Western Bluebirds nest in cavities, unlike many other members of the thrush family. While many secondary cavity nesting birds have suffered from loss of natural cavities, bluebirds have been aided in some parts of their ranges by the installation of nest boxes. Cavity nests have some advantages over open-cup nests, nestlings are sheltered from rain, snow and extreme temperatures, brood parasitism is infrequent and nestlings can develop further in the cavity, with less risk of weather and perhaps predation-related mortality..
Although primary foods of these bluebirds are fruits and berries, gathered by perch foraging, they also eat insects and feed them to nestlings. Insects are captured primarily primarily in mid-air and by gleaning.(Guinan et al. 2000).
In Texas, distinguishing this species from Eastern Bluebirds (Sialia sialis) can be a problem in the narrow area separating their usual ranges.. While the brightly-plumaged males are easy to sort out, the plainer females can be difficult. Consult Pyle (1997) and Guinan et al. (2000) for plumage details. and distinguishing characteristics.
DISTRIBUTION. During the 1987-1992 field work of the TBBA project, volunteers found 3 confirmed and a probable breeding record for Western Bluebird in latilong 31104 (the Guadalupe Mountains) and 2 confirmed, 2 probable and a possible records in 30104 (the Davis Mountains), the locations where Lockwood and Freeman (2004) report this species is a resident. A possible record in 33097 in the Rolling Plains region is an enigma. This location is within the area where TBBA workers found Eastern Bluebirds breeding. The record is also within the area where Western Bluebird is an irruptive winter resident (Guinan et al. 2000) and thus could be a late-departing migrant.
Outside Texas Western Bluebirds breed in two ranges. One is in British Columbia , Washington, Idaho, Oregon, California, western Nevada and northern Baja California. Individuals breeding in the first 4 states and province and inland northern California migrate south and to lower elevations while other breeders are year-round residents. Another breeding area is in Wyoming, Utah, Colorado, Arizona, New Mexico and the highlands of north and central mainland Mexico. Only the most northerly of individuals in this range migrate south (Howell and Webb 1995, Guinan et al. 2000).
SEASONAL OCCURRENCE. Western Bluebirds are resident in the Davis and Guadalupe mountains and irruptive winter residents in the Trans-Pecos region from late October to mid-March. The species is much rarer further east (Lockwood and Freeman 2004). In Arizona almost all breeding evidence was obtained during May, June and July (Spence 2005) and in Colorado breeding reports other than fledged young were obtained from May 11 to July 23 (Dexter 1998).
BREEDING HABITAT. Oberholser (1974) describes the breeding habitat of Western Bluebird in Texas as forested canyons and slopes of Trans-Pecos mountains. In Arizona half of these bluebirds breed in pinyon-juniper habitats (pure, mixed with ponderosa pine or grassland) and 36% in ponderosa pine habitats (pure, mixed with Gambel’s oak or
mixed with Douglas fir; Spence 2005). In Colorado about 60% of breeding reports came from coniferous woodland, mainly ponderosa pine and pinyon- juniper, about 20% came from shrub habitats and almost all the rest cane from deciduous woods, riparian and rural habitats (Dexter 1998). This bluebirds often use cavities in rotted wood, old woodpecker holes or a crevice between the bark and trunk on a dead tree, and readily accept nest boxes (Fiehler et al. 2006). Bluebirds compete with Violet-green Swallows (Tachycineta thalasina) for nest cavities. The female fills the cavity with grasses, forb stems, moss, needles and other available materials, forms a cup and lines it with fine grasses. The nest is difficult to distinguish from that of Eastern Bluebird (Harrison 1979, Guinan et al. 2000).
The average clutch is 5 (range 2-6) smooth, glossy, pale blue to white, unmarked eggs (indistinguishable from the eggs of Eastern Bluebird). The female incubates them for12-18 (average 13-14) days. Young bluebirds leave the nest between 16 and 26 days after hatching. Adult male helpers at the nest are common and adult females, adult pairs, juveniles and even Tree Swallows (T. bicolor) have been observed bringing food to nestlings. Two broods per season are common (Harrison 1979, Guinan et al. 2000).
STATUS. Western Bluebirds are uncommon and local in the Davis and Guadalupe mountains (Lockwood and Freeman 2004). Earlier they were considered fairly common in the Guadalupes (Oberholser 1974). The North American Breeding Bird Survey found this bluebird on 2 routes in the Trans-Pecos region at a relative abundance of 1 bluebird per route per year. This amount of data is inadequate to provide a biologically meaningful trend estimate. Data from 286 BBS routes on which this species was detected in the United States and Canada produced a 95% confidence interval (There is a 95% chance that the actual population trend will be between these two numbers.) of -1.5 to +1.3% population change per year for the period 1966-2005. Data from 1980-2005 gave a statistically significant trend of -4.0% per year (Sauer et al. 2005). This latter trend is worrisome.
Text by Robert C. Tweit (2007)
Dexter, C. 1998. Western Bluebird (Sialia mexicana). In Colorado breeding bird atlas, pp. 384-385 (H. E. Kingery, ed.). Colorado Bird Atlas Partnership, Denver.
Fiehler, C. M., W. D. Tietje and W. R. Fields. 2006. Nesting success of Western Bluebirds (Sialia mexicana) using nest boxes in vineyards and oak-savannah habitats of California. Wilson J. Ornithol. 118: 532-537.
Guinan, J. A., P. A. Gowaty and E. K. Eltzroth. 2000. Western Bluebird (Sialia mexicana). In The birds of North America, No. 510 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.
Harrison, H. H. 1979. A field guide to western birds’ nests. Houghton Mifflin, Boston, MA.
Howell, S. N. G. and S. Webb.1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.
Lockwood, M. W. and B. Freeman. 2004. The TOS handbook of Texas birds. Texas A&M University Press, College Station.
Oberholser, H. C. 1974. The bird life of Texas University of Texas Press, Austin.
Pyle, P. 1997. Identification guide to North American birds, part 1. Slate Creek Press, Bolinas, CA.
Sauer, J. R., J. E. Hines, and J. Fallon. 2005. The North American Breeding Bird Survey, results and analysis 1966-2005. Version 6.2 2006. USGS Patuxent Wildlife Research Center, Laurel MD < http://www.mbr-pwrc.usgs.gov/bbs>
Spence, J. R. 2005. Western Bluebird (Sialia mexicana). In Arizona breeding bird atlas, pp. 430-431 (T. E. Corman and C. Wise-Gervais, eds.). University of New Mexico Press, Albuquerque.