BIOLOGICAL DATA AND HABITAT REQUIREMENTS
WILDLIFE SPECIES: Sialia mexicana | Western Bluebird 



TIMING OF MAJOR LIFE HISTORY EVENTS : 
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].


PREFERRED HABITAT : 
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).


COVER REQUIREMENTS : 
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].


FOOD HABITS : 
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.


PREDATORS : 
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].


MANAGEMENT CONSIDERATIONS : 
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

Related categories for Wildlife Species: Sialia mexicana | Western Bluebird