California Partners in Flight—Oak Woodland Bird Conservation Plan
Western Bluebird
    Sialia mexicana
 

PREPARED BY:

SPECIES: SUBSPECIES STATUS: Only one subspecies, Sialia mexicana occidentalis, breeds in California.

S. m. bairdi, the Rocky Mountain Western Bluebird may occasionally mix into S.m. occidentalis winter flocks in the Joshua Tree National Monument (Mojave bioregion).

 MANAGEMENT STATUS: No official special status.

Listed on the National Audubon Society's Blue List in 1972 and from 1978 to 1981. Declines continue in parts of range (Washington/Oregon) and in California where available habitat is destroyed or the species must compete with European starlings and house sparrows (Ehrlich et al. 1988, Roberson 1993)

 range map: I. Historical range:
Central Coastal:
Common breeder (Small 1994).
Los Angeles Co.: Bent (1949) relays correspondence reporting bluebird feeding young in Pasadena and another pair with newly fledged young in El Monte.
Point Lobos, Monterey Co.: Grinnell and Lindsdale (1936) describe western bluebirds as "a most common resident" with large flocks in the winter.

Klamath:
Rare to uncommon breeder along Oregon-California border south to the North Coast range (Small 1994).
Detected as breeders in the Klamath National Forest (Raphael 1986).

Modoc:
Lassen area: Common breeders in the foothills around Lassen Volcanic National Park (Stebbins and Stebbins 1953). Grinnell et al. (1930) report bluebirds in the Lassen region in clearings and areas with little tree cover, usually lighting on the tops of dead or dead-topped trees.

Sacramento Valley:
Formerly a common breeder, now only seen mainly in winter (Small 1994).

San Joaquin Valley:
Common throughout foothills and sub-alpine elevations to Kern River (Small 1994).

Sierra Nevada:
Yosemite Region: Grinnell and Storer (1924) report bluebirds to be "confined to blue oak belt of the western foothills, hence within the Upper Sonoran Zone." Bluebirds were not found to breed in the Transition Zone or in the Yosemite Valley.
Mono Basin: Rare to common breeders (Small 1994).

South Coastal:
Common breeder in San Diego mountains (Small 1994).
Riverside Co.: Weathers (1983) reports bluebirds breeding in the Santa Rosa Mountains.

Other Sites:
None reported on offshore islands (Ziener et al. 1990).

II. Current breeding distribution:
Only nesting data collected in the previous ten years (1988-1998) were used to establish "current" breeding status.

Counties and sites with known breeding populations:
1. Type of method used in determining breeding status (by site and year). See appendix for summary of distribution by all methods.
a. Expert opinion:
Bay/Delta:
Sacramento Co: Found breeding at the Cosumnes River Preserve, 1995-1998 (DiGaudio pers. comm. 1998)

b. Point count (singing individual encountered on 2 or more different days of census-at least one week apart):
Bay/Delta:
Sacramento Co: Detected as a breeder in the grasslands of the Valensin property of the Cosumnes River Preserve in 1997 (PRBO data).

Klamath:
Detected during the breeding season in the Mendocino National Forest (PRBO data).

c. Mist netting (female with brood patch, female with eggs in oviduct, juvenile with no skull ossification before 1 August):
No data.

d. Nest searching:
Central Coastal:
Monterey Co.: Studied as a breeding species for years on the Hastings Natural History Reservation in Carmel Valley. (Eichholz and Koenig 1992, Dickinson 1997)
San Luis Obispo Co.: Breeding at Camp Roberts Military Base (Tietje and Vreeland 1997).

Sacramento Valley:
Colusa Co.: Breeding near the East Park Reservoir (PRBO data).

San Joaquin:
Madera Co.: Purcell (1997) observed 86 bluebird nest attempts between 1989 and 1991 (47 in boxes, 39 in natural cavities) on the San Joaquin Experimental Range (SJER).

South Coast:
Riverside Co.: Breeding in the San Bernadino National Forest (PRBO data). Also nesting in Garner Valley of the San Jacinto Mountains (33 deg. 47'N, 116 deg. 58'W; 1,375m altitude) (Mock et al. 1991).

e. Spot mapping: No information.
f. Area search: No information.
g. Breeding Bird Atlas:
Bay/Delta:
Marin Co.: Regular breeders (Shuford 1993).

Central Coast:
Monterey Co.: Formerly more widespread breeder. Continues to be a common resident of oak savanna and open oak/pine woodland of inner coast range and eastern Monterey Co. Once bred on the Monterey Peninsula, but there are no recent records of breeding bluebirds there. Currently, only breed in Corral de Tima/Carmel Valley. Formerly bred at Pt. Lobos, with numbers increasing in the winter, but are now rare there. No other winter reports from the county. Western bluebirds on the coastal slope remain as breeders only in areas free from European starlings (Roberson 1993).
Santa Cruz Co.: Suddjian (1990) reports that fewer bluebirds breed in the county due to changes in land use.

Sierra Nevada:
Lake Tahoe Region: Orr and Moffitt (1971) reports breeders on slopes, often away from human settlement.

South Coast:
San Diego Co.: Atlas data report that bluebirds continue to regularly breed in the oak woodlands in the county. Recently, observers recorded western bluebirds breeding in Anza-Borrego desert for the first time (Philip Unitt, pers. comm. 1999).

h. BBS route:
Bluebirds are too rarely detected on BBS routes to allow for a route-by-route analysis.
.
i. Other/Local opinion: no information.

ECOLOGY:

I. Average territory size:

In Arizona, territories averaged 1.13 ha (range 0.3-0.8 ha) in a conifer forest (Balda 1975). II. Time of occurrence and seasonal movements.
A. Arrival date on breeding grounds:
        Initiates breeding from March to early May. See below. Central Coastal: Hastings Reservation, Monterey Co.: Between 1990-1994, the date of first laying ranged from March 8 to May 2. Annual variations in weather may have influenced nest initiation (Dickinson and Leonard 1996). B. Departure date from breeding grounds:

C. Spring migration period: NA

D. Fall migration period:

Not true migrants, but may shift range after breeding in July and August. Some movement southward and down-slope occurs in early winter, apparently affected by weather and food availability (Grinnell and Miller 1944, Small 1994).

Western bluebirds at the San Joaquin Experimental Range in Madera Co. (San Joaquin Bioregion) are year-round residents (Purcell pers. comm.)

 E. Extent of wintering in CA: Winter range may be dependent on local native berry crops (See Part XVI. Winter grounds needs and distribution below).

Central Coastal:

37 birds/ 100 ha in Morongo Valley, San Bernadino Co. in winter (Wilson 1978) Mojave: Miller and Stebbins (1964) report western bluebird flocks of about 3-10 birds in the Joshua Tree National Monument. Occasaionally, flocks were as large as 25-50 birds, with mountain bluebirds and S.m. bairdi occasionally mixed in. Most reported have been seen from Twenty-nine Palms westward, feeding on mistletoe and occasionally drinking water. Sacramento Valley: Gaines (1977) reports 37 birds/km2 at the Bobelaine Audubon Sanctuary in Sutter Co.

Common in winter (Small 1994).

 San Joaquin Valley: Grinnell and Storer (1924) found "moderate numbers" in the Valley below La Grange, Stanislaus Co., and Snelling, Merced Co.                     Sierra Nevada: Grinnell and Storer (1924) explain that the non-breeding range of bluebirds (at least in fall and early winter) is affected more by availability of mistletoe berries than weather.

Gaines (1988) reports 0-187 western bluebirds detected during Christmas Bird Counts in Yosemite.

Very rare visitors to Imperial Valley (Small 1994).

 South Coast: Santa Rosa Mountains winters in the chaparral, often moving in search of berries (Weathers 1983). Rare winter visitors to the lowlands of southern coastal California (Small 1994). III. Migration stop-over needs/characteristics:

A. Stop-over period: NA

B. Habitat use: NA (See Part II Section E: Extent of wintering in CA)

C. Routes: NA

IV: Nest type:
                    Secondary cavity nester, using old woodpecker holes and natural cavities. Builds a small cup inside cavity.

Bluebirds readily in artificial nest boxes, in some cases with higher nest success than in natural cavities. Purcell (1997) found that bluebirds nesting in boxes fledged more young than did those nesting in natural cavities at the San Joaquin Experimental Range in Madera Co., CA (San Joaquin bioregion), though clutch sizes were the same in both the cavities and the boxes. V. Foraging strategy: Mainly insectivorous when insects are available. Relies heavily on winter berries, especially mistletoe during the winter months. While foraging for insects, western bluebirds prefer to use a low perch from which to sally. Lacking perches, bluebirds will hover above ground and swoop down upon prey (Bent 1949). VI. Displays: Pair may sit close to each other and male will mount the female, then the female may mount the male and fly away (Bent 1949). Occasionally, the female will perform a wing flutter and beg, after which the male may feed her. Also, females may solicit breeding by raising her tail and tilting her body upward (Dickinson and Leonard 1996). VII. Social Organization:
  1. Typical breeding densities:
Breeding density may be dependent upon availability of suitable nest sites, and, correspondingly, the presence of nest boxes on a plot (Zarnowitz and Manuwal 1985, Brawn and Balda 1988). A study of mountain bluebirds and tree swallows has shown that density is not an adequate indicator of habitat quality; productivity varies between areas of equal densities, apparently dependent on other habitat characteristics (Holt and Martin 1997).

Central Coastal:

Camp Roberts, San Luis Obispo Co: Tietje and Vreeland (1997) found an average of 3.7 western bluebird territories per 100 acres on 9 study plots from 1994-1996.

Hastings Reservation, Monterey Co.: 363 nest boxes have been placed on the 7 km2 study plot. While boxes were about 78 m (se=31m) apart, the mean distance to nearest neighbor ranged from 120-240m (Dickinson and Leonard 1996).

 Klamath: Mendocino County: At the Hopland Field Station, Wilson et al. (1990) detected 10.2 bluebirds per 40 ha (se=1.3).

 

 B. Mating system: Monogamous. Extra-pair copulations are common and result in offspring in over one third of nests at the Hastings Reservation, Monterey Co. (Central Coastal bioregion) (Dickinson and Leonard 1996). C. Delayed breeding: Adult males will occasionally forego breeding and instead help at their parents' or siblings' nests, providing defense and food for nestlings (Dickinson et al. 1996). D. Post fledging biology of offspring (where do they go and when?): Family groups often remain together in flocks after breeding. Often, western bluebirds will move upslope after breeding and move lower once weather and food sources push them down (Grinnell et al. 1930, Grinnell and Miller 1944). E. Post breeding social behavior: Often stay in family groups and create loose flocks after breeding. Flocks of 6-25, sometimes greater, are seen in the fall and during the winter (Bent 1949, Gaines 1988). VIII. Clutch size:
4-6, occ. 3-8 (Baicich 1997).

IX. Incubating sex:
Female.

X. Incubation period:
13-14.5 days (Baicich 1997, Martin and Lu 1992).

XI. Nestling period:
17.5-20 days (Baicich 1997, Martin and Lu 1992). At the SJER in Madera Co. (San Joaquin Bioregion), the nestling period ranges from 21-23 days (SJER unpubl. data)

XII. Development at hatching:
Altricial.

XIII. Number of broods:

1-2 (Dickinson and Leonard 1996). At the Hastings Reservation in Carmel Valley, CA (Central Coastal bioregion), researchers found that in 67% of years studied about 20% of the studied pairs attempted a second brood after successfully fledging their first (Dickinson et al. 1996). At the SJER, western bluebirds 6 out of 40 nests were second broods between 1989 and 1994 (SJER unpubl. data). Availability of food may influence the decision to double-brood (Brawn 1990). XIV. Who tends the young: Both parents feed nestlings. Male tends fledglings while female renests. At the Hastings Reservation in Monterey Co. (Central Coastal bioregion), pairs occasionally (less than 10%) have an adult male helper at the nest who is usually an offspring of the pair (Dickinson et al. 1996). XV. Diet: A. Major food items (by season): Summer: Insects. Bent (1949) reports that bluebird stomachs contained 81.9% animal and 18.1% vegetable matter. Animal matter consisted of insects, mainly grasshoppers (21.3%) and caterpillars (20.3%). In Arizona, Brawn (1990) also noted that grasshoppers were the primary food item brought to nestlings.

Winter: Bluebirds rely heavily on winter berry crops, and their winter range depends on the availability of such food, especially mistletoe (Grinnell and Miller 1944, Bent 1949, Gaines 1988).

 B. Drinking: Gander (1960) describes western bluebirds often drinking during the fall and winter months in his garden and Miller and Stebbins (1964) report bluebirds drinking while wintering in the Joshua Tree National Monument. However, Williams and Koenig (1980) failed to observe a bluebird drink during 162 hours over 26 days. They add that insectivores are generally less dependent on water than granivores. XVI. Wintering ground needs and distribution: Often, western bluebirds shift upslope after breeding along California's mountain ranges, then move lower as weather grows harsher (Grinnell and Storer 1924). They winter at low altitudes throughout the state. Availability of winter berries seems to be a primary factor in wintering ground selection and distribution (Grinnell and Miller 1944, Verner et al. 1980, Gaines 1988). BREEDING HABITAT AND NEST SITE CHARACTERISTICS:

I. Overview of breeding habitat:

Bluebirds appear partial to areas characterized by widely spaced understory vegetation and major tree species usually clustered to form areas of dense cover adjacent to, or within, more open spaces (Herluson 1969, Shuford 1993). Bluebirds may be found in oak savanna, oak woodlands, oak-conifer, riparian corridors and conifer stands adjacent to open, grassy areas (Mock et al. 1991, Small 1994, Dickinson 1996, Purcell 1997, Suddjian 1999).

In thickly forested areas, such as the Sierran bioregion of California, bluebirds nest in open meadows ringed with trees (Gaines 1988, Mock 1991). Western bluebirds are often seen in edge habitat and burned areas, avoiding agricultural lands, subalpine areas, treeless grasslands, and clearcuts—though there are some reports of bluebirds using the latter (Herlugson 1969). In Washington, Power (1966) observed mountain bluebird adults foraging in more open areas while dense patches provided cover for their young. Western bluebirds may use a similar strategy.

Zarnowitz and Manuwal (1985) observed that western bluebirds in Washington only nested in the early successional stages of forests (25-50 years), which tended to be more open, with more saplings and shrub cover (which provide perches).

Bluebirds are dependent on previously excavated or natural cavities for nesting. In Washington, western bluebirds had about 2-5 times higher breeding density in areas with an abundance of large snags than areas with fewer or smaller snags (Zarnowitz and Manuwal 1985).

Bay/Delta

San Francisco Co.: Sibley (1952) reports bluebird in south San Francisco Bay breeding in valley oaks (Quercus lobota) and blue oaks (Q. douglasii) in grassy areas with widely spaced trees. He describes them more as a "savanna" rather than a "woodland" species.

Marin Co.: Bluebirds make use of extensive grasslands bordered by oaks and oak savanna (Shuford 1993).

 Central Coastal: Bent (1949) reports two pairs of bluebirds feeding young in Los Angeles Co. One pair was observed along a golf course bordered by oaks and the other in a riparian area in Los Angeles County at an altitude of 300 feet.

Monterey Co.: Bluebird breeding habitat in Monterey Co. consists of oak savanna and open pine-oak woodland of the inner coast (Roberson 1985). Atlas data reveal that they breed in clearings of oak woodlands, riparian and chaparral. However, bluebirds avoid extensive open grasslands, agricultural fields, dense woodland, chaparral, and "much of the degraded riparian along the Salinas River" (Roberson 1993). He adds that bluebirds are missing from areas invaded by starlings. At the Hastings Natural History Reservation, bluebirds breed in oak woodland and oak savanna dominated by valley oak, blue oak, and coast live oak (Q. agrifolia) (Eichholz and Koenig, 1992, Dickinson 1996).

Santa Cruz Co.: Suddjian (1999) reports that bluebirds mainly breed at upper elevations of local mountain ridges, in open areas abutted with conifers or blue oaks.

Camp Roberts, San Luis Obispo Co.: Tietje and Vreeland's (1997) study plots, which included breeding western bluebirds, were comprised of extensive oak woodland and savanna over steep hills. Grasses included many introduced annuals. Some shrubs (Artemesia sp. and Manzanita sp.) grew under the oaks and chaparral often abutted the study plots.

Santa Barbara Co.: Lehman (1994) reports breeding bluebirds in oaks, open riparian, golf courses and cemeteries.

 Klamath: Mendocino National Forest: Detected as a breeding species in areas dominated by Brewer's oak (Q. garryana var. breweri), black oak (Q. kelloggii), Douglas fir (Pseudotsuga menziessii) and white fir (Abies concolor) (PRBO data). Modoc: Historically common in foothills in summer, lowlands in winter (Stebbins and Stebbins 1953).

Grinnell et al. (1930) observed bluebirds in summer inhabiting areas with little tree cover, often perched on snags.

 San Joaquin: Purcell (1997) found abundant bluebirds nesting at the San Joaquin Experimental Range in Madera Co. There, foothill pine (Pinus sabiniana), blue oak, and interior live oak (Q. wislizenii) dominate the overstory while buck brush (Ceanothus cuneatus), chaparral whitethorn (C. leucodermis), redberry (Rhamnus crocea), and Mariposa manzanita (Arctostaphylos viscida mariposa) comprise the understory. Sierra Nevada bioregion: Grinnell and Storer (1924) note that bluebirds are confined to the blue oaks in the foothills of the Yosemite region in the summer (considered the Upper Sonoran zone).

Gaines (1988) reports western bluebirds breeding exclusively in meadows and woodlands west of Sierran crest (rare visitors to the east side). In the foothills, they nest in blue and live oaks in dry, grassy areas. In mid-elevations, they tend toward forest meadows or dry rocky ridges with a grassy understory and few trees.

Others describe breeding habitat in the Sequoia and Kings Canyon National Parks as closed-canopy forests (AB 28:946). But these were possibly detected after breeding occurred (Gaines 1988).

In the Lake Tahoe area, bluebirds are found in thick woods and along mountain slopes, often away from human disturbance (Orr and Moffitt 1971).

 

 South Coast: Riverside Co.: Weathers (1983) describes western bluebirds in Deep Canyon of the Santa Rosa Mountains saying they avoid the dry, desert slopes and prefer areas with denser vegetation. They are most abundant in pinyon-juniper and coniferous forest habitats (preferring the pinyon in winter and conifers in summer).

Mock (1991) found bluebirds in nest boxes in stands of Jeffrey pine (P. jeffreyi), with wet and dry meadows comprised of native and exotic grasses.

 Other sites: Arizona: Known breeder in ponderosa pine (P. ponderosa) and pinyon-juniper habitat (Balda 1975, Brawn 1990). II. Nest Site.
    A.  Substrate (species):
      Bluebirds readily nest in nestboxes placed on any nearly substrate (live or dead trees, poles, fence posts, etc.) provided the surrounding habitat is suitable (Eichholz and Koenig 1992, Radunzel et al. 1997).

      Bluebirds often have higher reproductive success in nestboxes, especially those constructed to deter interspecific competition or depredation (Purcell 1997, Radunzel et al. 1997).

      In an Arizona forest dominated by ponderosa pines, Cunningham et al. (xxxx) found that bluebirds nested in snags most frequently (70%, n=33). They generally preferred cavities with 81-100% bark cover around the cavity entrance (a preference shared by other cavity nesters).

      Central Coastal:
         Camp Roberts, San Luis Obispo Co.: Western bluebirds mainly nest in blue oaks (pers. obs.).

      Klamath:
          Mendocino County: Wilson et al. (1986) found a western bluebird nesting in a Pacific madrone (Arbutus menziessii).

      Sacramento Valley:
          East Park Reservoir, Colusa Co.: 100%, 6 of 6, nests were found in blue oak (PRBO data).

      San Joaquin Valley:
          Madera Co.: 29 (83%) of natural cavity or excavated nests were in blue oak, 2 were in interior live oak, and 4 were found in snags. Interior live oak typically dominated the canopy. Cavities in live trees tend to last a long time and are reused for many years while snags do not remain standing for very long (SJER unpubl. data, K. Purcell pers. comm.).

      South Coast:
          San Bernadino National Forest, Riverside Co.: natural cavity nest substrates included 34% Jeffrey pine, 24% stumps, 17% black oak, 10% snags, 5% logs, 5% willows (Salix spp.), and 5% buildings (n=41) (PRBO data).
       

    B.  Height of (natural or excavated) nest cavities:
      Central Coastal:
          Pt. Lobos, Monterey, CA: Grinnell and Linsdale (1936) report nests in pines and pine stumps ranging in height from 5 to 25 feet.

      Sierra Nevada:
          Yosemite: Two nests found near LaGrange were in blue oaks, 9 feet and 14 feet from the ground (Grinnell and Storer 1924).

      San Joaquin Valley:
          SJER, Madera Co.: mean nest height was 4.11 m (range 1-8.1, n=38) from 1989 to 1984 (SJER unpubl. data).

       South Coast:
          San Bernadino National Forest, Riverside Co.: nest height averaged 1.36m, (range 0-6.4m, sd=1.6m, n=43) (PRBO data).

      Other sites:
          In New Mexico, nests were found to average approximately 4.1m, range=1-7 m, (n=17) (Goguen 1995).
          In Arizona, nests in oaks averaged 3.7 m in height (range 1.3-6.1 m) (Cunningham et al. xxxx).

    C.  Height of plant:
      Sacramento Valley:
          East Park Reservoir, Colusa Co.: mean=8.4m, range=3-7.0m, sd=1.4m, (n=6) (PRBO data).

      San Joaquin Valley:
          SJER, Madera Co.: mean=9.48 m (range=1.9-15 m, n=34) (SJER unpubl. data).

      South Coast:
          San Bernadino National Forest, Riverside Co.: mean = 3.9m, range=0-30m, sd=6.2m (n=41) (PRBO data).

      Other sites:
          In New Mexico, the height of the nest substrate averaged 10.1m, range 1-14m, n=17 (Goguen 1995).
       

    D.  Nest concealment: NA
III.  Vegetation surrounding the nest
    A.  Canopy cover:
      Bay/Delta:
          Sibley (1951) lists the western bluebird as a savanna species, preferring grassy areas with a few clumped blue oaks in south San Francisco Co. Shuford (1993) also notes their preference for open grassy areas in Marin Co.
          Cosumnes River Preserve, Sacramento Co.: Seen in the oak savanna and along fences and power lines in adjacent agricultural fields (pers. obs.)

      Central Coast:
          Camp Roberts, San Luis Obispo Co.: Tietje and Vreeland (1997) report that the average canopy cover by trees and shrubs on their plots was 40-70% (over all plots studied).

      Klamath:
          Mendocino County: At the Hopland Field Station, Wilson et al. (1990) found that bluebirds preferred plots with low tree densities (less than 100 trees per ha).

      San Joaquin Valley:
          SJER, Madera Co.: In grazed plots, canopy cover averaged 62% (range=2.5-100%, n=28) (SJER unpubl. data).

      Sierra Nevada:
          Yosemite: Gaines (1988) reports bluebirds in dry grassy areas with few trees, in forest meadows, and along rocky ridges.

      South Coast:
          San Bernadino National Forest, Riverside Co.: Average canopy cover within 11.3 m of nests was 17.9% (sd=22%, range=0-81%, median=8%, n=3) (PRBO data).

      Other Sites:
          Arizona: Brawn (1990) describes western bluebirds nesting in boxes in clearcut (with 69 trees/ha remaining) and "thinned" (225 trees/ha) ponderosa pine forests in northern Arizona. Both annual productivity (percent young fledged) and feeding rates of young were both higher in the thinned plots than in clearcuts.

       

    B.  Dominant plant species in canopy:
      Bay/Delta:
          Palomarin Field Station, Marin Co.: Nest in a box on a pole surrounded by coastal scrub habitat, with coast live oak, Douglas fir, Monterey pine (P. radiata), and California bay (Umbellularia californica) in area (PRBO data).

          South SF Bay: Blue oaks (Sibley 1951).

          Cosumnes River Preserve, Sacramento Co.: Isolated or clumped valley oaks in savanna (PRBO data).

      Central Coastal:
          Camp Roberts, San Luis Obispo Co.: Tietje and Vreeland's (1997) study plots consisted mainly of blue oaks (56-100% of tree species per plot) and coast live oaks (0-44% per plot). (Note: these were for all plots in the study, not just those utilized by bluebirds.)
          Hastings Natural History Reservation, Monterey Co.: Canopy dominated primarily by blue oak, valley oak, and coast live oak (Eichholz and Koenig 1992).

      Klamath:
          Mendocino National Forest, Sonoma Co.: The dominant plant species in the canopy consisted of Brewer's oak, black oak, ponderosa pine and Douglas-fir (PRBO data).
          Mendocino County: Raphael's (1986) study plot was dominated by Douglas-fir, tanoak (Lithocarpus densiflorus), and Pacific madrone.

      San Joaquin:
          Madera Co.: At the San Joaquin Experimental Range, gray pine (P. sabiniana), interior live oak, and blue oak dominate the canopy (Verner 1980, Purcell 1997).

      South Coast:
          San Bernadino National Forest, Riverside Co.: Common tree species around 31 nests include white fir, incense cedar (Calocedrus decurrens), Jeffrey pine, ponderosa pine, and willow spp. (PRBO data). Bluebirds nest success was significantly negatively correlated with presence and abundance of willows within 11.3m of the nest (P<0.05) (Geupel et al. 1996).

          Garner Valley, Riverside Co.: Jeffrey pine dominated the canopy (Mock et al. 1991).
       

    C.  Average shrub cover:
      San Joaquin Valley:
          SJER, Madera Co.: Mean shrub cover was 0.4% (n=29). Only 3 of 29 bluebird nests had shrubs within 11.3 m (SJER unpubl. data).

      South Coast:
          San Bernadino National Forest, Riverside Co.: mean=13%, sd=23.4%, range=0-78%, median=0 (n=31) (PRBO data).

      Other sites:
          Manitoba, Canada, Munro and Rounds report that eastern bluebirds prefer sites with a higher percentage of shrub cover than those with less shrub cover.
       

    D.  Dominant shrub species:
      South Coast:
          San Bernadino National Forest, Riverside Co.: white fir, sage brush (Artemesia sp.) Mountain Whitethorn (Ceanothus cordulatus), chinquapin (Chrysolepis sempervirens), Jeffrey Pine, willows, snowberry (Symphoricarpas spp.), and wild rose (Rosa californica) were found within 5m of 31 nests in the San Bernadino Natl. Forest (PRBO data).

          Riverside Co.: Mock (1991) describes the shrubby layer of understory as consisting of sagebrush.

    E.  Average forb cover:
      San Joaquin Valley:
          SJER, Madera Co.: Forb cover within 11.3 m of nests on a grazed plot averaged 20.8% (range=1-39%, n=26). Grass cover in the same area averaged 84.5% (range=38-100%, n=26%).

      Other sites:
          Manitoba, Canada: Eastern bluebirds selected nest sites with the most grass and wooded pasture. They selected against long grass and fallow fields (Munro and Rounds 1985).
       

    F.  Dominant forb species:
      South Coast:
          Mock (1991) reports a dominate grassy understory around nest boxes consisting of native (Stipa sp.) and exotic (Bromos sp.) grasses.
       
    G.  Ground cover:
San Joaquin Valley:
    SJER, Madera Co.: South Coast:
San Bernadino National Forest, Riverside Co.:
    H.  Slope:
      Bluebirds are commonly found on hillsides and ridges as well as flat grasslands throughout their breeding range in California.

      San Joaquin Valley:
          SJER, Madera Co.: slope averaged 12.5% (n=28) (SJER unpubl. data).

      South Coast:
          San Bernadino National Forest, Riverside Co.: The average slope on which nest trees were found was 17.6deg (med=8.5 deg , sd=21.95 deg , range=0-75 deg , n=26) (PRBO data).
       

    I.  Aspect:
      San Joaquin Valley:
          SJER, Madera Co.:
              The average slop on which nest trees were found was 178.2 deg. (range=3-363deg. )
              Nest typically faced northward, about 4 deg. (SJER unpubl. data).

      South Coast:
          San Bernadino National Forest, Riverside Co.:
          The slopes on which nests were found generally faced southwest, about 213 deg. (sd=118 deg. , range=16-360 deg. , n=26) (PRBO data).
          The nests themselves typically faced south-southwest, about 196 deg. (med=220 deg. , sd=121 deg. , range=10-360 deg. , n=37).
       

    J.  Tree Diameter-at-Breast-Height (DBH):
San Joaquin Valley:
    SJER, Madera Co.: Mean DBH of nest trees (all species) was 61.4 cm (range=15-156 cm, n=32). Blue oaks averaged 55 cm (range=15-99 cm, n=27). Interior live oaks averaged 132 cm (n=2). Snags averaged 70.3 cm DBH (n=3).   South Coast:
    San Bernadino National Forest, Riverside Co.: Nests had an average of 6.5 trees within 11.3 m (med=4, sd=7.1, range=0-26, n=25). Species richness of the trees averaged 1.7species, (sd=1.2 spp., range=0-4, n=25) (PRBO data).

Number of trees within 11.3 m of nest by DBH:

 Klamath:
    Mendocino County: Western bluebirds were associated with areas with large trees (average DBH was 45 cm) (Raphael 1986).

Other sites:
    In Arizona, the bluebirds preferred oaks with mean DBH of 35.6 cm (range=25.4-65.0 cm) (Cunningham et al. xxxx).
    K.  Snags and Stumps.
      For many cavity nesters, nest site availability is strongly associated with snag abundance, especially in areas without nest boxes. Reduction of snags may have contributed to the decline in eastern bluebird populations over recent decades (Radunzel et al. 1997). The population size of many cavity nesters may be limited by nest site availability (Balda 1975, Brawn and Balda 1988, Brawn 1990). Cunningham et al. (xxxx) found that half the variability in the density of breeding secondary cavity nesters in their study area could be explained by snag density.

      Brawn and Balda (1988) found evidence that a population of bluebirds in Arizona was limited by nest site availability. However, not every available cavity is a suitable nest site (Cunningham et al. xxxx). Retaining a few snags or trees with cavities from being removed may not be a sufficient conservation measure.

      This may be the case for bluebirds in nest boxes as well. Waters et al. (1990) failed to observe a reduction in density after blocking available nest boxes at the San Joaquin Experimental Range in Madera Co. and only about 33% of available nest boxes are occupied at the Hastings Reservation in Monterey County (Dickinson and Leonard 1996). In these areas, other factors may be limiting population sizes.

      Western bluebirds may be less reliant on snags as nest substrates than are some other species. Though they preferred snags in ponderosa pines (70%) in Arizona, they shifted to cavities in live trees in areas with low snag densities (Cunningham et al. xxxx). At the San Joaquin Experimental Range in Madera Co. (San Joaquin Bioregion), Purcell (1999) observed that live trees, especially large, older ones, tend to have more excavated and natural cavities than snags. Furthermore, cavities in live trees remain available to nesting birds for longer than those in snags.

      Finally, the age of the snag may play an important role in selection of a snag for nesting. Cunningham et al. (xxxx) found that secondary cavity nesters in their study preferred snags 5-20 years in age with greater than 40% bark around the nest. The oldest and largest snags are not necessarily the best for nest sites for many species.

      South Coast:
          San Bernadino National Forest, Riverside Co.: Bluebird nests in this area had an average of 0.84 stumps (med=0, sd=2.6, range=0-13, n=25), and 0.08 snags (med=0), sd=0.4, range=0-2, n=25) within 11.3m (PRBO data).
       

    L. Distance to water: No data.
IV. Landscape factors

A. Elevation:

Bluebird breeding range generally spreads from sea level to about 7000 feet, occasionally reaching higher elevations (Grinnell and Miller 1944, Bent 1949, Small 1994).

Bluebirds utilize habitats at various elevations at different time of the year, often moving upslope after the breeding season in search of berries (Bent 1949, Verner et al. 1980, Gaines 1988). This should be considered when drawing range maps or making similar conclusions about the habitat requirements of bluebirds.
 

 B. Fragmentation: Fragmentation of closed-canopy habitat may benefit bluebirds. Verner et al. (1980) qualify the western bluebird as an "edge" species; therefore, some fragmentation may provide preferable habitat for western bluebirds. C. Patch size: No information for this species. Does not seem to be particularly patch-dependent. D. Disturbance (natural or managed): (e.g. floods, fires, logging)
    1.  Population growth, with urban and suburban expansion (especially in Sonoma county, which holds 38% of the north coast's oaks and is growing by about 4000 residents a year).
    2.  Industrial logging ventures are clearing oaks to make way to grow more conifers.
    3.  Expansion of agricultural land, specifically vineyards into woodlands.
LOGGING

Bluebirds may colonize areas opened up by thinning, if suitable nesting habitat remains (Zarnowitz and Manuwal 1985).

The importance of snags: According to Zarnowitz and Manuwal (1985), "species richness, densities, and diversities of cavity nesters increased as snag densities increased" in a conifer forest in Washington. Western bluebirds bred in 2-5 times higher densities in plots with many large snags than they did in plots with fewer and smaller snags (see Section III-K. Snags and Stumps). Also, bluebird breeding distribution in the study was restricted to areas considered "early successional," stands 25-50 years in age, which were more open and had more saplings and shrub cover (suitable bluebird perches). It is noteworthy that species richness and breeding density of all cavity-nesting breeding birds were also highest in the snag-rich plots. This information describes not only the need to preserve older, decadent stands with large and abundant snags, but to preserve dynamic mosaics of woodland at different successional stages. However, snags may be of less importance in oak woodlands (Purcell 1999). More information on nest site selection of natural and excavated cavities would illuminate what dependency (if any) western bluebirds have on snags in oak woodlands.

Thinning vs. clear-cutting: Aigner et al. (1998) found no short-term change negative impacts from "light" fire-wood harvesting in a California oak woodland. They reduced the total basal area in their study plots by 23%, keeping old growth, snags, trees with cavities, and granaries. Point counts failed to detect significant changes in most of the species occurring in the area. However, they only censused for one year following the harvest. Furthermore, they carefully selected which trees to remove--a consideration often lacking during wood harvests, especially those on private lands. Finally, while sparing trees with nest cavities and snags, cutting younger trees may reduce available nest sites for future generations of breeding birds.

The practice of "thinning" appears favorable for bluebirds over clear-cutting. Brawn (1990) found that western bluebirds in thinned plots of conifers (225 trees/ha remaining) had higher annual productivity (percent young fledged) and higher feeding rates of young than bluebirds in clear-cut plots (69 trees/ha remaining).

In a study of mountain bluebirds and tree swallows in British Columbia, Holt and Martin (1997) found that clear-cutting reduced the population density of cavity nesters. While nest site availability was the primary factor affecting density, the structure of the vegetation also played an important role. Furthermore, they found that nest success increased as the age of the stand increased. It is important to note too, that breeding density was not dependent on stand age, while productivity was; density estimates (such as those from spot mapping or point counts) may not be sufficient to gauge habitat quality.

FIRE
Hutto (1995): "For species that were relatively abundant in or relatively restricted to burned forests, stand-replacement fires may be necessary for long-term maintenance of their populations…In addition, salvage-cutting reduces the suitability of burned-forest habitat for birds by removing the most important element--standing, fire-killed trees needed for feeding, nesting, or both by the majority of bird species that used burned forests."

Creates successional areas and mosaic stands: In a study in northern Rocky Mountain conifer forests, Hutto (1995) found that mountain bluebirds were found to be fairly restricted to early post-fire conditions. Furthermore, it created the early successional level of forest that western bluebirds are known to frequent (Zarnowitz and Manuwal 1985).

Fire suppression: It is unclear the effect that fire suppression has on snag densities, since fires create as well as destroys them (Cunningham xxxx). However, because many cavity nesters seem to prefer "young" snags (aged 5-20 years in ponderosa pine forests), regular fires may create more snags fitting this category.

GRAZING
Grazing and nest site selection and nest success: Purcell (1999) found no significant difference in the breeding densities of western bluebirds on grazed and ungrazed plots. However, this is likely due to the size of many bluebirds' territories; many bluebirds seen in the ungrazed plots also held part of their territory in neighboring grazed areas. The thick understory on the ungrazed plot seems less preferred than the more open, grazed areas.

Goguen and Matthews (1998) did not detect a significant difference in nest success between ungrazed (for at least 20 years) and moderately grazed (from Nov. to June, 1.3 ha/animal) pinyon-juniper woodlands in New Mexico (1998). Their data suggest that moderate grazing did not alter the vegetation structure of the woodlands in their study area.

Grazing and regeneration: Grazing may negatively impact oak regeneration and could potentially result in an unhealthy woodland system (Dahlgren et al. 1997).

 E.  Adjacent land use: Urbanization leads to destruction of nesting and foraging habitat, introduces exotic species, such as the European starling and the house sparrow, which often out-compete the bluebird for nest sites. See Special Factors: Sections V. and VI. below.

Logging can have both positive and negative effects on abundance. See Section IV. E: Disturbance above.

 V. Other:

SPECIAL FACTORS: Factors influencing a species occurrence and viability.

A.  Brood parasitism:

B. Dietary:
    As insectivores, may be affected by pesticide use (McClusky et al. 1977). Winter range strongly affected by local native berry crops (Gaines 1988). Range may be reduced or altered when native berry-bearing species are cleared.
C.  Sensitivity to human-induced disturbance:
    Western bluebirds often tolerate human disturbance. They readily nest on or near buildings. However, because European starlings and house sparrows, potential nest predators and nest site rivals, increase with human habitation, urbanization may lead to the reductions in the range of western bluebirds.
D.  Pesticide use:
    McClusky et al. (1977) found no effect on western bluebird reproductive success one year after aerial application of DDT on their study plot. However, their study did not investigate long-term effects of pesticides on bluebird populations.
E.  Predators:
    Western bluebirds at the Hastings Reservation in Carmel Valley, CA are commonly depredated by snakes, chipmunks, western scrub-jays, chipmunks, squirrels, and other small mammals (Eichholz and Koenig 1992).

    In a study of eastern bluebirds in Wisconsin, Radunzel et al. (1997) found that depredation of nests by house wrens, house sparrows, domestic cats, and raccoons was the main source of reproductive failure.

    House wrens, documented as a major predator of eastern and mountain bluebirds (Radunzel et al. 1997, Holt and Martin 1997), often prefer edge habitats, such as those created by forest clearing. This should be considered when putting up nest boxes intended for western bluebirds.

F.  Exotic species invasion/encroachment:
    European starlings negatively impact western bluebirds though nest site competition (Purcell 1995). Roberson (1993) reports that bluebirds remain in Monterey Co. only in areas without starlings.

    House sparrows, associated with human settlements, can displace bluebirds from their territories and nest cavities. Additionally, house sparrows will destroy bluebird nests, kill young, and even occasionally kill bluebird adults (Radunzel et al. 1997).

G.  Other: Nest boxes: Nest boxes can be effective tools in the conservation of bluebirds throughout the country. However, it is very important that nest boxes are built to exclude starlings and house sparrows (Purcell 1995, 1997). Use of Open-top nest boxes, as opposed to Standard boxes, results in higher reproductive success in an eastern bluebird population in Wisconsin (for a description of nest box types, see Radunzel et al. 1997). POPULATION TREND: Negative trend detected in Oregon (see Eltzroth et al 1979 and Prescott 1980) and in some parts of California (bioregional and county), yet no overall negative BBS decline (Sauer et al 1997). DEMOGRAPHICS:

A.  Age and sex ratios:
        No data.

B.  Productivity measure(s):
Sacramento Valley:

  • East Park Reservoir, Colusa Co.: Out of 6 nests found in 1997, 50% (3) successfully fledged young, 33% (2) were depredated, and 33% (2) were abandoned during incubation (PRBO data).

    • South Coast:

  • San Bernadino National Forest, Riverside Co.: from 1992-1996, PRBO biologists found 38 western bluebird nests in natural or excavated cavities: 73% (28) successful, 16% (6) abandoned, 5% (2) depredated, and 5% (2) unknown outcome.

    • Central Coastal:

  • Hastings Reservation, Monterey Co.: From 1986 to 1988, western bluebirds had a total nest success of about 41% (54% in 1986, 63% in 1987, and 37.2% in 1988) (Eichholz and Koenig 1992). In these years, snakes apparently depredated an average of 21% of all nests.

    • San Joaquin:

  • Madera Co.: Purcell (1997) reported a Mayfield estimate of nesting success for western bluebirds to be 59% in boxes and 29% in natural cavities at the San Joaquin Experimental Range.

    • Other sites:

  • Arizona: Martin and Li (1992) report western bluebirds succeeded in 65.5% (6 of 8 nests) of their nesting attempts.
  • In New Mexico, Goguen (1995) found total western bluebird success to be about 39% (9 of 23 nests) from 1992-1995.

    • C.  Survivorship:

  • No data.

    • D.  Dispersal:

  • No data.

    • MANAGEMENT ISSUES:

    1. Preservation of oaks and other hardwoods to maintain mosaics of habitat that include both regenerating and old growth stands. Also, see Disturbance: Logging above. Disturbance regimes, such as stand-replacement fires and post-fire logging, should attempt to mimic natural conditions (Hutto 1995).

    2. European starling and house sparrow range expansion threatens the populations of many cavity nesters, including bluebirds.

    3. Nest boxes are effective for the conservation of western bluebirds, but they need to be widely placed and properly built to exclude starlings and house sparrows.

    4. Seasonal habitat use: Western bluebirds are well known to utilize different habitats and tree species at different times of the year (Grinnell and Lindsay 1930, Block and Morrison 1990). This difference in temporal habitat use should be considered when drawing up a conservation plan for bluebirds (as well as for many other oak species).

    5. Oak woodland regeneration: long thought to be a problem…others believe oaks to be "more resilient" than previously assumed (quote CERES report).

    6. Habitat loss: reduction of available habitat due to development may reduce local populations, as has likely occurred in Santa Cruz County (Suddjian pers. comm.)

    MANAGEMENT RECOMMENDATIONS:

    Cunningham et al. (xxxx) outline the following recommendations to benefit secondary cavity nesters in ponderosa pine forests:

      The authors point out that many cavity nesters and secondary cavity nesters, prefer "young" snags.
    Snags also provide important roosting sites in the winter. It is important to have many possible cavities for the birds to choose from, as some may not be suitable--in size or insulation--for individual species (Cunningham et al. xxxx).

    Roberts (1986) makes the following management recommendations to maintain avian diversity in oak woodlands:
    Structural diversity, as used by Roberts, appears to refer to plant community diversity in age and species. He also recommends ceasing fragmentation of oak woodlands until its effects on avian diversity can be further studied.

    Some of the management recommendations made by Wilson et al. (1990):
    ASSOCIATED SPECIES: Other species that would benefit from the preservation of oak woodland and other hardwoods in California include (neotropical migrants noted with "n"):

    Cavity nesters:

    acorn woodpecker

    ash-throated flycatcher (n)

    Bewick's wren

    blue-gray gnatcatcher (n)

    downy woodpecker

    house wren

    oak titmouse

    Nutall's woodpecker

    red-shafted flicker

    violet-green swallow (n?)

    white-breasted nuthatch
     

     Cup-nesters dark-eyed junco

    Hutton's vireo

          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.
     

    ACTION PLAN SUMMARY:. Click here.

    ACKNOWLEDGEMENTS:

    I'd like to thank John Cavitt of the Montana Cooperative Wildlife Research Unit at University of Montana for gathering the BBIRD and passing it along. Also thanks to BBIRD contributors Randy Dettmers, Lindsey Garner, Chris Goguen, Melinda Knutson, Tom Martin, Donald Whitehead, and Petra Wood.

    Phil Unitt informed passed along information about bluebirds in San Diego County and Kathryn Purcell of the USFS Forestry Sciences Lab in Fresno, California also offered advice and gave me useful manuscripts.  Janis Dickinson, of the UC Hastings Natural History Reservation reviewed and commented on this document as well.

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