Download Oak Woodlands - Point Blue Conservation Science

California Partners in Flight—Bird Conservation Plan
Oak Woodlands
Author: Diana Humple, PRBO
Section 1: Species account outline.
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).
RANGE MAPS/DISTRIBUTION (California): Information on historical distribution and
abundance. Information on current distribution (especially breeding). Please include
copies of historic range maps if available or easily obtainable references.
I. Historical references: According to Grinnell and Miller (1944), bred in California in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.
A. UTM or lat long coordinates of sites known to contain breeding populations:
1. Type of method used in determining breeding status (by site and year).
a. Expert opinion:
 Believed to have been greatly reduced in lowlands due to
Brown-Headed Cowbird parasitism, and to no longer breed in the
Central Valley (Gaines 1974). Grinnell and Miller (1944) said
BGGNs bred locally in riparian habitat in Central Valley
riverbottoms, but Gaines (1974) did not find them in the
Sacramento Valley during his 1973 breeding census, and believes
they are no longer there. Speculates that this is due to cowbird
parasitism. No observations of breeding BGGNs in the San
Joaquin Valley or the Sacramento River Valley during extensive
point count censuses of riparian habitat along these rivers and their
major tributaries in 1998 (PRBO data).
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 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, 1997-1998).
b. Point count (singing individual encountered on 2 or more
different days of census-at least one week apart):
 Inyo County, east side of the Sierras (PRBO data 1998).
 Marin County (PRBO data 1996-1998).
 Mendocino National Forest, Mendocino County (PRBO
data 1996-1997).
 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).
c. Mist netting (female with brood patch, female with eggs in
oviduct, juvenile with no skull ossification before 1 August):
no information.
d. Nest searching:
 Inyo County, eastern Sierras: common breeder in shrubsteppe habitat; during study of riparian drainages in eastern Sierras,
3 of BGGN 4 nests found were in adjacent shrub-steppe, 1 of 4
nests was in riparian habitat.
e. Spot mapping:
 Camp Roberts, central coastal region, San Luis Obispo
County (Tietje and Vreeland 1997).
f. Area search: No information
g. Breeding Bird Atlas:
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 Marin BBA - breeds only on relatively dry ridges in the
interior (Shuford 1993).
 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 pineoak 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).
h. BBS route:
 most birds per route in west are from inner coast ranges
and Sierran foothills of California (S. Droege pers. comm. in
Ellison 1992).
i. Other/Local opinion:
 Small (1994) states that this species has been eliminated
from former breeding range in lowlands of Orange County.
However, Hamilton and Willick (1996) make no mention of this,
saying only that BGGNs nest locally in mixed oak/chaparral
habitat in mountains, foothills, and in coastal chaparral.
 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).
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 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).
ECOLOGY:
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).
A. Arrival date on breeding grounds: 1st males sighted between 24 February
and 30 March on Hastings Reserve (Root 1969). Root believes pair bonds
established either prior to arrival or within first 24-hr after arrival in chaparral, but
that bond forms later in oak woodland. One of earliest nesting small
insectivorous songbirds (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).
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 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).
III. Migration stop-over needs/characteristics:
A. Stop-over period: no information.
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.
IV. Nest type: Cup with relatively high walls, outside of which consists of lichens and
spider webs or caterpillar silk, the lichen which camouflages it in most circumstances
(Weston 1949). Interior consists of fibrous materials such as forb and grass stems,
bark strips, becoming progressively finer towards interior of the nest, where materials
include willow catkins, plant down, paper, cocoons, hair, feathers (Weston 1949,
Root 1969).
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,
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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:
A. Typical breeding densities:
 12 pairs on a 56.1 acre (22.7 hectare) plot at Hastings Reserve in California (Root
1969).
 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).
A. Mating system: Monogamous.
B. Delayed breeding (where are SY birds?): no information.
C. 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
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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).
D. Post breeding social behavior (mixed species flocks, or simply migrate
away?): no information.
VIII. Clutch size: At Hastings in California, mean = 4.25, n=20 (Root 1969); across
multiple states, mean = 4.35, n=160 (Ellison 1992).
IX. 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.
XIV. Who tends the young: Brooding done mostly by females, male occasionally covers
young (Root 1969, Ellison). Both parents feed young (Ellison), male feeds more in
earlier nestling phase when female does more brooding, although males appear to
feed second broods more than first broods (Root 1969).
XV. Diet:
A. Major food items (by season): Small insects and spiders. Most frequent
families include Membracidae, Cercopidae, Cicadellidae (Homoptera), Miridae
(Hemiptera), Chrysomelidae, Mylabridae, Curculionidae (Coleoptera) and
Lycosidae (Araneae) (Root 1967).
B. Drinking: no information. Many areas they inhabit in California are relatively
arid.
XVI. Wintering ground needs and distribution:
 Overall wintering distribution of CA subspecies: winters from Central-Western
California (rarely) to southern CA, Southern Nevada, Western and Central AZ,
Southern New Mexico (rarely,) south to Jalisco, Mexico.; east rarely to SW and S
Texas Phillips (1991). Apparently fairly common wintering in southeastern deserts,
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and uncommon to common winterer west of the southeastern deserts (Grinnell and
Miller 1944)
 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, oakpine 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.
BREEDING HABITAT AND NEST SITE CHARACTERISTICS:
I. Overview of breeding habitat: (e.g. oak woodland vs. oak savannah, age of stand,
dominant species, plant species diversity, structural diversity/variability):
 Most favorable habitat in California is thought to be blue-oak covered hill slopes
and chaparral edges mingled with oaks of several species or with diversified arroyoedge cover. Occurs sparingly in the adjacent chaparral (Root 1967).
 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).
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 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).
A. Substrate (species):
 Nest in a variety of substrates, suggesting that plant species may not be a critical
factor in nest site selection (Root 1967, 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).
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 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).
B. Height of nest:
 Variability suggests that height may also not be a critical factor in selection of
nest sites. Height at which nests were placed above ground is related to the height
of the nest tree, with placement towards the top of the shrub or tree. Height of 66
nests at Hastings Reservation varied from 3to 34 feet, w/ 79% between 7 and 23
feet high. Nests in chaparral were near top of the shrub, while oak nests were at
least 3 feet below the top, and usually at least a third of the way up the tree (Root
1967).
 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).
C. Height of plant: No data for oak-woodland. However in shrub steppe/riparian
habitats in Inyo County, 3 ARTRs were b/w 1 and 2 meters high; black
cottownwood 17 meters high (PRBO data).
D. Nest concealment: Appears to not be based on actual foliage concealment,
but based on camouflage, as lichen covered nests might be completely exposed
but easily unnoticed (Root 1967, Ellison 1992). Lichen is standard, but in burned
areas, pieces of scorched bark are used that may serve the same purpose
(Chamberlin 1901 in Shuford 1993). In Inyo County, the 4 nests were poorly
concealed in terms of percent foliage cover, supporting above idea (PRBO data
1998).
II. Vegetation surrounding the nest (Importance of each category may differ by
species)
A. Canopy cover:
 At Hastings Reservation, deciduous oaks occur in open stands, with more
extensive areas dominated by oak scrub (8 to 24 feet tall) (Root 1967).
 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 (74100 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
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canopy openings, but more study is needed (Ambuel and Temple 1983 in Ellison
1992, Robbins et al. 1989).
B. Dominant plant species in canopy:
 At Hastings Reservation (Root 1967): Quercus Douglasii (deciduous oak) with
some Quercus lobata, some coast live oaks (Quercus agrifolia). Territorial
boundaries extended throughout breeding season to include the dense clumps of
evergreen vegetation to which fledglings are led by the adults; these areas may
provide the fledglings better protection from predators than the more open
deciduous oaks and chaparral (Root 1967).
 At Mendocino: Brewers Oak, followed by Oregon White Oak, with some Black
Oak and Ponderosa Pine (PRBO data).
C. Average shrub cover:
 Shrubs cover considered an important aspect (see Breeding Habitat Overview
above).
 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).
D. Dominant shrub species:
 At Hastings, chaparral is dominated by chamise (Adenostoma fasciculatum) on
dry slopes, and buckbrush (Ceanothus cuneatus) where it is wetter (Root 1967). 
 In eastern Sierras, shrubs are predominately Big Sagebrush (Artemisia
tridentata), Rabbitbrush, Coffeeberry (PRBO data).
 In desert ranges, dominant shrub is Antelope Brush (Small 1994).
 At Mendocino National Forest, dominant species were Brewers Oak and
manzanita (PRBO data).
E. Average forb cover: Inyo County, eastern Sierras, cover <10%, n=4 (PRBO
data). No other information.
F. Dominant forb species: No information.
G. Ground cover: No information.
H. Slope: Inyo: 2-10%. In Maryland a significant negative correlation was found
between BGGN abundance and slope (Robbins et al. 1989).
I. Aspect: Inyo County, eastern Sierras, 18-90 degrees (n=4) (PRBO data).
J. Tree DBH:
 In pinyon-juniper habitat in New Mexoci, mean dbh=17.4cm, n=92 (Bbird data).
 In Arizona, mean dbh=22.8cm, n=67 (Bbird data).
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 In Minnesota, mean dbh=46.7 (Bbird data).
 In Ohio, mean dbh=38.4, n=3 (Bbird data).
K. Snags:
 Only data comes from Inyo County, eastern Sierras, n=4. Mean percent snags =
0%; mean percent logs/stumps=12.5% (PRBO data).
 In Maryland a significant negative correlation was found between BGGN
abundance and number of standing dead trees (Robbins et al. 1989).
L. Distance to water: Ellison (1992) reports no published information.
III. Landscape factors:
A. Elevation:
 Prefer foothills and low mountains (Grinnell and Miller 1944).
 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).
B. Fragmentation: Has been suggested that BGGNs affinity to edge habitats
may be partially responsible for large amounts of nest predation by jays and
for the high parasitism rates (USDA Forest Service 1994). However, a study
in oak woodland in San Mateo County, CA revealed that abundance of
BGGNs from point count data was neither positively or negatively associated
with amount of edge (Sisk et al. 1997).
C. Patch size: No detailed information.
D. Disturbance (natural or managed): (e.g. floods, fires, logging):
 The clearing of woodland (eliminating substrates) would probably reduce
BGGN populations; fires might open up habitat to their liking (Shuford 1993).
 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).
E. Adjacent land use: Frequently adjacent to grazing, as California’s oak
woodlands are predominately privately owned and grazed (Aigner 1998).
IV. Other:
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SPECIAL FACTORS: Factors influencing a species occurrence and viability.
I. Brood parasitisim:
 One of the smallest regular hosts of the BHCO (Weston 1949).
 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)
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 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)?
II. Dietary: Purely insectivorous. See pesticide use below.
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.
V. Predators:
 Probable predation by Western Scrub-jay was said to be the major factor responsible
for nest loss at Hastings Reserve (Root 1969).
 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).
VI. Exotic species invasion/encroachment: abundance of BGGNs in Marin as
determined by point count censuses does not appear to be negatively affected by
presence of Scotchbroom (PRBO data).
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 twoweek 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.
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 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).
DEMOGRAPHICS:
I. Age and sex ratios: no information.
II. Productivity measure(s):
 24% of nests fledged BGGN young at Hastings Reserve, CA (Root 1969).
 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.
III. Survivorship: Oldest recaptured bird was 4yr. 2 mo. old when last encountered
(Klimkiewicz et al. 1983 in Ellison 1992).
IV. Dispersal: no data.
MANAGEMENT ISSUES:
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 Woodpewee, Solitary Vireo, Western Tanager, Bushtit, Spotted Towhee, Plain Titmouse, and
Western Scrub-jay (Goguen and Matthews 1998).
15
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
16
returning to the lowlands of San Diego County in response to an active cowbird control
program (implemented for Least Bell’s Vireos).
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
17
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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