Western Bluebird

Maximum longevity: 6.1 years (wild) Observations: One banded male was alive after 6 years (http://bna.birds.cornell.edu/).

We do not have information on lifespan/longevity for this species at this time.

Average lifespan
Status: wild: 73 months.

Western bluebirds are often found at the edges of forests. Because of the availability of perch sites, they tend to live in burned and logged areas. Western bluebirds tend to build their nests in conifer trees (pine and firs) and deciduous trees (oak). They do not live in open areas, such as meadows. They can be found at elevations up to 2,900 m.

Range elevation: 2,900 (high) m.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: forest ; scrub forest

Sialia mexicana is found throughout parts of western North America, including southeastern British Columbia, Alberta, Washington, Oregon, Idaho, California, Nevada, Utah, Colorado, New Mexico, Arizona, northern Baja California, and the central Mexican states.

Biogeographic Regions: nearctic (Native )

Western bluebirds eat a variety of foods; their main food source varies depending on the season. During the spring and summer months, they are insectivorous. They are "perch-foragers" at this time; they will look for prey from a perch and then drop down on the ground to pick it up. During the winter, western bluebirds mainly eat fruit. Their main source of food in the winter is mistletoe berries (Phoradendron).

Foods eaten include: spiders, flies, grasshoppers, bees, sowbugs, beetles, termites, mistletoe berries, cherries, blackberries, raspberries and figs.

Animal Foods: insects; terrestrial non-insect arthropods

Plant Foods: fruit

Primary Diet: omnivore

Western bluebirds live in areas that are occupied by other birds. Conflicts arise between swallows, wrens, woodpeckers, and flycatchers for nest space and food. Swallows and wrens have taken over some western bluebird nest sites because space is limited. Western bluebirds and mountain bluebirds have overlapping ranges, but western bluebirds establish their nests earlier, so they usually win disputes over nesting sites.

Bluebirds are beautiful birds and people often put out nest boxes to attract them to their area.

There are no known adverse affects of western bluebirds on humans.

Western bluebirds are not endangered; however, their habitat is threatened. Western bluebirds do not live in open areas; they live in the forest (Guinan et al., 2000). However, increases in logging have led to increases in the amount of open area in the forest, which has led to a decrease in the number of available nesting sites for western bluebirds. In addition to the loss of nesting sites, the suppression of forest fires has led to a decrease in edge habitat (Guinan et al., 2000). To prevent further declines in western bluebird populations, nest boxes have been set up throughout California to provide breeding sites (Dickinson, 2001). Western bluebirds are protected under the US Migratory Bird Treaty Act.

US Migratory Bird Act: protected

US Federal List: no special status

CITES: no special status

IUCN Red List of Threatened Species: least concern

Western bluebirds use a variety of calls to communicate. Communication begins as early as 14 days as nestlings call for food. Calls become more complex as western bluebirds grow older. There are a variety of calls that are used to determine the location of mates as well as to establish territorial boundaries. The "Kew" call and the "Che-check" call are used by mates to determine each other’s location. These calls are particularly important during breeding season. Additional calls are used to establish territorial boundaries. When building a nest, males produce a chatter call to establish their territories. If a foreign male invades another birds’ territory, the defending male will produce a squawk-like call. Calls are the common method of communication among western bluebirds and are used to establish boundaries as well as position.

Communication Channels: acoustic

Perception Channels: visual ; tactile ; acoustic ; chemical

The genus Sialia has three species: western bluebirds (Sialia mexicana), mountain bluebirds (S. currucoides), and eastern bluebirds (S. sialis). These three species differ in their coloration and body shape. Males of the different species are easily recognizable; females are more difficult to distinguish. For example, male eastern bluebirds have a red-brown throat and a white belly instead of a blue throat and grayish-blue belly. Female eastern bluebirds have an orange-brown throat and a white belly with a pale brown outline. Mountain bluebirds are longer and have a thinner beak. The males and the females have a paler blue coloration. Body size and color patterns are important features in distinguishing between the different bluebird species.

Western bluebirds are small thrushes ranging in length from 16.5 to 19 cm and weighing from 24 to 31 g. Male and female adult western bluebirds differ in their coloration. The females are less dramatic than the males. They tend to have a brown abdomen and a gray head, throat and back. Their tails and wings are a gray-blue color. The males are brighter. Cobalt-blue is found on their head, chin, throat, and tail. The males have a brown breast and a gray-blue belly. Juveniles exhibit the same patterns as adults, except their coloration is not as intense.

Range mass: 24 to 31 g.

Range length: 16.5 to 19 cm.

Sexual Dimorphism: sexes colored or patterned differently; male more colorful

Other Physical Features: endothermic ; bilateral symmetry

Average basal metabolic rate: 0.423 W.

Chipmunks (grey-necked chipmunk (Eutamias cinereicollis), townsend chipmunk (Eutamias townsendii) and yellow-pine chipmunk (Eutamias amoenus)), squirrels (Douglas squirrel (Tamiasciurus douglasi), red squirrel (Tamiasciurus hudsonicus) and tufted-eared squirrel (Sciurus aberti)) and mice (deer mouse (Peromyscus maniculatus)) are predators of western bluebirds. They will attack chicks in a nest and then take over and live in the nest. Male western bluebirds stand guard at the nests and chase intruders away. While an intruder is running away, the male will grasp the intruder's legs causing it to fall. Then, the bird will attack the predator with his beak.

Known Predators:

• grey-necked chipmunks (Tamias cinereicollis)
• Townsend's chipmunks (Tamias townsendii)
• yellow-pine chipmunks (Tamias amoenus)
• Douglas's squirrels (Tamiasciurus douglasii)
• red squirrels (Tamiasciurus hudsonicus)
• tufted-eared squirrels (Sciurus aberti)
• deer mice (Peromyscus maniculatus)

Western bluebirds are monogamous; they are also co-operative breeders ("helpers" or birds that are not the parents help to raise young). In some instances extra-pair copulation occurs (matings outside of the monogamous relationship). Helpers reduce the chance of extrapair copulations. They tend to fend off male intruders when the male mate is away from the nest. The females often use display signals to deter extrapair copulations (Dickinson et al., 2000). Females have been seen leaving their perch sites and attacking the breast of an invading male, as well as flattening themselves against a branch. The invading males typically flap their wings rapidly and call in a high-pitched tone (Dickinson et al., 2000). Most attempts at extrapair copulation fail because the female is not receptive. However, in a small percentage of cases extrapair copulations occur. If the intruding male is older than the female, then the female is more likely to mate with him (Dickinson, 2001).

Mating System: monogamous ; cooperative breeder

Both male and female western bluebirds can begin reproducing once they are a year old. Breeding occurs from May to July. Females typically lay one to two clutches (approximately 5 eggs per clutch (range 3 to 8)) during the breeding season. Copulation occurs from 10 days prior to egg laying through the last day of egg laying (Dickinson et al., 2000).

Incubation lasts from 12 to 18 days (Guinan et al., 2000). The nestlings fledge after 21 days, but they remain close. After two weeks, the young birds are fully independent (Guinan et al., 2000).

Breeding interval: Females typically lay one to two clutches per year

Breeding season: May to July

Range eggs per season: 3 to 8.

Average eggs per season: 5.

Range time to hatching: 12 to 18 days.

Average fledging age: 21 days.

Average time to independence: 2 weeks.

Average age at sexual or reproductive maturity (female): 1 years.

Average age at sexual or reproductive maturity (male): 1 years.

Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization ; oviparous

Average eggs per season: 4.

Female western bluebirds are responsible for incubating the eggs. Incubation lasts from 12 to 18 days (Guinan et al., 2000). During incubation, males guard the nest while the female searches for food. The female does not leave the nest for too long because the males do not incubate the eggs. Once the eggs hatch, both parents are responsible for taking care of the altricial young by cleaning the nest and providing food. Females also brood the nestlings. The chicks are able to leave the nest after 21 days, but they remain close. After two weeks, the young birds are fully independent (Guinan et al., 2000).

In addition to the mother and the father caring for their young, western bluebirds often have helpers at the nest (Kraaijeveld and Dickinson, 2001). These helpers are older offspring, who are non-breeders. The helpers may have been reproductively active at one point in time, but became helpers because their mate died or their nest failed. They increase their fitness by ensuring the survival of the fledglings who are related to them. For example, if a male bird does not reproduce, he is not passing on any of his genes so his fitness is zero. However, if a male bird helps at his parents nest, then some of his genes will be passed on because he is related to the fledglings. A helper increases his fitness by taking care of his siblings (Kraaijeveld and Dickinson, 2001).

Parental Investment: no parental involvement; altricial ; pre-hatching/birth (Protecting: Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Female); pre-independence (Provisioning: Male, Female)

More info for the terms: cone, woodland

Western bluebirds exhibit a strong preference for ponderosa pine (Pinus
ponderosa) communities [43].  In Montana western bluebirds occur in the
Douglas-fir (Pseudotsuga menziesii)-ponderosa pine zone and the cedar
(Thuja spp.)-hemlock (Tsuga spp.)-spruce (Picea spp.)-grand fir (Abies
grandis) zones [26].  In Rocky Mountain, Sierran, and Madrean conifer
forests, western bluebirds occur in montane meadow grasslands, usually
over 8,580 feet (2,600 m) elevation.  They are not usually found in
subalpine communities [13].

In central and southern Utah western bluebirds are uncommon summer
residents of ponderosa pine, aspen (Populus spp.), and Gambel oak
(Quercus gambelii) woodlands [29].

In Oregon and Washington western bluebirds' primary habitat is in low to
mid-elevation conifer and conifer-hardwood forests [28].  In Washington
western bluebirds occurred on a study site in a ponderosa pine-Oregon
white oak (Quercus garryana) woodland and in big sagebrush (Artemisia
tridentata)/bluebunch wheatgrass (Pseudoroegneria spicatum)-cheatgrass
(Bromus tectorum)-rubber rabbitbrush (Chrysothamnus nauseous)
communities ranging from 2,325 to 6,320 feet (705-915 m) elevation [33].

In California western bluebirds are characteristic of oak (Quercus spp.)
woodlands from 198 to 3,465 feet (60-1,050 m) elevation [14].  In El
Paso County, Arizona, western bluebirds breed mainly in the yellow pine
region from 7,000 to 8,000 feet (2,134-2,438 m) [8].

Western bluebirds winter in pinyon (Pinus spp.)-juniper (Juniperus
spp.) woodlands in north-central Arizona.  They are numerous in some
years but not in others, depending on weather and juniper cone crops [6].


REFERENCES :
NO-ENTRY

western bluebird

The western bluebird was on the Audubon Society's Blue List in 1972 and
was listed again from 1978 to 1981 [60]. It was considered of special
concern in 1982 and of local concern in 1986. Population declines
continue in some areas [21]. Atwood [3] compiled listings of western
landbirds; the western bluebird is listed as a candidate species in
Washington, a sensitive (vulnerable) species in Oregon, and sensitive
(declining population due to limited range or habitat) in Utah.

More info for the terms: cover, density, natural, selection, snag

Nesting:  Preferred nesting cavities are usually abandoned woodpecker
(Picidae) holes in dead or dying trees [22].  Other nest cavities
include holes in dirt banks, abandoned swallow (Hirundinae) and American
dipper (Cinclus mexicanus) nests, hollow fenceposts, mailboxes, holes in
buildings, and nest boxes [32].  In Monterey County, California, western
bluebird nests were in pine or pine stumps, 5 to 40 feet (1.5-12.1 m)
above the ground, at an average of 22 feet (6.7 m) [8].  Characteristics
of snags used by secondary cavity nesters including western bluebirds in
western Oregon Douglas-fir are as follows:  average diameter 28 inches
(71.3 cm), range of diameters 10 to 54 inches (25.4-137.2 cm); average
height of snags 30.4 feet (9.2 m), range of heights 12 to 55 feet
(3.7-16.8 m); average bark cover 16 percent, range of bark cover 0 to
100 percent [49].  The average diameter of ponderosa pine snags used by
western bluebirds (23 nests) was 26.5 inches (67.6 cm), ranging from 12
to 45 inches (29.5-114.6 cm).  The average diameter of oaks used by
western bluebirds (9 nests) was 14 inches (35.6 cm), ranging from 10 to
26 inches (25.4-65.0 cm) [16].  Balda [5] recommended a snag density of
2.6 per acre (6.5/ha); however, a study by Cunningham and others [16]
indicated that snag density of 2.1 per acre (5.2/ha) is sufficient.

Nest Boxes:  In northern Arizona nest boxes appeared to be preferred
over natural cavities in ponderosa pine forests [11].  Western bluebirds
are more restricted in nest box selection than mountain bluebirds.  In
south-central Washington western bluebirds always selected nest boxes
within 0.3 mile (0.5 km) of pine-oak stands (or other cover) [32].

Foraging:  Perches near open areas with sparse ground cover are
preferred foraging sites; dead branches are preferred to living ones,
presumably for greater prey visibility.  Western bluebirds obtain less
food on the ground than the other two bluebirds and may be found in
relatively dense pine woodlands [43].

The breeding range of western bluebird extends from southern British
Columbia, western and south-central Montana south through the mountains
to northern Baja California, Michoacan, Puebla, and central Veracruz,
Mexico [1].

Ranges of subspecies are as follows [1,8]:

The chestnut-backed bluebird breeds from southern Nevada, central Utah,
and Colorado south through the mountains to west-central and
southeastern Arizona, northern Chihuahua, and western Texas.  It winters
throughout the breeding range at lower elevations, wandering to
southeastern California, eastern Sonora, and central Texas.

The western bluebird (ssp. occidentalis) breeds from southern British
Columbia, western and central Montana, south through eastern Oregon,
northern Idaho, northwestern Wyoming, and through the mountains to
southern California and western Nevada.  It winters mostly within its
breeding range at lower elevations, wandering to southern California
including Santa Catalina and San Clemente islands.

The San Pedro bluebird is resident in northern Baja California,
wandering in winter to lower altitudes and to the coastal islands.

Western bluebirds have been reported as winter residents in Vancouver,
British Columbia [57].

More info for the term: fruit

Western bluebirds are primarily insectivorous but also consume fruit
from late summer to early spring. An early study (1915) established that
82 percent of the diet was animal (insect) foods and 18 percent was
plant matter.  The largest single item was grasshoppers, followed by
caterpillars, beetles, ants, bees, and wasps [7].

Plant Foods:  Most of the plant foods eaten by western bluebirds are
fruits.  Elderberries (Sambucus spp.) and mistletoe berries are favored
foods.  Weed seeds and a few seeds of other species are eaten in small
amounts; however, western bluebirds do not usually consume grain [7,39].
Western bluebird diet from fall to early spring includes mistletoe,
juniper, coffeeberry (Rhamnus spp.), and toyon (Heteromeles arbutifolia)
berries [32].  One of Bent's [8] contributors reported western bluebirds
eating the fruit of California palm (Washingtonia filifera) in
California.

Nestlings are fed mostly grasshoppers, crickets, caterpillars, beetles,
bugs, and spiders [8,32,39].  In south-central Washington, caterpillars,
spiders, bees, and wasps became more important late in the season and
beetle use decreased [32].

Foraging Techniques:  Western bluebirds forage primarily from perches,
dropping to the ground to capture prey spotted from the perch.
Occasionally (2.7% of observations), hover foraging is employed; the
bird flies from a perch, hovers 3.3 to 33 feet (1-10 m) above the ground
and flutters down to capture prey after it has been sighted [33].
Pitelka [44] observed western bluebirds soaring in an updraft as a means
to spot prey.  Soaring is usually employed by large birds with broad
wings; small birds can efficiently soar if air conditions are right.

More info for the terms: cover, fire exclusion, forest, fruit, prescribed fire, snag

Postfire successional communities are usually attractive to western
bluebirds, especially in the first few years following the fire [48].
In Kern County, California, western bluebirds were recorded prior to a
July 1954 prescribed fire in scrub woodlands dominated by blue oak
(Quercus douglasii), gray pine (Pinus sabiniana), interior live oak (Q.
wislizenii), and wedgeleaf ceanothus (Ceanothus cuneatus).  By 3 years
after the fire, western bluebird densities had nearly doubled [37].  In
southwestern Idaho a ponderosa pine-Douglas-fir forest that burned in
1992 was censused in 1994.  Most of the trees on the plot were dead.
Cavity nesters in the area included western bluebirds (17 nests on
7,813 acres [3,163 ha]) [48].  Based on data from Brawn and Balda [12],
Hejl [30] hypothesized a western bluebird population decrease for
southwestern ponderosa pine forests since fire exclusion has led to
increased amounts of closed forest. 

Snag availability in postfire communities is a substantial positive
influence on western bluebird activity [49].  On the Olympic Peninsula
western bluebirds were detected on 30- to 50-year-old Douglas-fir
plantations in dense patches of fire-killed snags; they were not
detected on plots without snags nor in younger and older plots with
snags [59].  Hutto [34] emphasizes the importance of snags and strongly
recommends against salvage cutting of burned forests; the abundance of
newly killed trees and concomitant woodpecker activity is particularly
valuable to western bluebirds for nest cavities.  If salvage logging is
unavoidable (for sanitary or firebreak reasons) then some areas should
be left untouched rather than thinning the entire unit [34].

Loss of vegetative cover due to fire is detrimental to most small birds,
leaving them vulnerable to raptors, especially in large, severe fires
which leave few unburned refuges.  In San Dimas, California, western
bluebirds initially declined in immediate postfire chaparral
communities, probably due to lack of fruit [58].

More info on this topic.

This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):

    66  Ashe juniper-redberry (Pinchot) juniper
    68  Mesquite
   201  White spruce
   202  White spruce-paper birch
   203  Balsam poplar
   204  Black spruce
   205  Mountain hemlock
   206  Engelmann spruce-subalpine fir
   207  Red fir
   208  Whitebark pine
   209  Bristlecone pine
   210  Interior Douglas-fir
   211  White fir
   212  Western larch
   213  Grand fir
   215  Western white pine
   216  Blue spruce
   217  Aspen
   218  Lodgepole pine
   219  Limber pine
   220  Rocky Mountain juniper
   221  Red alder
   223  Sitka spruce
   224  Western hemlock
   225  Western hemlock-Sitka spruce
   226  Coastal true fir-hemlock
   227  Western redcedar-western hemlock
   228  Western redcedar
   229  Pacific Douglas-fir
   230  Douglas-fir-western hemlock
   231  Port-Orford-cedar
   232  Redwood
   233  Oregon white oak
   234  Douglas-fir-tanoak-Pacific madrone
   237  Interior ponderosa pine
   238  Western juniper
   239  Pinyon-juniper
   240  Arizona cypress
   241  Western live oak
   242  Mesquite
   243  Sierra Nevada mixed conifer
   244  Pacific ponderosa pine-Douglas-fir
   245  Pacific ponderosa pine
   246  California black oak
   247  Jeffrey pine
   248  Knobcone pine
   249  Canyon live oak
   250  Blue oak-foothills pine
   251  White spruce-aspen
   252  Paper birch
   253  Black spruce-white spruce
   254  Black spruce-paper birch
   256  California mixed subalpine

More info on this topic.

This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):

More info for the term: shrub

FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES22 Western white pine
FRES23 Fir-spruce
FRES24 Hemlock-Sitka spruce
FRES25 Larch
FRES26 Lodgepole pine
FRES27 Redwood
FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES31 Shinnery
FRES32 Texas savanna
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES37 Mountain meadows

More info on this topic.

This species is known to occur in association with the following plant community types (as classified by Küchler 1964):

More info for the terms: forest, shrub, woodland

   K001  Spruce-cedar-hemlock forest
   K002  Cedar-hemlock-Douglas-fir forest
   K003  Silver fir-Douglas-fir forest
   K004  Fir-hemlock forest
   K005  Mixed conifer forest
   K007  Red fir forest
   K008  Lodgepole pine-subalpine forest
   K009  Pine-cypress forest
   K010  Ponderosa shrub forest
   K011  Western ponderosa forest
   K012  Douglas-fir forest
   K013  Cedar-hemlock-pine forest
   K014  Grand fir-Douglas-fir forest
   K015  Western spruce-fir forest
   K016  Eastern ponderosa forest
   K018  Pine-Douglas-fir forest
   K019  Arizona pine forest
   K020  Spruce-fir-Douglas-fir forest
   K021  Southwestern spruce-fir forest
   K022  Great Basin pine forest
   K023  Juniper-pinyon woodland
   K024  Juniper steppe woodland
   K026  Oregon oakwoods
   K027  Mesquite bosque
   K028  Mosaic of K002 and K026
   K029  California mixed evergreen forest
   K031  Oak-juniper woodlands
   K032  Transition between K031 and K037
   K033  Chaparral
   K038  Great Basin sagebrush
   K055  Sagebrush steppe
   K060  Mesquite savanna
   K061  Mesquite-acacia savanna
   K062  Mesquite-live oak savanna
   K071  Shinnery
   K086  Juniper-oak savanna
   K087  Mesquite-oak savanna

More info on this topic.

This species is known to occur in association with the following Rangeland Cover Types (as classified by the Society for Range Management, SRM):

More info for the terms: association, forb, shrubland, woodland

   107  Western juniper/big sagebrush/bluebunch wheatgrass
   109  Ponderosa pine shrubland
   110  Ponderosa pine-grassland
   201  Blue oak woodland
   202  Coast live oak woodland
   207  Scrub oak mixed chaparral
   208  Ceanothus mixed chaparral
   208  Ceanothus mixed chaparral
   314  Big sagebrush-bluebunch wheatgrass
   315  Big sagebrush-Idaho fescue
   316  Big sagebrush-rough fescue
   409  Tall forb
   411  Aspen woodland
   412  Juniper-pinyon woodland
   413  Gambel oak
   503  Arizona chaparral
   504  Juniper-pinyon pine woodland
   509  Transition between oak-juniper woodland and mahogany-oak association
   718  Mesquite-grama
   729  Mesquite
   733  Juniper-oak
   734  Mesquite-oak

More info for the terms: competition, density, fire exclusion, forest, herb, natural, shrub, snag, species richness, tussock

Population Status:  Breeding Bird Survey (BBS) data indicates that there
were no nesting western bluebirds west of the Cascade Range in
Washington for the period 1968 to 1976 [31].  Nationally, BBS data
indicated a decreasing trend for western bluebirds between 1968 and
1991, but a more recent trend (1982-1991) showed a slight but
nonsignificant increase [30].  Hejl [30] hypothesized that species
associated with burns and/or snags, such as western bluebirds, are less
abundant in the United States than they were 100 years ago.  Populations
in the southwest have probably declined due to forest closure as a
result of fire exclusion.  Based on data from Raphael and others [47],
she also hypothesized a local population increase in northwestern
Douglas-fir forests because logging has resulted in increased amounts of
early successional habitats [30].  Herlugson [31] suggested that the
mountain bluebird has replaced the western bluebird as the predominant
bluebird in the Northwest during this century.  Grazing reduces herb
density and may benefit mountain bluebird at the expense of western
bluebird.  Mountain bluebird probably benefits more from logging as well
[43].  In Arizona Szaro [52] recorded the replacement of western
bluebird by mountain bluebird following clearcutting in ponderosa pine
forests.  Western bluebirds also declined in Nevada [18].

There is often a high degree of inter- and intraspecific competition
among cavity nesters for nest sites.  Competition for nest sites has
increased with the invasion of European starlings and house sparrows
introduced from Europe in this century [32].  On a burned site in
southwestern Idaho, Lewis' woodpeckers (Melanerpes lewis) frequently
usurped western bluebird nests, sometimes ejecting nestlings [48].
Competition with European starlings has probably contributed to western
bluebird population declines in Utah [29].  Competition with
violet-green swallows (Tachycineta thalassina) for nest sites has
probably contributed to western bluebird movement to higher elevations
for nesting.  It may also be responsible for the near extinction of
mainland populations west of the Cascade Range in Oregon and Washington,
and reduced numbers on nearby islands.  Other factors contributing to
western bluebird population declines in these areas include competition
for nest sites with house sparrows, changes in farm practices (larger
areas with no hedgerows, etc.) and destruction of potential nest
cavities.  There was a severe drop in population after DDT use in 1947
in the Willamette Valley, and western bluebird numbers never completely
recovered [26].

Population Density:  During the breeding season western bluebird density
in northern Arizona ponderosa pine forests at 6,930 to 7,590 feet
(2,100-2,300 m) elevation was observed in three different habitat
structures, comparing plots with nest boxes to plots with no nest boxes
(control plots).  The three treatment plots consisted of ponderosa pine
stands that had been severely thinned (open), moderately thinned
(thinned), and uncut for 60 years (dense).  There were higher densities
of breeding western bluebirds on open and thinned plots with nest boxes
than on similar habitat with no nest boxes.  The amount of increase was
about the same open and thinned plots.  Breeding densities were similar
on dense plots with and without nest boxes.  The authors concluded that
nest site availability influenced breeding density in areas with limited
nest sites.  Brawn [10] found no negative effects on fledging success
with increased breeding population density with added nest boxes.
However, other factors such as availability of foraging perches may
affect breeding density when nest sites are plentiful [12].  In western
Oregon young Douglas-fir plantations, breeding western bluebirds were
common on or near plots with snags.  On these plots snags were usually
the only component taller than the shrub layer [49].

Nest Box Programs:  In Washington west of the Cascade Range, nest boxes
placed above 600 feet (182 m) elevation were more successful (had fewer
competitors) in providing western bluebird nest sites than those at
lower elevations.  Nest boxes placed on trees were more successful for
western bluebirds than those placed on fenceposts [45,46].  Nest box
success in western Montana was improved when nest box entrance hole
sizes were increased to 1.56 inches (4 cm) presumably from 1.5 inches
(3.8 cm), the standard size used to exclude starlings from nest boxes.
The nest box program appears to have attracted many western bluebirds to
breed; local population sizes are perceived (anecdotal evidence) to have
increased [4].  Boxes for western bluebirds in southwestern ponderosa
pine forests should be placed along forest edges or in grassy glades
within open forests.  Western bluebirds use large nest boxes with 3 inch
(7.6 cm) diameter entrance holes if no other sites are available [25].
Nest boxes may attract western bluebirds to atypical habitat [33].

Management Recommendations:  Szaro and Balda [53] suggested that removal
of one-sixth to two-thirds of available foliage in ponderosa pine
communities either in strips or by thinning is not detrimental to
breeding bird communities (in terms of species richness, density, or
diversity), but caution that post-treatment bird communities are not
composed of the same species.  Western bluebirds apparently benefit from
moderate disturbance [54].  Snags are a necessary component of western
bluebird nesting habitat and should be retained wherever possible
[26,49].  Mature and decadent trees should be left during timber harvest
for future snag production, particularly where existing snags are few
[43].  Diem and Zeveloff [19] suggest that western bluebirds may be
especially sensitive to timber harvest in ponderosa pine habitats
because of the need for snags.  Bull and Partridge [15] recommend that
retention of natural snags is preferable to killing trees.  For snag
production in Oregon ponderosa pine forests they recommend topping trees
50 to 83 feet (15-25 m) above the ground.  Trees should be greater than
20 inches (50 cm) d.b.h. [15].

Pesticide Studies:  In northeastern Oregon Douglas-fir forests treated
for Douglas-fir tussock moth, western bluebird hatching rates increased
significantly in areas sprayed with DDT; the difference was not
attributed to DDT use.  Other nesting parameters showed no significant
differences between sprayed and unsprayed plots [40].

In the southwestern United States a large portion of western bluebird
winter diet consists of juniper and mistletoe berries; therefore western
bluebirds may be a major dispersal agent for those species [6].


REFERENCES :
NO-ENTRY

AZ
CA
CO
ID
MT
NV
NM
OR
TX
UT

WA
WY













BC






MEXICO

There are numerous potential predators of western bluebird including
hawks (Buteonidae) and owls (Strigidae).  Nest competitors including
European starlings (Sturnus vulgaris) and house sparrows (Passer
domesticus) destroy eggs, nestlings, and occasionally adult western
bluebirds [32].

More info for the terms: cover, density, forest, snag, woodland

Western bluebirds normally occupy open woodland or edge habitat with
exposed perches and fairly sparse ground cover [43].  There is a great
deal of similarity between western and mountain bluebird habitat
preference; however, western bluebirds are less tolerant of large
treeless areas and disturbance [6].  Szaro and Balda [54] listed western
bluebirds as preferring lightly or moderately disturbed areas in
northern Arizona ponderosa pine communities.  Highest densities of
western bluebirds were observed in plots that had been irregularly cut
in strips.  Severely thinned plots had lower western bluebird densities
than irregular strip cut plots, but lowest western bluebird densities
occurred in untreated plots.  There were fewer individuals and fewer
species present after wet winters (heavy snowfall) than after mild
winters.  The combination of lower temperatures and more precipitation
during the winter and early spring is important in determining the
survival of permanent residents including western bluebird.  In a
Jeffrey pine (Pinus jeffreyi)-white fir (Abies concolor) forest in
California, yearly variation in cavity-nester density was negatively
correlated with yearly precipitation [54,55].  Hansen and others [28]
listed western bluebird in Oregon and Washington as preferring open
canopy stages of conifer and conifer-hardwood stands, and western
bluebird response to edge as "generalist" (uses edges but not
exclusively).  They also rated western bluebird sensitivity to landscape
change as 13 (on a scale from most sensitive-20 to least sensitive-11).
In ponderosa pine-oak forests of Arizona, western bluebirds are less
sensitive to low snag densities than other cavity nesters, using oaks
more often in areas with few ponderosa pine snags [16].  Hutto and
others [35] listed five studies which reported increased numbers of
western bluebirds in partially cut forests.

Western bluebirds usually breed in areas near the lower treeline [20].
However, the breeding range extends from lowlands with bunchgrasses to
upper timberline [32].  In Zion National Park, Utah western bluebirds
are summer residents above 7,500 feet (2,286 m) elevation and are common
in open forest glades near meadows [8].

Wintering Habitat:  Western bluebirds are frequent drifters in
pinyon-juniper woodlands in winter; density depends on availability of
mistletoe (Phoradendron spp.) and juniper berries [6,8].  Bent's [8]
contributors reported that in the area around Yosemite National Park
flocks move to higher elevations in early fall, apparently following
availability of mistletoe berries.  Availability of this food is
probably more of a determining factor in western bluebird movement than
is weather (i.e., they move up in elevation in spite of colder weather,
and move down again only when food supplies are exhausted).

More info on this topic.

This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):

    1  Northern Pacific Border
    2  Cascade Mountains
    3  Southern Pacific Border
    4  Sierra Mountains
    5  Columbia Plateau
    6  Upper Basin and Range
    7  Lower Basin and Range
    8  Northern Rocky Mountains
    9  Middle Rocky Mountains
   10  Wyoming Basin
   11  Southern Rocky Mountains
   12  Colorado Plateau

The currently accepted scientific name for western bluebird is Sialia
mexicana Swainson [1,2]. Subspecies listed in the 1957 American
Ornithologists' Union Checklist (the last to include subspecies) include
[1,8]:

S. m. mexicana
S. m. bairdi Ridgway, chestnut-backed bluebird
S. m. occidentalis Townsend, western bluebird
S. m. anabelae Anthony, San Pedro bluebird

The breeding ranges of western and mountain bluebird (S. currucoides)
overlap extensively in the western United States, but the two species
have slightly different habitat preferences [1,32].

Spring Migration and Seasonal Movements:  The western bluebird is a
short-distance migrant; individuals of some populations do not migrate
at all [28].  Western bluebirds and mountain bluebirds arrive in
south-central Washington at the same time of year (February to March) in
mixed flocks with approximately equal sex ratios.  Western bluebirds
select and defend nest boxes within a week of arrival [32].  In the
Huachuca Mountains of Arizona, western bluebirds are numerous in the
lower foothills in early March and absent from the higher parts of the
mountain range.  As the weather warms they move up in altitude to the
pine regions above 8,000 feet (2,438 m) for the breeding season but move
down again beginning about the middle of August [8].

Territoriality:  Western bluebirds and eastern bluebirds are close in
their habitat requirements and exhibit interspecific territoriality in
areas of sympatry [38], as do western bluebirds and mountain bluebirds
in the areas where they exist together [43].

Clutch Size and Incubation:  Typical western bluebird clutches consist
of four or five eggs (range 3-8) [21].  The female incubates, probably
for about 14 days [8].

Development of Young:  Both parents feed the nestlings but usually only
the female broods.  Fledging occurs in about 23 days, and fledglings
continue to be fed by the parents for up to 30 days [21].  Almost
always, a second clutch is started as soon as the first brood is
fledged.  The parents continue to feed the first brood almost until the
second brood hatches.  After the second brood fledges, both families
form a single flock and remain in the area until migration [8].

Fall Migration:  In areas of sympatry, mixed flocks of western bluebirds
and mountain bluebirds have been observed during migration and in winter
[32].  Resident flocks often move up or down in elevation with the
season [8].

Winter Flocks:  Western bluebirds form small flocks in winter (average 7
birds).  In the southwestern United States western bluebird flocks are
nomadic in winter, traversing large areas in search of berries and
water.  There is some evidence that they follow regular foraging routes [6].

More info for the terms: fire regime, forest, natural, prescribed fire, shrubs, understory fire

Savannas and open stands of mature pine are natural western bluebird
habitat that require recurrent fire for maintenance.  Prescribed fire is
usually beneficial to western bluebird especially if it reduces shrubs
and understory trees [43].


The Research Project Summary Effects of understory fire on cavity-nesting
birds in Arizona pine forests provides information on prescribed fire effects
in Arizona pine (Pinus ponderosa var. arizonica) forest where western
bluebird occurred on both burned and unburned plots.

FIRE REGIMES :
Find fire regime information for the plant communities in which this
species may occur by entering the species name in the FEIS home page under
"Find FIRE REGIMES".

The western bluebird (Sialia mexicana) is a small North American thrush.