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).
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)
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).
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.
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).
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).
Common breeder in San Diego mountains (Small 1994).
Riverside Co.: Weathers (1983) reports bluebirds breeding in the Santa Rosa Mountains.
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:
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):
Sacramento Co: Detected as a breeder in the grasslands of the Valensin property of the Cosumnes River Preserve in 1997 (PRBO data).
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):
d. Nest searching:
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).
Colusa Co.: Breeding near the East Park Reservoir (PRBO data).
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).
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:
Marin Co.: Regular breeders (Shuford 1993).
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.
Lake Tahoe Region: Orr and Moffitt (1971) reports breeders on slopes, often away from human settlement.
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.
I. Average territory size:
C. Spring migration period: NA
D. Fall migration period:
Western bluebirds at the San Joaquin Experimental Range in Madera Co. (San Joaquin Bioregion) are year-round residents (Purcell pers. comm.)
Common in winter (Small 1994).
Gaines (1988) reports 0-187 western bluebirds detected during Christmas Bird Counts in Yosemite.
Very rare visitors to Imperial Valley (Small 1994).
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.
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).
IX. Incubating sex:
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:
XIII. Number of broods:
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).
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).
Marin Co.: Bluebirds make use of extensive grasslands bordered by oaks and oak savanna (Shuford 1993).
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.
Grinnell et al. (1930) observed bluebirds in summer inhabiting areas with little tree cover, often perched on snags.
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).
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.
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).
Camp Roberts, San Luis Obispo Co.: Western bluebirds mainly nest in blue oaks (pers. obs.).
Mendocino County: Wilson et al. (1986) found a western bluebird nesting in a Pacific madrone (Arbutus menziessii).
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.).
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).
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).
San Bernadino National Forest, Riverside Co.: nest height averaged 1.36m, (range 0-6.4m, sd=1.6m, n=43) (PRBO data).
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).
San Joaquin Valley:
SJER, Madera Co.: mean=9.48 m (range=1.9-15 m, n=34) (SJER unpubl. data).
San Bernadino National Forest, Riverside Co.: mean = 3.9m, range=0-30m, sd=6.2m (n=41) (PRBO data).
In New Mexico, the height of the nest substrate averaged 10.1m, range 1-14m, n=17 (Goguen 1995).
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).
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).
Yosemite: Gaines (1988) reports bluebirds in dry grassy areas with few trees, in forest meadows, and along rocky ridges.
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).
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.
South SF Bay: Blue oaks (Sibley 1951).
Cosumnes River Preserve, Sacramento Co.: Isolated or clumped valley oaks in savanna (PRBO data).
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).
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.
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).
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).
San Bernadino National Forest, Riverside Co.: mean=13%, sd=23.4%, range=0-78%, median=0 (n=31) (PRBO data).
Manitoba, Canada, Munro and Rounds report that eastern bluebirds prefer sites with a higher percentage of shrub cover than those with less shrub cover.
Riverside Co.: Mock (1991) describes the shrubby layer of understory as consisting of sagebrush.
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).
San Joaquin Valley:
SJER, Madera Co.: slope averaged 12.5% (n=28) (SJER unpubl. data).
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).
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).
Number of trees within 11.3 m of nest by DBH:
In Arizona, the bluebirds preferred oaks with mean DBH of 35.6 cm (range=25.4-65.0 cm) (Cunningham et al. xxxx).
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.
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).
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.
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.
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 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).
Logging can have both positive and negative effects on abundance. See Section IV. E: Disturbance above.
SPECIAL FACTORS: Factors influencing a species occurrence and viability.
A. Brood parasitism:
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.
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).
A. Age and sex ratios:
B. Productivity measure(s):
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.)
Cunningham et al. (xxxx) outline the following recommendations to benefit secondary cavity nesters in ponderosa pine forests:
Roberts (1986) makes the following management recommendations to maintain avian diversity in oak woodlands:
Some of the management recommendations made by Wilson et al. (1990):
ash-throated flycatcher (n)
blue-gray gnatcatcher (n)
violet-green swallow (n?)
ACTION PLAN SUMMARY:. Click here.
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.
Aigner, P. A., W.M. Block, and M.L. Morrison. 1998. Effect o firewood harvesting on birds in a California oak-pine woodland. J. Wildl. Manage. 62(2):485-497.
Aylesworth, A. 1987. Mountain x Western bluebird hybrids. Sialia 9:9-21.
Balda R. P. 1975. The relationship of secondary cavity nesters' snag densities in western coniferous forests. USDA Forest Service Wildlife Habitat Technical Bulletin 1, 37p. Southwestern Region, Albuquerque, NM.
Beal, F.E.L. 1915. Food of the robins and bluebirds of the United States. US Dep. Agric. Bull. 171.
Bent, A.C. 1949. Life histories of North American thrushes, kinglets, and their allies. U.S. Natl. Museum Bull. 196.
Block, W.M. and M.L. Morrison. 1990. Influence of scale on the management of wildlife in California oak woodlands. In Proc., Symposium on oak woodlands and hardwood rangeland management (Davis, CA Oct.31-Nov 2 1990). USDA For. Serv. Gen. Tech. Rep., PSW-126, p. 96-104.
Brawn, J.D. 1990. Environmental effects on variation and covariation in reproductive traits of western bluebirds. Oecologia 86:193-210.
Brawn, J.D. and R.P. Balda. 1988. Population biology of cavity nesters in northern Arizona: Do nest sites limit breeding densities?" Condor 90: 61-71.
Cunningham, J.B., R.P Balda, and W.S. Gaud. XXXX. Selecting and use of snags by secondary cavity-nesting birds of the ponderosa pine forest. Jour. For.
Dahlgren, R.A., M.J. Singer, and X. Huang. 1997. Oak tree and grazing impacts on soil properties and nutrients in a California oak woodland. Biogeochemistry 39: 45-64.
Dickinson, J. L. and M. L Leonard. 1996. Male attendance and copulatory behaviour in western bluebirds: evidence of mate guarding. Anim. Behav. 52(5):981-992.
Dickinson, J.L., W.D. Koenig and F.A. Pitelka. 1996. Fitness of helping in western bluebirds. Behav. Ecol. 7(2):168-177.
Dickinson, Janis. 1997. Extra-pair copulation in Western Bluebirds. Anim. Behav. 53(3):561-571.
DiGaudio, R. 1998. Conversations regarding the breeding birds of the Cosumnes River Preserve, Sacramento County.
East, M.L. and C.M. Perrins. 1988. The effect of nestboxes on breeding populations in broadleaved temperate forests. Ibis 130:393-401.
Ehrlich, P.R., Dobkin, D.S, and D. Wheye. 1988. The birder's handbook: a field guide to the natural history of North American birds. Simon and Schuster Press, NY.
Eichholz, M. W. and W. D. Koenig. 1992. Gopher snake attraction to birds' nests. The Southwestern Naturalist 37(3):293-298.
Elgroth, E. K. Violet-green Swallows help Western Bluebirds at nest. Journal of Field Ornithol. 55(2):259-264
Gaines, D. A. 1977. The valley riparian forests of California: Their importance to bird populations. In Ann Sands (editor) Riparian Forests in California: Their ecology and conservation. Institute of Ecology Publication 15, Univ. of California, Davis, CA. 57-85.
Gaines, D. A. 1988. Birds of Yosemite and the eastern slope. Artemesia Press, Lee Vining, CA.
Gander, F. F. 1960. Western bluebirds in my garden. Audubon 62: 70-71, 83.
Gardali, T, G.R. Geupel, and G. Ballard. 1996. Songbird census in Brewer's oak forest in the Mendocino National Forest: Results from the 1996 field season. Unpublished report of the Point Reyes Bird Observatory to the Mendocino National Forest Service.
Geupel, G., N. Nur, G. Ballard, and A. Kiener. 1996. Monitoring nests of songbirds and their associated vegetation in montane meadows of the San Bernardino National forest, results of the 1992-1995 field seasons. Unpublished report to the USFS. Point Reyes Bird Observatory, Stinson Beach, CA.
Grinnell, J. and T.A. Storer. 1924. Animal life in the Yosemite. University of California Press.
Grinnell, J. J. Dixon, and J.M. Lindsdale. 1930. Vertebrates and natural history of a section of northern California through the Lassen Park region. University of California Press.
Grinnell, J. and J.M. Lindsdale. 1936. Vertebrate animals of Point Lobos Preserve 1934-1935. Carnegie Institute of Washington, WA.
Grinnell, J. and A.H. Miller. 1944. The distribution of the birds of California. Artemesia Press, Lee Vining, CA.
Goguen, C. Department of Wildlife Ecology, University of Wisconsin. 1995. Data submitted to the BBIRD program.
Gowaty, P.A. and W.C. Bridges. 1991. Nestbox availability affects extra-pair fertilizations and conspecific nest parasitism in Eastern Bluebirds. Anim. Behav. 41:661-675.
Harris, S.W. 1991. Northwestern California birds. Humboldt University Press, Arcata, CA.
Herlugson, C.J. 1978. Comments on the status and distribution of Western and Mountain bluebirds in Washington. West. Birds 9:21-31.
Herlugson, C.J. 1981. Nest site selection in Mountain Bluebirds. Condor 83: 252-255.
Herlugson, C.J. 1982. Food of adult and nestling Western and Mountain bluebirds. Murrelet 63:59-65.
Holt, R.F. and K. Martin. 1997. Landscape modification and patch selection: the demography of two secondary cavity nesters colonizing clearcuts. Auk 114(3):443-455.
Hutto, R.L. 1995. Composition of bird communities following stand-replacement fires in northern Rocky Mountain (USA) conifer forests. Conservation Biology 9(5):1041-1058.
Integrated Range and Hardwood Management Program. 1998. http://danr.ucop.edu/ihrmp/prog98.html #134372:
Landres, P. B. and J. A. MacMahon. 1980. Guilds and community organization: analysis of an oak woodland avifauna in Sonora, Mexico. Auk 97:351-365.
Lehman, P.E. 1994. The birds of Santa Barbara County, California. Vertebrate Museum of University of California, Santa Barbara.
Martin, T. E. and P. Li. 1992. Life history traits of open-vs. cavity-nesting birds. Ecology 73(2):579-592.
McClelland, B.R., S.S. Friswell, W.C. Fischer, and C.H. Halvorson. 1979. Habitat management for hole-nesting birds in forests of western larch and Douglas-fir. J. Forestry 77:480-483.
McClusky,D.C., J.W. Thomas, and B.C. Meslow. 1977. Effects of aerial application of DDT on reproduction in House Wrens, Mountain Bluebirds and Western Bluebirds. Res. Pap. PNW-228, U.S. For. Serv., Portland, OR.
Miller, A.H. and R.C. Stebbins. 1964. The lives of desert animals in Joshua Tree National Monument. University of California Press, Berkeley and Los Angeles.
Mock, Patrick J., M. Khubesrian, and D. M. Larcheveque. 1991. Energetics of growth and maturation in sympatric passerines that fledge at different ages. Auk 108: 34-41.
Munro, H. L. and R. C. Rounds. 1985. Selection of artificial nest sites by five sympatric passerines. J. Wild. Manage. 49(1)
Ohmann, J. L., W.C. McComb, and A.A. Zumrawi. 1994. Wildl. Soc. Bull. 22:607-620.
Orr, R.T. and J. Moffit. 1971. The birds of Lake Tahoe. California Academy of Science SF, CA
Pinkowski, B.C. 1979. Foraging ecology and habitat utilization in the genus Sialia. Pp. 165-190 in The role of insectivorous birds in forest ecosystems (J. G. Dickinson, R.N. Conner, R.R. Fleet, J.C. Kroll, and J.A. Jackson, eds.) Academic Press, New York.
Pogue, D.W. and G.D. Schnell. 1994. Habitat characterization of secondary cavity nesters in Oklahoma. Wilson Bull. 106(2): 203-226.
Power, H. W. and M. P. Lombardo. 1996. Mountain Bluebird (Sialia currucoides). In The Birds of North America, No. 222 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D.C.
Power. 1966. Biology of the Mountain Bluebird in Montana. Condor 68:351-371.
Purcell, K. 1995. Reproductive strategies of open- and cavity-nesting birds. Dissertation for University of Nevada, Reno.
Purcell, K. L. J. Verner and L. W. Oring. 1997. A comparison of the breeding ecology of birds nesting in boxes and tree cavities. Auk 114(4):646-656.
Purcell, K.L. 1999. Discussions via phone and in person.
Radunzel, Lorissa A., Donna M. Muschitz, Vincent M. Bauldry, Peter Arcese. 1997. A long-term study of the breeding success of eastern bluebirds by year and cavity type. Journal of Field Ornithology. 68(1) 7-18.
Raphael, M.G. 1986. Use of Pacific Madrone by cavity-nesting birds. In Proc., Symposium on multi-use management of California's hardwood resources [San Luis Obispo, CA, Nov. 12-14, 1986] USDA For. Serv. Gen. Tech. Rep., PSW-100, p. 12-14.
Ridgeway, R. 1907. the birds of North and Middle America: a descriptive catalogue of the higher groups, genera, species, and subspecies known to occur in North America. Vol. 4. U.S. Natl. Mus. Bull. 50.
Roberson, D. 1985. Monterey Birds. Monterey Peninsula Audubon Society.
Roberson, D. and C. Tenney (eds.) 1993. Atlas of breeding birds of Monterey County. Monterey Peninsula Audubon Society.
Roberts, R.C. 1986. Preserving oak woodland bird species richness: suggested guidelines from geographical ecology. . In Proc., Symposium on multi-use management of California's hardwood resources [San Luis Obispo, CA, Nov. 12-14, 1986] USDA For. Serv. Gen. Tech. Rep., PSW-100, p. 190-196.
San Joaquin Experimental Range. Unpublished data. Provided by K. Purcell, March 1999.
Sauer, J.R, J.E. Hines, G. Gough, I. Thomas, and B.G. Peterjohn. 1997. The North American Breeding Bird Survey results and analysis. Version 96.4 Patuxtent Wildlife Research Center, Laurel, MD.
Scott, V. E. 1978. Characteristics of ponderosa pine snags used by cavity-nesting birds in Arizona. Jour. For. 76: 26-29.
Sibley, C. S. 1952. The birds of the south San Francisco Bay region. Unbound copy available at the Point Reyes Bird Observatory library.
Small, Arnold. 1994. California birds: their status and distribution. Ibis Publishing Co. Vista, CA. 342 pp.
Stebbins, C.A. and R.C. Stebbins. 1953. Birds of Lassen Volcanic Park and Vicinity. Loomis Museum Association, Mineral, CA.
Stebbins, C.A. and R.C. Stebbins. 1954. Birds of Yosemite National Park. Yosemite Naturalist Division, Yosemite Natural History Association.
Strong, T. R. and C. E. Bock. 1990. Bird species distribution patterns in riparian habitats in southeastern Arizona. Condor 92:866-885.
Suddjian, D.L. 1990. Atlasser's assistant: a preliminary analysis of the status and distribution of breeding birds in Santa Cruz County, 2nd ed. D.L. Suddjian, Santa Cruz, CA.
Suddjian, D. L. 1999. Phone conversation regarding breeding birds in Santa Cruz County.
Tietje, W. D. and J. K. Vreeland. 1997. Vertebrates diverse and abundant in well-structured oak woodland. California Agriculture 51(6):8-14.
Unitt, P. 1998. Phone and email conversations regarding the San Diego County Breeding Bird Atlas effort.
Verner, J. and A.S. Boss. 1980. California Wildlife and their habitats: Western Sierra Nevada. Gen. Tech. Rep. PSW-37. 439 pp. Pacific Southwest Forest and Range Experimental Station, Forest Serv., USDA, Berkeley, CA.
Verner, J. and L. V. Ritter. 1985. A comparison of transects and point counts in oak-pine woodlands of California. Condor 87:47-68.
Verner, J. and L. V. Ritter. 1988. A comparison of transects and spot mapping in oak-pine woodlands of California. Condor
Waters, J.R., B. R. Noon, and J. Verner. 1990. Lack of nest site limitation in a cavity-nesting bird community. Journal of Wildlife Management 54:239-245.
Weathers, W.W. 1983. Birds of southern California's Deep Canyon. University of California Press, Berkeley and Los Angeles, CA.
Wilson, R.A., P. Manley and B.R. Noon. 1990. Covariance patterns among birds and vegetation in a California oak woodland. In Proc., Symposium on oak woodlands and hardwood rangeland management (Davis, CA Oct.31-Nov 2 1990). ] USDA For. Serv. Gen. Tech. Rep., PSW-126, p. 126-135.
Williams, P. L. and W. D. Koenig. 1980. Water dependence of birds in a temperate oak woodland. Auk 97:339-350.
Zeiner, D.C., W. Laudenslayer jr, K. Mayer and M. White. eds. 1990. California’s wildlife, Vol. 2, Calif. Dep. Fish and Game, Sacramento. 732pp.