distance by juvenile is 16 km (Braden 1992). Dispersal across highly man-modified landscapes, including major highways and residential development, occurs often (Bailey and Mock 1998, Gavin 1998, Lovio 1996, Haas and Campbell 2003, Atwood unpublished data). Many examples of occupied habitat patches isolated by extensive development (e.g., Dana Point Headlands, Oceanside; Bailey and Mock 1998). Extensive movements by breeding adults are relatively rare (Bailey and Mock 1998). Longest documented dispersal distance by an adult is 9 km (Atwood and Bontrager 2001). Types of habitat used during dispersal are highly variable (Campbell et al. 1998).
Ground- and shrub-foraging insectivore.
Orthoptera, Araneae, Coleoptera, Homoptera (Burger et al. 1999); Hymenoptera,
Lepidoptera larva (Grishaver et al. 1998), Arachnids (Atwood and Bontrager 2001).
Obtains most of water through diet
Generally "prefers open sage scrub with California sagebrush (Artemisia californica) as a dominant or co-dominant species (summarized in Atwood and Bontrager 2001). More abundant near sage scrub-grassland interface than where sage scrub grades into chaparral. Dense sage scrub occupied less frequently than more open sites. Mostly absent from coastal areas dominated by black sage (Salvia mellifera), white sage (S. leucophylla), or lemonadeberry (Rhus integrifolia). Nest placement typically in areas with less than 40 percent slope gradient. Gullies and drainages, when available within territory, used as nest sites. See also Braden et al. 1997.
Use proportional to shrub species availability: typically California sagebrush, California buckwheat (Eriogonum fasciculatum), California sunflower (Encilia californica), broom baccharis (Baccharis sarothroides), and laural sumac (Malosma laurina). Many other less common sage scrub species used less frequently.
HEIGHT OF NEST:
Mean: 82 ± 2.9 SE cm (range 30-292, n = 101).
HEIGHT OF PLANT:
135 cm ± 3.6 SE (range 62-155, n=103).
Nests placed lower than 70 cm above the ground less successful than higher placed nests. Nest success varied significantly between host shrub species (Grishaver et al. 1998. See also Braden et al. 1999.). Site selection affects risk of nest predation (Sockman 1997).
VEGETATION SURROUNDING THE NEST:
AVERAGE SHRUB COVER:
Typically between 20 and 60 percent. Inter-shrub gap 153 to 176 cm (Bontrager 1991, Mock and Bolger 1992, Grishaver et al. 1998).
DOMINANT SHRUB SPECIES:
California sagebrush dominant or co-dominant.
Prefers nesting in areas with less than 40 percent slope gradient (Mock and Bolger 1992, J.L. Atwood, unpublished data).
Preferentially sings from taller shrubs (e.g., Malosma larina) (Mock and Bolger 1992; Preston et al. 1998b).
Mean varies between site and year, influenced by rainfall prior to egg laying (Grishaver et al. 1998, Patten and Rotenberry 1999). Clutch size typically 3 or 4 eggs (summarized in Atwood and Bontrager 2001).
Both sexes incubate; female incubates 61 percent of time during day, Female on nest at night. Female controls duration of incubation shifts (Grishaver et al. 1998).
14 ± 0.13 SE days from completion of clutch (Grishaver et al. 1998).
DEVELOPMENT AT HATCHING:
Altricial, naked, blind, uncoordinated.
13.3 ± 0.29 SE days after hatching, range: 10 to 15 days (Grishaver et al. 1998).
Both parents feed young. Male brings food more frequently (Grishaver et al. 1998)
POST FLEDGING BIOLOGY OF OFFSPRING:
Offspring disperse from natal territory 3-4 weeks after fledging to unoccupied habitat. Young-of-Year typically paired and with established territories by October (Mock and Bolger 1992, Preston et al. 1998a, Grishaver et al. 1998).
POST BREEDING SOCIAL BEHAVIOR:
Remain on territory throughout the year, expand home range during non-breeding season. Forage with neighboring individuals in habitats not defended (Preston et al. 1998a, Grishaver et al. 1998).
Young of year breed during spring following birth.
NUMBER OF BROODS:
Highly variable between sites and year. Very persistent breeder. Typically 3 or 4 clutches laid per pair per year; maximum number: 10 nests per season. Number of successful broods per successful pair ranges from 1 to 3 (summarized in Atwood and Bontrager 2001).
Nest have been parasitized by Brown-headed Cowbirds (Molothrus ater). Both abandonment and the raising of cowbird young have been observed. Demographic effects of nest parasitism may be small due substitution for typically high natural rates of predation on gnatcatcher nests (Braden et al. 1997). Reduced number of nesting attempts in a season likely an adverse effect of nest parasitism.
Sea Level to 500 m (Atwood and Bolsinger 1992). Most core populations in coastal counties below 300 m (Mock 1993).
Occurrence or nest success of California Gnatcatcher is not reduced near edges with human development (Mock 1993, San Diego County Bird Atlas 2002, Mock and Preston 1995, Lovio 1996, Atwood 1998, Atwood et al. 1998b).
Presence of California Gnatcatcher not related to patch size in coastal areas of range, although smaller patches less consistently occupied over time (Atwood et al. 1998). Successful breeding reported from patch as small as 0.2 ha. Larger patch size requirements in inland portions of range (Mock and Bolger 1992, Famarlaro and Newman 1998, Preston et al. 1998a).
DISTURBANCE: (natural or managed):
Disturbances that reduce shrub cover, such as frequent fire, mechanical disruption, livestock grazing, off-highway vehicle use, and military training activities appear to reduce habitat suitability for California Gnatcatcher (Bontrager et al. 1995b, Mayer and Wirtz 1995, Beyers and Wirtz 1997, Wirtz et al. 1997, Atwood et al. 1998c). Construction monitoring studies suggest California Gnatcatchers are tolerant of adjacent construction activities (Atwood and Bontrager 2001) and high noise levels (Famarlaro and Newman 1998, Awbrey 1993, Awbrey et al. 1995, Awbrey and Hunsaker 1997, URS Corporation 2004).
ADJACENT LAND USE:
Abundance not reduced near urban edges (see "fragmentation" above).
Like most other open cup nesters, California Gnatcatchers are extremely vulnerable to nest predation. Snakes, birds, rodents, medium-sized mammals, and ants are reported nest predators. Nest predation rates vary from 26 to 68 percent of nests monitored (summarized in Atwood and Bontrager 2001).
EXOTIC SPECIES INVASION/ENCROACHMENT:
In coastal scrub habitats, increased fire frequency can cause extensive habitat conversion to grassland (Zedler et al. 1983), and reductions in gnatcatcher populations (Bontrager et al. 1995a, Atwood et al. 1998c).
DEMOGRAPHY AND POPULATION TRENDS:
AGE AND SEX RATIOS:
Equal sex ratios at birth.
Varies between sites and year: Palos Verdes: 3.0 ± 0.62 SD fledglings per female per year (n = 5 years); Orange Co.: 2.5 ± 0.48 SD (n = 5); San Diego Co.: 2.4 ± 1.48 SD (n = 4); Riverside Co.: range 1.4 to 3.8 at 3 sites during 3 years Summarized in Atwood and Bontrager 2001).
Palos Verdes, First-year survivorship 29 ± 5.4 SE percent (n = 4 years); Adults: 52 ± 10.2; Orange Co., Adults 57 ± 7.2 SE (n = 4 years). Mortality greatest during winter and immediately after fledging (Atwood and Bontrager 2001, Atwood et al. 1998a, Grishaver et al. 1998, Mock 1998, Erickson and Miner 1998).
Majority of relocated banded juveniles found within 3 km of banding location; maximum juvenile dispersal distance reported: 16 km (Braden 1992). Longest documented dispersal distance by an adult is 9 km (Atwood and Bontrager 2001). Gnatcatcher distribution in isolated habitat patches suggest that maximum long-range dispersal could extend to nearly 22 km (Braden 1992, Mock and Bolger 1992, Gavin 1998, Bailey and Mock 1998, Atwood and Bontrager 2001).
In California, BBS analysis of 52 routes shows no significant trend between 1966 and 2000. However, for individual regions in California, there is too little BBS data available (4-12 routes per region) to reliably estimate trends (http://www.mbr-pwrc.usgs.gov/bbs/). Based on estimates of suitable habitat and survey data, U.S. population likely exceeds 3,000 pairs and maybe as high as 5000 pairs during years with favorable weather conditions (USFWS 1996, Mock 1993).
FIRE AND EXOTIC VEGETATION:
Fire frequency and the invasion of exotic vegetation, especially grasses and annual forbs, interact to pose potentially serious threats to suitable gnatcatcher habitat. In much of coastal southern California, where these exotic plants are well-established and where the irreversible conversion of shrublands to grasslands is likely, fire frequency and burn size should be kept low. Where possible, flammable exotics should be removed or reduced in shrubland habitats. The recent series of wildfires in 2003 in southern California affected 4% of known gnatcatcher occurrences, 16% of designated critical habitat acreage, and 28% of USFWS modeled habitat for the California Gnatcatcher (Bond and Bradley 2004).
California Gnatcatchers do not appear to be especially sensitive to fragmentation and development at the landscape scale. Primary concern is the chronic reduction in habitat carrying capacity due to development and need to develop a network of habitat reserves linked by habitat linkages. A sufficient number of "core" populations for California Gnatcatcher are extant to allow for a viable network of habitat reserves to be conserved though NCCP/HCP subregional planning processes that are ongoing throughout southern California.
Predation is the most common cause of gnatcatcher nest failure. Information is lacking as to whether this predation rate is influenced by anthropogenic factors, although typical edge effects on gnatcatcher breeding is not evident (Atwood 1998).
Greater Roadrunner, Bewick's Wren, Rufous-crowned Sparrow, Wrentit, Cactus Wren, California Towhee, San Diego pocket mouse (Chase et al. 2000).
MONITORING METHODS AND RESEARCH NEEDS:
Trend monitoring: The BBS method does not monitor California Gnatcatchers well in the areas of California where they are most likely to be declining due to habitat loss and degradation. Off-road monitoring methods should be applied in coastal scrub habitats in coastal California. Monitoring plans for NCCP efforts in southern California includes the monitoring plots of conserved California Gnatcatcher populations.
Demographic monitoring and research: Nest success and the factors that influence it should be monitored directly (through nest monitoring) in replicate sites to evaluate management options. Additional data on survivorship, productivity, and dispersal capability could be obtained from color-band re-sightings. Confirmation of assumed metapopulation and source-sink dynamics, and sensitivity to local weather conditions would be valuable (Mock 1993, Akçakaya and Atwood 1997, Mock 1998).
ACTION PLAN SUMMARY
SPECIES: California Gnatcatcher (Polioptila californica)
One subspecies occurs in the California shrublands covered by this plan. The coastal California Gnatcatcher (P. c. californica) is a characteristic coastal sage scrub bird found mainly in the coastal plain of southern California. Given the amount of loss of habitat in coastal California (Atwood 1993), its distribution in coastal scrub habitat has been reduced. The coastal California Gnatcatcher has been listed as a Species of Special Concern in California and a federal Threatened Species.
HABITAT NEEDS and CONCERNS:
California Gnatcatcher requires variable amounts of semi-open sage scrub co-dominated by California sagebrush on shallow slope gradients. Too frequent fires in sage scrub habitats can convert shrubland habitat to grassland and has probably contributed to the decline in California Gnatcatcher throughout southern California. Other disturbances that eliminate shrubby vegetation should also be avoided or minimized. Large areas of suitable habitat should be conserved to benefit California Gnatcatcher populations. Reduced localized productivity due to nest predation and nest parasitism is a cause for concern.
· Maintain current distribution.
· Conserve healthy, interconnected local breeding populations that exceed 50-100 pairs (Mock 1993).
· Guide conservation planning efforts to benefit California Gnatcatcher.
· Improve trend and demographic monitoring efforts (Atwood 2000).
· Gather information on the effects of management practices, habitat restoration, and dispersal capacity (Rotenberry and Scott 1998).
Habitat protection recommendations:
Habitat preservation for the Bells' Sage Sparrow should focus on coastal sage scrub associations that have California sagebrush as a co-dominant species. The habitat and area requirements of California Gnatcatcher should be addressed in multi-species conservation planning efforts (NCCPs and HCPs) throughout the range in southern California.
Management and restoration recommendations:
Manage fire frequency and other disturbances to maintain a semi-open shrub structure in coastal scrub.
MONITORING AND RESEARCH NEEDS:
As is true of many coastal shrubland bird species, California Gnatcatchers are not well monitored by Breeding Bird Survey counts. Given their sensitivity to habitat degradation, monitoring to determine population trends and demographics should be a high priority. Further studies of gnatcatcher dispersal capability through man-modified landscapes will help in the design of adequate habitat linkages between core populations.
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