SPECIES: Blue-gray Gnatcatcher, Polioptila caerulea
SUBSPECIES STATUS: P. c. amoenissima
This subspecies breeds in the western United States and NW Mexico, and winters from southern California (casually from Marin County), western and central Arizona, and western Texas south to latitude 28 degrees N. in Mexico (AOU checklist 1957).
MANAGEMENT STATUS: No special status. Protected under the migratory Bird Act.
Considered a neotropical migrant landbird of special concern in California (Laymon 1995).
I. Historical references: According to Grinnell and Miller (1944), bred in California in the foothills and low mountains surrounding Sacramento and San Joaquin River Valleys, locally in river bottoms of the valleys, in the interior coast ranges, the mountains and coastal plain of southern California south to Mexican boundary, and desert mountain ranges from Providence Mountains north to White Mountains.
II. Current breeding distribution: Attempt to obtain the most current information.
· Extirpated from the coastal lowlands of San Diego County (Garrett and Dunn 1981, Small 1994, San Diego BBA). However this may be in the process of reversal, most likely due to cowbird trapping in efforts to save the Least Bell’s Vireo (Phillip Unitt, pers. Communication). See San Diego Breeding Bird Atlas below.
· Mill Creek, Lassen National Forest,
Tehama County. Birds were not detected on point count (PRBO data 1997)
or ever observed in riparian habitat, but were observed during breeding
season in adjacent oak savannah and oak scrub (Anne King, pers. communication,
· San Luis Obispo County, central coastal region (Mike Lynes, pers. comm.).
· San Mateo County, Santa Cruz Mountains (Sisk et al. 1997).
· Tehama County, detected along Dye Creek in Sierran foothills; plots covered riparian habitat and adjacent oak savannah (PRBO data 1998).
· Yuba County, foothills of northern Sierra Nevada range (Aigner et al. 1998).
d. Nest searching:
g. Breeding Bird Atlas:
· San Diego BBA - breeds sparingly in San Diego County chaparral above 2000 feet, and occasionally in desert riparian. Recent observations in Spring Valley and in Miramar Naval Air Station suggests the species may be beginning to reoccupy this lowland habitat in response to lessening cowbird parasitism in the area (San Diego BBA). Throughout the county, far more BGGNs are currently seen during the breeding season than 20 years ago, and it is likely that the Least Bell’s Vireo cowbird trapping program is responsible for this change (Phillip Unitt, pers. communication).
· Monterey BBA - Local declines have occurred in Monterey County. Were considered abundant in early 1900’s in upper Salinas Valley (Willet 1908 in Tenney 1993), and the numbers now are very low and local. Declines believed to be due to habitat loss and cowbird parasitism. Tenney (1993) recommended a cowbird control project. Fairly common breeder in chaparral and arid woodlands throughout the Santa Lucia Mountains, Sierra de Salinas, and northern Galibar. Also partial to upland foothills covered with small oaks and open chaparral. Few reside east of Salinas River, which is mostly grassland and pine-oak woodlands. Seldom nest in coastal scrub of coast (Tenney 1993).
· Sonoma BBA – Breed in eastern portion of county, and are for the most part absent from the central and western parts (Wigh 1995).
· Local, and uncommon to fairly common breeder in Santa Barbara County. Most numerous in hills bordering the Upper Santa Ynez River, and along the crest of the Santa Ynez Mountains (Lehman 1994).
· Very rare breeder in California’s Deep Canyon (between Colorado Desert and Santa Rosa Mountains); only a few pairs nesting in the pinyon-juniper and chaparral (Weathers 1983).
· Said to be found year-round in the
following National Forests: Angeles, Cleveland, Los Padres and San Bernadino;
summers (breeds?) in El Dorado, Inyo, Klamath, Lassen, Mendocino, Modoc,
Sequoia, Shasta-Trinity, Sierra, Six Rivers, Stanislaus, Tahoe and Toiyabe
National Forests (Timossie 1990, in USDA Forest Service 1994).
I. Average territory size: At Hastings Reserve, located in Central Coast Ranges of California, Monterey County, territories averaged 4.5 (2.2-7.4) acres or 1.8 (0.9-3.0) hectares, n=9 (Root 1969).
II. Time of occurrence and seasonal movements: Northerly populations show long distance migration (Ellison 1992).
B. Departure date from breeding grounds: Appear to depart from breeding areas as soon as young are independent, mid- to late August. Observed leaving nesting areas in northern California by late September (McCaskie et al. 1979 in Ellison 1992). Post-breeding, may wander upslope as far as the mixed conifer zone in the Sierras (Grinnell and Miller 1944).
C. Spring migration period: begins late February to early March. (Ellison1992).
D. Fall migration period: Data from PRBO’s Palomarin Field Station, along coast in western Marin County where they do not breed. Dates for the relatively few captures (n=39, 1972-1998) range from July 25 to October 30 over 26 years.
E. Extent of wintering in CA:
· common in winter on southeastern CA deserts (Grinnell and Miller, 1944). Relatively small numbers winter in the Sonoran Desert of California and Arizona (Ellison 1992). Winters fairly commonly from the southern Mohave Desert and southward, in screwbean mesquite & possibly creosote scrub (Garrett 1981).
· common (although sparse) winterer in lowlands of southern coastal district (Grinnell and Miller 1944).
· sparse winter visitant in the north at low elevation and coastal region (Grinnell and Miller 1944).
· Relatively uncommon winterer in Orange County (Hamilton and Willick 1996).
· Winter Bird Population study plots reveal relatively low numbers in the US. Highest comes from brushy or disturbed sites such as young riparian woodland in Orange County, CA, 62/km squared (Hays 1983 in Ellison 1992). However since few Christmas Bird Counts take place in Central America, these relatively low numbers are only suggestive as there are little data to compare with (Ellison 1992).
· Sparse but regular winterer in Monterey County (Tenney 1993). Not observed at Hastings Reservation in winter (Root 1967); at nearby Carmel, in very low numbers, usually in stands of riparian willows or coastal chaparral (L.O. Williams, pers. comm., in Root 1967).
· Fairly common in winter along southern coastal portion of Santa Barbara County, but decidedly uncommon in northern coast of county, as determined by Christmas Bird Counts (Lehman 1994).
· Common in Deep Canyon (between Colorado Desert and Santa Rosa Mountains) in winter (Weathers 1983).
B. Habitat use: Very little data. Not mentioned in BNA (Ellison 1992). At PRBO’s Palomarin Field Station, Marin County, 1972-1998, fall captures yielded 4x as many BGGNs were captured in coastal scrub habitat than in mixed evergreen riparian woodland, despite fewer nets in coastal scrub (6 nets in scrub versus 14 in forest). Coastal scrub is structurally more similar to both breeding and wintering habitat in California than is the mixed woodland.
C. Routes: Both sexes appear to migrate synchronously in California (Root 1969). Uncommon as migrant in Monterey County, but do occur along the coast there (Tenney 1993). Is rare along north coast during migration from Mendocino County and northward (Harris 1991). No other information.
V. Foraging strategy: Forage throughout height of the vegetation but concentrate in the foliage zone; in chaparral they tend to use the subcanopy zone more. (Root 1967). Hop rhythmically from perch to perch, capture prey by gleaning, lunging, hovering, and hawking (Shuford 1993). They subdue large prey by beating them against branches. In CA feed more in evergreen foliage in spring than later in summer. When deciduous oaks are barren or in the process of developing new foliage, i.e. during early spring, live oaks were used as foraging substrates more frequently than at other times (Root 1967).
VI. Displays: To other males, male assumes tail-spread posture, tail is fanned out exposing white outer rectrices; tail held horizontally and wagged from side to side; sometimes does short undulating flights w/ spread tail displayed; during this sings sometimes but more often gives it’s "peeew" call. During most intense disputes, one male flies directly at the other, where often they meet each other in midair and ascend w/ breasts nearly touching in vertical flight (Root, 1969).
VII. Social Organization:
· As determined by spotmapping at Camp Roberts, San Luis Obispo County, from 1994-1995, 4.4 territories per 100 acres (Tietje and Vreeland 1997).
· Home range of 4 hectares found in Florida (Fehon 1955 in Ellison 1992). Area patrolled and defended earlier in nesting season larger than area defended later in season (Root 1969). Discrepancies have been found in territory size, this might reflect spacing (Ellison 1992).
· In pinyon-juniper habitat in NM, found in relatively low densities, averaging less than 2 pairs per 35 hectares (Goguen 1996).
· Highest densities overall were found in southeastern U.S. (Ellison 1992).
· Highest densities in west were recorded on a BBC plot in blue oak woodland in central California with 71 territorial males/km squared and in pinyon-juniper woodland in Utah (Williams 1979 in Ellison 1992). Territories which contain large areas of open woodland tend to be larger, suggesting that territory size is related to the amount of tree foliage present (Root 1967).
· Lowest densities on Breeding Bird Census plots were in disturbed upland habitats such as clear cuts, reclaimed strip mines, and abandoned agricultural land (Ellison 1992).
D. Post fledging biology of offspring (where do they go and when?): Adults feed young infrequently after day 16 and at times up to 4 weeks; young may stay in parents’ territory up to a month after fledging even when not being fed. During June, most fledglings led to chaparral, while during July fledglings spent more time in the live oak woodlands (Root 1967). Some immatures shift into nearby areas and/or habitat not used during breeding season. No data on initial dispersal from natal site; may wander upslope as far as the mixed conifer zone in the Sierras (Grinnell and Miller 1944).
E. Post breeding social behavior (mixed species flocks, or simply migrate away?): no information.
VIX. Incubating sex: Both sexes incubate, roughly equal amounts of time, although only female develops brood patch. Female incubates at night (Ellison 1992)
X. Incubation period: mean 13 days, range 11-15 (Ellison 1992)
XI. Nestling period: Across multiple states: 10-15 days (Ellison 1992); at Hastings in CA, 12-13 days(Root); in east, 10 to 12 days (Weston 1949).
XII. Development at hatching: altricial
XIII. Number of broods: More pairs are single brooded than double
(Ellison 1992). One third of 12 pairs at Hastings, California had second
broods, and pairs had multiple attempts when nests failed: 2 pairs observed
making 7 attempts in one season (Root 1969). At Hastings breeding season
goes from late March to late August (Root 1969), allowing multiple attempts.
B. Drinking: no information. Many areas they inhabit in California
are relatively arid.
· Slight discrepancy between sources of where CA breeders winter. According to most sources (AOU checklist 1957, Phillips 1991), western subspecies winters from southwestern US to west Mexico. This conflicts with information presented in the Forest Service text (Timossi 1990 in USDA Forest Service 1994) which states that California BGGNs are local migrants only and do not winter in the neotropics. However is unlikely that all CA breeders also winter in CA.
· Winter habitat: Primarily a scrub wintering species, although found wintering many habitat types in Mexico, including mangrovers, gallery forest, thornscrub, oak-pine woodlands, shade coffee plantations (Hutto 1980, and personal observations). In Sonoran Desert, observed foraging during winter in plants with "typical" shrub or tree-form, such as palo verde, mesquite, screw-bean, bur-sage, catclaw, willows, and cottonwoods. Never observed foraging in cacti (Root 1967). Little known about habitat requirements in southern part of its winter range, in Central America (Root 1967).
· Densities/territory size: winter home range near Tucson,AZ estimated at 8.8 hectares; population density in screwbean mesquite woodland near Yuma much higher (Root 1969).
· Ellison (1992) suggests that the apparent rise in overall BGGN numbers might imply little current effect of neotropical deforestation on wintering populations, as is not primarily a forest-wintering species. However this suggestion grossly overlooks the numbers that do breed in various forest habitat types in Mexico.
I. Overview of breeding habitat: (e.g. oak woodland vs. oak savannah, age of stand, dominant species, plant species diversity, structural diversity/variability):
· Occurs in the open chaparral that normally borders stands of pinyon-juniper and/or oak woodlands (Grinnell and Miller 1944). Some territories although adjacent to woodland may contain entirely chaparral (Root 1967); however Root (1967) found that the one territory which did so was centered around a gulch where the chaparral (chamise and buckbrush) grew much larger, up to 9 feet, so the vegetation actually structurally resembled oak scrub. Breeds in blue oak woodland in interior coast ranges of CA.
· In Marin County, is attracted to slopes w/ open stands of small valley oaks w/ adjacent patches of coast live oaks and openings w/ low brush (Shuford 1993).
· In Monterey County, prefers extensive stands of oaks varying from live oak woodland, mixed live oak-deciduous oak woodland, dense oak scrub, and open stands of mature deciduous oaks (Root 1967).
· In Mendocino County, Mendocino National Forest, habitat is Brewers Oak Forest, dominated by small, scrubby Brewers Oaks.
· Less commonly occurs (and often locally) in riparian habitat, i.e. in willow thickets, sycamores and cottonwoods, and then usually where there is adjacent chaparral (Grinnell and Miller 1944). Root found the one pair which occured in riparian woodland at Hastings made frequent foraging trips into the chaparral and oak woodland nearby.
· In desert mountains, pinyon, juniper and Purshia shrubs provide comparable conditions (Grinnell and Miller). BGGN is a common species in pinyon-juniper habitats in these desert ranges (Garrett 1981).
· Also breeds in California’s shrub-steppe habitat (i.e. east side of the Sierras, Inyo County). Occasionally nests in the riparian strips that cut through the shrub-steppe. When nesting in shrub-steppe that has adjacent riparian, may utilize this wetter, perhaps more diverse (for prey species?) riparian habitat for foraging (Sacha Heath pers. comm., PRBO data).
· In the Sierras, breeds in blue oak savannah, digger pine-oak, chaparral edges mingled with oak, and in riparian deciduous forests if adjacent to oak woodland or chaparral (Verner and Boss, 1980 in USDA Forest Service 1994).
· Mendocino National Forest: Brewer’s Oak forest. Brewers Oak, the dominant tree and shrub, is a small, winter-deciduous scrubby oak (Tom Gardali, pers. comm., PRBO data).
· Broad-sclerophyll "canyon" forest of California, which structurally resembles the flood plain forests which this species prefers in the east, is only marginal habitat in CA (Root 1967).
· Generally rare or absent from habitats dominated by needle-leaved conifers. Also absent from live oak woodlands in the fog belt along the Pacific Coast, just 14 miles west of Hastings Reservation where they breed (Root 1967).
· Have been found to shift their habitat use in response to arthropod prey availability (Root 1967) - when arrive in Monterey County in March, April, they concentrate foraging efforts in the evergreen foliage of live oaks and chaparral. By late April, when deciduous oak foliage is well developed, they shift most foraging to these woodlands through July. Fledglings led to dense evergreen foliage partly for protection from predators, and later wander to nonbreeding habitats. By August adults and juveniles leave deciduous oak woodlands for adjacent live oak woodlands and chaparral.
· Breeds in very different habitats elsewhere in US, particularly in east (Ellison 1992).
· In Monterey (Hastings Reservation), 90% of nests were in deciduous oaks, n=64 (Root 1967). Interestingly, 2/3 nests that were in live oaks had been partially defoliated by tent caterpillars and superficially resembled deciduous oaks.
· Also use chamise shrubs at Hastings, in 1 case even when several deciduous oaks occured in their territory (Root 1967).
· One pair with territory centered in chaparral built 2 nests in live oaks, 1 in a buck brush shrub, and one in deciduous oak (Root 1967).
· In Inyo County, where research and monitoring was being conducted primarily in riparian habitat immediately adjacent to shrub-steppe, 3 of 4 nests found were in Artemisia tridentata (Big Sagebrush), and one was in a Black Cottonwood in the riparian habitat (PRBO data).
· In foothills of Sierra Nevada (Tuolumne County), BGGNs have been reported to build nests in pine and alder trees (Chamerlin 1901 in Root 1967).
· Even reported using eucalyptus (Weston 1949).
· In non oak-woodland: Inyo
County, 1 nest in riparian was 10 meters high in Black Cottonwood, 3 nests
shrub-steppe in Big Sagebrush were just over 1 meter high (upper third
of shrub) (PRBO data).
· At Mendocino National Forest, tree cover (determined by vegetation assessment taken at point count stations where BGGNs occur) EXPAND
· In shrub-steppe, no canopy cover. In adjacent riparian, high canopy cover (74-100 percent).
· However, in northeastern US they prefer closed canopy forests (Ellison 1992). May have specific needs in upland eastern US and midwestern US forests, such as canopy openings, but more study is needed (Ambuel and Temple 1983 in Ellison 1992, Robbins et al. 1989).
· At Mendocino: Brewers Oak, followed by Oregon White Oak, with some Black Oak and Ponderosa Pine (PRBO data).
· In a point count study in the Santa Cruz Mountains of central coastal California, abundance not significantly different between oak patches surrounded by chaparral and oak patches surrounded by grassland (Sisk et al. 1997).
· In Inyo County, east side of Sierras, shrub cover averaged 63% (n=4, 3 nests in shrub-steppe, 1 nest in adjacent riparian).
· In desert ranges, dominant shrub is Antelope Brush (Small 1994).
· At Mendocino National Forest, dominant species were Brewers Oak and manzanita (PRBO data).
F. Dominant forb species: No information.
G. Ground cover: No information.
J. Tree DBH:
· In Arizona, mean dbh=22.8cm, n=67 (Bbird data).
· In Minnesota, mean dbh=46.7 (Bbird data).
· In Ohio, mean dbh=38.4, n=3 (Bbird data).
· In Maryland a significant negative correlation was found between BGGN abundance and number of standing dead trees (Robbins et al. 1989).
· In western Sierra Nevada, said to occur below 4000 feet (approx. 1300m); in eastern Sierra Nevada below 2300 feet (approx. 800m) (Zeiner et al. 1998 in USDA Forest Service 1994); other literature stated that they breed up to 6000 feet (approx. 2000m) in oak woodlands (Small, 1994).
· In coast range breeds up to about 1500m (Garrett and Dunn 1981).
· In desert slopes of mountains and in desert ranges, occurs up to 7000 feet (Small 1994).
C. Patch size: No detailed information.
D. Disturbance (natural or managed): (e.g. floods, fires, logging):
· In a 2 year post-thinning study of blue oak woodlands in the foothills of the Northern Sierra Nevada range, BGGN abundance was not shown to be positively or negatively affected by thinning for firewood harvesting; however relative abundances overall were low and this might account for the lack of relationship (Aigner 1998).
SPECIAL FACTORS: Factors influencing a species occurrence and viability.
I. Brood parasitisim:
· As early as the 1910’s and 1920’s, this species was observed being frequently parasitized, with 10 out of 12 nests in Mississippi parasitized (Weston 1949).
· At Hastings Reserve, Monterey County, 6 of 22 nests in 1963 consisted of only 1 cowbird young and no host young; this is only 4 years after BHCOs were first observed at the Hastings Reserve (Root 1967).
· 2 of 4 nests in Inyo County, Eastern Sierras (shrub-steppe/riparian) were parasitized in 1998 (PRBO data).
· In pinyon-juniper woodland in New Mexico, where cowbirds and gnatcatchers have existed sympatrically for a long time, 76% of 83 gnatcatcher nests found were parasitized. Parasitism was responsible for the failure of 58% (48 of 83) of the nests. Two nests that were parasitized w/ one egg before laying buried the cowbird egg into their nest bottom but did not desert. Of nests parasitized during egg-laying, 45% were deserted. None of the 25 nests that accepted cowbird eggs during laying fledged host young; 16 successfully fledged a cowbird. All cowbird eggs laid after clutch completion were accepted. Some which accepted cowbird eggs during incubation fledged their own young. Parasitism rates were lower late in the breeding season, and nesting success of unparasitized nests was higher (Goguen and Matthews 1996).
· Goguen and Matthews (1996) suggested desertion as the strategy used by BGGNs in response to cowbirds, as pairs that deserted were sometimes able to raise cowbird-free clutches. Gnatcatchers are very common hosts and typically raise none of their own young when parasitized, yet no other anti-parasite behaviors have been reported. Of 48 parasitized pairs, 19 pairs deserted at least one nest; however, only 7 deserters were relocated by nest searchers. 6 of the 7 deserters experienced at least one subsequent unparasitized nest, although it often took 2 or 3 more attempts before this was achieved. Nest desertion is often more common response of smaller hosts that perhaps cannot physically eject the eggs. Also, BGGNs show nest moving - break down of old nest as primary material source of next attempt.(Weston 1949). However, effect of late nesting due to desertion parasitized nests on adult survival or on fledgling survival is unknown (Goguen and Matthews 1996). Also, desertion is not always the response to parasitism; Goguen and Matthews (1996) suggest that they may only desert when the parents observe the nest being disturbed by a cowbird. Also suggest that this species may be in an evolutionary transitional stage..
· Parasitism rates and cowbird abundance did not differ b/w grazed plots and plots ungrazed for 20 years, but the ungrazed plots had nearby feedlots/grazing which were ideal for BHCOs (Goguen and Matthews 1998). Reflects need for landscape approach to deal with indirect affects of grazing (i.e. cowbirds) on songbird populations.
· Parents fed BHCO nestlings (which often meant just one BHCO in a nest) at rate similar to that of a normal brood. Two individual cowbird fledglings (separate pairs/nests) each averaged 28.0g in body weight, while the total weight of 5 BGGN fledglings (max normal brood size) was 29.3g (Root 1967)
· Pressure this high cannot be applied to the population as a whole without causing it’s demise (Goguen and Matthews 1996). Perhaps is only being applied to restricted areas (lowlands adjacent to grazing)?
III. Sensitivity to human-induced disturbance: Said to desert nests easily (Root 1969).
IV. Pesticide use: Many of its prey items are agricultural pests (Terres 1980 in USDA Forest Service 1994), so would be affected by pesticides. No reported studies/information on this subject.
· In 1961 after a population outbreak of tent caterpillars that defoliated the oaks, all located BGGN nests failed, and many individuals did not seem to even be engaged in nesting activity, however nest searching was not as intense as in 1963 (Root 1969). ·· Documented nest predators in west include WESJ, YBMA (Ellison 1992).
· Inferred predators include American Crow, raccoons, squirrels, rat snakes. Also probable are snakes and other small climbing mammals such as Peromyscus mice (Ellison 1992), and possible predator on adults are Loggerhead Shrikes (personal observation).
· Adults mob potential predators.
· Predation appears important in regulating nest success but apparently has little effect on adult populations (Ellison 1992).
VII, Other: Temperature and potentially vegetation appear to be the major factors limiting the winter range (Root 1988 in Ellison 1992). Gnatcatchers wintering in temperate an subtropical areas may be sensitive to severe winter weather (Weston 1949), as has been found for other temperate winterers the same size (Laurenzi et al. 1982). A two-week freezing spell in Florida in 1940 caused the complete disappearance of BGGNs that winter, and none were seen until spring migrants returned (Weston 1949).
POPULATION TREND: http://www.mbr.nbs.gov/bbs/bbs.html
· BBS Data: nonsignificant positive trend over entire state. However, this trend comes with warning on the website, that results for low relative abundance species such as BGGN must be viewed with caution, and especially may be less accurate over long-term. Coupled with the fact that the trend is not statistically significant, this trend may not be meaningful.
· Said to have recently expanded range into northeastern CA in Lassen County, but source of this uncertain (USDA Forest Service 1994).
· Nesting range expansion in North/Northeast US (Ellison 1992, Ellison 1993).
· Recent pop trends in US (1966-1987) include increases in E and W regions and a decline in the Central region, none judged significant (S. Droeg pers. comm in Ellison 1992).
I. Age and sex ratios: no information.
II. Productivity measure(s):
· Of 12 females studied, 58% fledged young from 1st nest attempt, and 33% fledged young from 2nd broods (Root 1969 in Ellison 1992).
· Over 64% of all nests found in Vermont in 1988 produced fledglings, but only 42% found mid-incubation or earlier fledged young. Note: Mayfield method (1961) not used to determine nest success at any of above examples.
· In Eastern Sierras, Inyo County, 1 of 4 nests believed to have fledged, 1 abandoned during building, 1 abandoned with 3 out of the 4 eggs being BHCO eggs, and 1 nest depredated with 2 of 4 eggs being cowbird eggs (PRBO data).
· Productivity in NM: 7 unparasitized pairs averaged 3.0 young fledged (all successful); 16 pairs that accepted cowbirds during laying on averaged fledged 0.00 BGGN and 1.00 BHCO young; 5 pairs that accepted cowbird eggs laid during incubation period averaged 1.60 BGGN and 0.40 BHCO young fledged; 4 pairs that deserted and moved nests in response to parasitism fledged 2.25 BGGN and 0.25 BHCO young on average.
IV. Dispersal: no data.
ASSOCIATED SPECIES: A list of other species that would benefit from management of the target species.
· In oak woodland, associated species may include: California Quail, Ash-throated Flycatcher, Red-shafted Flicker, Western Scrub-jay, Bushtit, Bewick’s Wren, Western Bluebird, Oak Titmouse, California Towhee, Spotted Towhee, and Hutton’s Vireo, among others (Sisk et al. 1997).
· In pinyon-juniper habitat in New Mexico, associated species included Western Wood-pewee, Solitary Vireo, Western Tanager, Bushtit, Spotted Towhee, Plain Titmouse, and Western Scrub-jay (Goguen and Matthews 1998).
MONITORING METHODS AND RESEARCH NEEDS: Recommend methods that will address immediate needs as well as those most appropriate to monitor how effective the proposed management recommendations will be.
· Certainly more comprehensive knowledge of statewide range is needed. Need extensive oak woodland point count surveys throughout the state, much as is being done with riparian habitat statewide.
· Need to locate specific areas for more intensive monitoring research to be conducted, specifically nest searching.
· It seems necessary to determine and monitor productivity of populations, both those which do not occur in the lowlands, and also any remaining populations in such lowlands. Found very little literature on reproductive success of BGGNs in CA, particularly any recent studies in oak woodland. Found nothing on reproductive success in populations free of BHCO parasitism.
· As it has been suggested that the parasitism rates found in BGGNs will not allow for a sustainable population, it is necessary to immediately determine the degree of parasitism of this species statewide and across habitats, to identify strong populations as well as once which are possibly declining due to cowbirds.
· If it appears that the degree of parasitism is non-sustainable, it may then be necessary to implement cowbird control in an extreme case. This is proving effective in San Diego County, where cowbirds are being controlled not to help BGGNs but to help Least Bell’s Vireo, and BGGNs are one of possibly many species gaining from this. In areas where cowbird control programs are put into affect, it is crucial to monitor the effectiveness of such a strategy.
· It is necessary to address landscape
issues which are currently affecting the degree of parasitism found.
Section 2: Action plan summary. Summarize the above information into concise statements under each section.
STATUS (from subspecies, trend, local extirpations, state and federal lists, etc.)
· No federal or state special status. However is classified as a neotropical migrant landbird of special concern in California (Laymon 1995).
· BBS data reveals a nonsignificant positive trend over entire state. However, this trend comes with warning on the website, that results for species with low relative abundance, such as BGGNs, must be viewed with caution, and especially may be less accurate over long-term. Coupled with the fact that the trend is not statistically significant, this trend may not be meaningful.
· believed to have been extirpated
and/or greatly reduced from the following regions: from riparian habitat
in the Central Valley where they once bred locally, from lowlands of San
Diego County, from Salinas Valley in Monterey County, and possibly in Orange
County. These lowland extirpations are all thought to be the result of
Brown-headed Cowbird parasitism, to which they are very susceptible. This
species appears to be returning to the lowlands of San Diego County in
response to an active cowbird control program (implemented for Least Bell’s
HABITAT NEEDS (e.g., elevation, patch size, breeding habitat characteristics, disturbance). The following is specific to BGGNs in California, as their habitat associations and requirements are very different in the eastern portion of the country.
· Elevation: Thought to prefer foothills and low mountains according to Grinnell and Miller (1944), probably as a result of habitat. In some areas of the Sierras may breed up to 4000-6000 feet; in Sierras may breed up to around 1500 meters, or 4500 feet. May occur even higher in desert ranges.
· Most favorable habitat is blue-oak covered hill slopes and chaparral edges mingled with oaks of several species or with diversified arroyo-edge cover; also occurs sparingly in the adjacent chaparral (Root 1967). Appear to prefer a matrix of woodland (often including both live and deciduous oaks) and shrubs, sometimes including grasslands as well or instead of the chaparral. Often the oaks/trees are fairly shrubby in structure.
· Little information on patch
size. Does not need extensive woodland patches and usually do not choose
closed canopy forests. Prefer open matrix of trees and shrubs.
CONCERNS (e.g., productivity, brood parasitism, habitat loss, lack of information, wintering distribution, pesticide use)
· This species is highly vulnerable to parasitism. Parasitism rates very high (too high) in areas populated by cowbirds, within this state and in others as well. Much of oak woodland habitat where they breed is of a nature suitable for cattle grazing, which increases BHCO range and consequent parasitism rates.
· Lack of current information exists on productivity as determined by nest finding and monitoring; most nest searching done in previous decades or in other habitat types and may not reflect status of this species in California's oak woodlands today.
· Many prey items are known agricultural pests, which may make pesticides an issue in the diet of this purely insectivorous species, although no study has specifically been done.
· Winters in chaparral and
desert scrubland in southern California. Also winters in a variety of both
scrub and forest habitats in Mexico. Habitat loss in California, i.e. conversion
of such habitat to agricultural or grazed lands, has the potential to affect
this species' overwintering survival, as does deforestation in Mexico.
OBJECTIVES (e.g., increase distribution, identify healthy breeding populations, increase available habitat, guide restoration efforts to benefit species)
· Determine more complete range distribution across state, as well as this distribution across different habitat types.
· Identify healthy breeding populations where parasitism rates are low or nonexistent and productivity levels are sustainable, and determine what makes those sites optimal (implement monitoring studies).
· Increase amount of oak woodland habitat that is not vulnerable to development or conversion into agriculture or grazing. Increase habitat acquisition of various types of oak woodland for preservation, including areas far enough from feedlots to prevent or reduce the problem of parasitism locally.
· Implement cowbird control locally,
if determined necessary, and in areas where associated species will benefit.
Implement this in conjunction with land acquisition and/or protection,
in areas which will not require long-term cowbird control in order for
initial cowbird control to be successful.
ACTION (e.g., acquire and restore habitat, specific management and restoration recommendations, specific research and monitoring needs, specific land protection recommendations):
· Statewide census of oak woodland habitats to determine abundance and distribution of BGGNs and associated oak woodland species.
· Statewide census of other habitat types within California in which BGGNs breed (desert, shrub-steppe, pinyon-juniper).
· Set up Breeding Bird study plots to monitor reproductive success and parasitism rates in select oak woodland. Ideally, set up some plots in lowland areas that contain BHCO populations, and others in more pristine habitat.
· Habitat restoration in historically grazed areas.
· Habitat acquisition.
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