California Partners in Flight

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I.   Historical references: According to Grinnell and Miller (1944), bred in California in the foothills and low mountains surrounding Sacramento and San Joaquin River Valleys, locally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).

· 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 shrub-steppe 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:

· 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 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).

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).

· 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).

· 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, 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 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.