Bluebird and Small Cavity Nester Conservation
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All About WesternBluebirds (Sialia mexicana)

Contents: Species, Interesting Facts, Identification, Song, Distribution, Preferred Nesting Habitat, Diet, Nesting Behavior, Nestboxes, Nestbox Location, Recommended Distance Between Nestboxes, Monitoring, Nesting Timetable, Longevity, More Info. Also see photos of nests, eggs and young

WEBL female.
WEBL female. Wikimedia Commons photo, taken in Cayucos CA by Mike Baird.
WEBL Male
WEBL male. Wikimedia Commons photo, taken by Paul McGuire of LA, CA.
WEBL male
WEBL male. Wikimedia Commons photo, by Peter Wallack

Species: There are three bluebird species: Eastern (EABL) (Sialia sialis), Mountain (MOBL) (S. currucoides) and Western (S. mexicana). The alpha code for the Western Bluebird is WEBL. There are also 6 tentatively recognized subspecies of the Western Bluebird - Sialis mexicana occidentalis, bairdii, jacoti, amabilis, nelsoni, and mexicana (see more info), which vary in location, size and coloration. Hybridization is considered extremely rare although overlap exists with Mountain Bluebirds in part of their range.

Interesting Facts:

  • Of the three bluebird species, WEBL's probably need the most help, as populations are not doing well (see data.)
  • WEBLs may sing in full darkness (called a dawn chorus) during the beginning of breeding season. Predawn singing stops shortly after hatching of the second brood. (Weydemeyer)
  • WEBLs are the least migratory of the three bluebird species (Dunn 1981). Migration is short to medium distance, and is mainly a change in altitude. They may be year-round residents in some areas (e.g., Zion in Utah, Grand Canyon in AZ) where residents mix with migrating flocks. (BNA)
  • Unlike EABLs, WEBLs do not favor large open meadows, but they do prefer an open overstory in wooded areas.
  • When their nest is disturbed, they may engagement in bill-wiping (vigorously drawing the sides of their bill across a perch) (BNA)
  • Where MOBL and WEBL ranges overlap, WEBLs generally establish their territories earlier. They fight most during nest-building. (BNA)
  • One study of nests noted that parents did not discriminate between feeding male or female nestlings, and basically just fed the one closest to the entrance or that begged first or highest. Parents will feed fostered young equally. (Leonard)
  • Bluebirds are still hunted on the Zuni reservation in New Mexico (considered a sovereign nation) for cultural or religious purposes. (Taylor and Albert 1999).

Identification: Males have brilliant, bright, deep cobalt-blue plumage on heads (including chin and throat), wings and tails, chestnut/rust colored breast (may be bisected by blue) and frequently smudges of chestnut back patches (scapulars). The rest of their underparts are grayish, becoming blue-gray on the belly and undertail coverts. Females are considerably duller with more brown and gray on their feathers. They do not have a lot of blue on them, except on the tail and wings. The head and throat are gray, back gray-brown, abdomen and undertail-coverts grayish. It can be hard to distinguish between female EABLs and WEBLs, but WEBL has a grayish throat (compared to pale cinnamon or rusty orange of EABL), and dull rufous below with graying belly (compared to EABL pale cinnamon underparts with whitish belly); a female MOBL is larger, and paler overall and never has chestnut or rufous lower breast and flanks. Length 16.5-19 cm weighing 24-31 grams. First-year birds look like adults but are slightly duller. Juveniles are gray-brown above, streaked with white, breast light rust, heavily streaked in white, blue in wings and tails with white eye ring. Juvenile females are duller blue in the wings and tail and more heavily streaked in white overall. They hop (vs. walk) on the ground.

Song: Simple combination of repeated calls - e.g., pa-wee few few, few, few fa-wee, which may be sung at dawn (called a dawn chorus), in daylight during pair formation and nest site selection. Females do not sing. Other descriptions of songs include a loud, vigorous f-few, few, f-few f-few, eh-eh, few, f-few, eh-eh, few, eh-eh, few few changing to ic-ic-te, tew, ic-ic-tew, ic-ic towee as season progresses or musical two-lee.

Distribution: WEBLs breed throughout much of the western U.S., Mexico (down to Baja California and Central Volcanic Belt of Mexico) and southwestern Canada (S British Columbia and SW Alberta). See BBS Map. U.S. States include (in some cases only parts of the state): NW Montana, N and W Idaho, Washington (mainly NE and SE), Oregon, California, W Nevada, S Wyoming, Colorado, E and S Utah, NW and SE Arizona, New Mexico, W Texas. WEBLs are medium to short-distance partial migrants. They may winter in lower elevations outside of their breeding range. Occasional sightings in Oklahoma, N Dakota and Kansas.

Preferred Nesting Habitat: Open canopy woodlands (coniferous and deciduous) and edge with scattered trees, moderately disturbed areas including moderately logged forests and burned areas with available foraging perches. Wooded riparian areas, grasslands, farmlands but does not prefer large open meadows like EABL, or deserts. Rural and suburban areas.

Diet: 68% of a bluebirds' diet is made up of insects: grasshoppers, crickets, beetles, spiders, and caterpillars (usually spied from a perch and then caught on the ground.) (Beal 1915). The rest is mostly small fruit - e.g., mistletoe (Phoradendron spp.) and juniper (Juniperus spp.) - see list. (Although they will eat the fruit of multiflora rose and Japanese honeysuckle, these are invasive species, and should be eradicated.)  Bluebirds love mealworms. They may eat suet (see link for recipes). Bluebirds rarely eat birdseed (they will occasionally take shelled sunflower and peanut chips). Occasionally they may eat shrews, snakes, salamanders, tree frogs and lizards. (BNA).

Nesting Behavior: Usually "socially monogamous," although if a mate is lost they will readily pair up with another available mate. Cooperative breeding (e.g., one male with two females and even 2 females in same cavity, or one female nesting with more than one male) is rarely reported. Sometimes fledglings from the previous brood will help tend to the subsequent brood (called "helping") (7.4% of pairs in one study by Dickinson 1996, 10.5% in Oregon, EKE and JAG). Adult males or adult pairs may also help. Females will "beg" from males (possibly to test their foraging skills). Males and females may fight with each other during nesting season.

Nestboxes: Build or purchase a nestbox designed specifically for bluebirds. WEBLs may need a larger entrance hole than EABLs. WEBL and MOBL ranges overlap in some areas, and MOBL's need a 1 9/16" hole. Linda Violett says "There are, of course, size differences amongst individual bluebirds regardless of species. Hole sizes for Eastern Bluebirds are usually recommended at 1.5" diameters. Hole sizes for Western and Mountain Bluebirds are usually recommended at 1&9/16". On my own trail, some of the Western population cannot or will not enter 1.5" holes. Monitors who consistently use smaller (1.5") hole sizes will effectively exclude the largest bluebirds from their nestboxes. Monitors who offer slightly larger holes will be able to accommodate the larger bluebird segment of the local population, regardless of bluebird species. My recommendation would be for bluebird monitors to offer the largest hole size possible that will still exclude Starlings." (Note: If you do use a box designed for EABLs, sand/round the edges of the hole to get closer to 1 9/16".)

Bluebird boxes are made of PVC or unpainted, untreated 3/4" - 1" wood, have an overhanging slanted roof (2-5", with a shallow saw kerf (groove) to keep rain from soaking into box), no perch, ventilation, drainage holes, deep enough so predators can't reach in and get to the eggs, and a door that opens for cleaning and monitoring (if rough wood is not used, add kerfs to inside of door to enable fledglings to climb out). MAY prefer boxes mounted on wooden posts vs. steel (Van Horn and Bacon 1989) - if mounted on wooden post, use a predator guard. Linda Violett has had success with larger, deeper boxes hung in trees in California.

Birds may roost in the boxes in cold weather, and the ground may be frozen in February/March when they start house hunting, so put boxes up in late fall or winter. See plans.

Nestbox Location: See breeding territory. Try placing nestboxes in areas covered by an open tree canopy (e.g., <20%) where there is a clearing within 3-20 meters. Experiments in Arizona showed that box selection varies by macrohabitat - in thinned habitat, boxes with large entrances on shorter trees with few live branches near the box were preferred. In open habitats, they preferred boxes with southern exposure on big diameter trees. In New Mexico they seemed to prefer cavities in ponderosa pine, gambrel oak and narrowleaf cottonwood. In a study in Arizona, they picked aspens over conifers or other deciduous trees; in California live oaks. They will nest in cavities in snags (standing dead trees - especially those with secondary branches, sapwood intact but rotting, with some bark on them), so placing nestboxes on snags may be more attractive. In Oregon, boxes in shade at midday were preferred over more exposed locations. For cavity orientation, go with East or South, but away from prevailing winds.

Recommended distance between nestboxes: A minimum of 100 yards apart. 200-300 may be better, per Linda Violett. Bluebirds may nest closer to each other if foraging habitat is good, cavities are plentiful and/or they cannot see the other pair from their nest site because something (like a building) blocks their view. If nesting bluebirds are harassed by Tree Swallows, or more than 50% of bluebird trail boxes are occupied by swallows, consider setting up a second, "paired" box 5-20 ft. from the first. The size of the territory depends on ground cover (e.g., smaller in lumbered areas than in pastures) and number of nesting cavities, and time of year (e.g., when flocking or roosting.) For WEBLs, it may range from .29-.61 hectares, with the average around .43 to .56 hectares. (BNA)

Monitoring: Parents generally very tolerant of monitoring. Incubating or brooding female may flush or sit tight. Young may fledge prematurely on day 13 and after. If you keep track of dates, you will be able to avoid opening the box after the young are 13 days old, to prevent premature fledging.  At this age, bright blue feathers are evident on males. Nervous parents may fly from perch to nest site and swoop, chatter or click/snap their bills during monitoring (especially when young are close to fledging), others just watch. During nesting season, monitor boxes 1-2 times per week. More on monitoring.

Nesting Timetable (typical):

  • Excavation or nest site selection (scouting): Migrants may arrive from mid-February through mid-May, depending on the location. (In some areas, bluebirds may overwinter.) Pairing may occur as early as late January or as late as mid-April. Can begin very early when winters are mild. February to Mid-March: Bluebirds start establishing breeding territory and checking out nesting sites. Male shows nest sites to female (may bring along a little nesting material); female makes final selection (good sign but no guarantee if both go in the cavity at the same time.) Late arrivals, or previously unpaired birds may nest as late as July or even August. If there are not very many nesting sites around, the search may take a long time. It's never too late to put up a nestbox, as they may be used for a subsequent nesting (see Number of Broods), for roosting, and are also often checked out in the fall or during a mild winter by birds that may return the following spring. Extremely rare to make an open cup nest (reported for EABLs; sometimes people confuse an open cup Robin's nest with a Bluebird nest due to blue eggs. A few WEB: nests have been reported in building crevices, mud swallow nests (Sims 1983), purple martin boxes (Richmond 1953) and a metal clothes pole (Arnold 1937.) Females do not breed until the year after they are born.
  • Nest construction: 3-13 days (more quickly during second broods; can be spread out over a longer period especially in early spring), primarily gathering nesting material from the ground. May take nesting material from nests of other birds. Timing varies depends on location - may start in mid-May, early to late April. Female does most of work, male may carry in a bit or rearrange nest or even take material out. (BNA). Occasionally a nest is started by abandoned, or is used as a "dummy" nest to keep other birds from using the box. Note: Occasionally, males may bring in a few pieces of nesting material, as part of a "nest demonstration display." Nest description: Collection of dry grasses/hay/straw, weed stems, and, sometimes, hair and feathers. Routinely add ribbons, strips of cellophane, thin bark (e.g., cottonwood fibers and cedar bark) and leaves to their nest, which may cause some to confuse it with a House Sparrow nest. Also conifer needles, moss, mammal fur/horse hair, dry rootlets. May be lined with finer grasses, nest not tightly woven. Linda Violett has also found straw wrappers (common), 8-track tape, shoestring, a large pink costume feather, flowers (Bougainvillea is "in" in 2007), plastic newspaper ties, polyester batting, leaves, cigarette filters, and plastic rings in WEBL nests. See photos.
  • Egg laying: 5-7 days (shorter in summer broods). See typical first egg dates by State. Depends on weather, first egg may be mid-March through early May. Usually laying one per day (skipping a day in cold weather is possible but uncommon), usually before 10 a.m., for a total of 3-7 eggs (4-5 is typical). Often start egg laying a day or few days after nest is completed. Egg laying can be delayed (sometimes for a week or two - 3 weeks is not unheard of) in cold weather, for young parents, or in cases where food is scarce. Later broods tend to have fewer eggs, and bluebirds tend to lay more eggs per nest in the north vs. south, but southern birds have a longer nesting season.Occasionally lay "runt" eggs. Egg description: Eggs are pale blue, bluish-white, or white. See photos.
  • Incubation: About 14 days (can be 12-18, 13-14 typical. Tends to be longer for early nests, colder temperatures and higher latitudes). Begins the day the last egg is laid. Only the female incubates (male bluebirds do not develop a brood patch). Female may come and go during the day (sometimes staying away for longer periods during warm weather). She usually stays on the nest at night. While they may sit on eggs occasionally during the egg laying period, "full-time" regular incubation doesn't start until all eggs are laid. They may wait about a week if weather is still cold. They may start incubating before the clutch is complete in warmer conditions. Hatching failure is highest during warmer conditions. See heat. Female often practices tremble-thrusting (possibly to shake parasites out of nesting material.)
  • Hatching: Usually over 1 day - may occur over 24-48 hours (rarely 72 hours). Usually in the first 2 hours after dawn but can occur at any time. Can take 1-6 hours from pipping. All eggs usually hatch within a half hour to 1 day of each other (in late summer they may hatch over a 2 day period.) Parents may remove egg shells and eat them. Female broods young until they are 5-7 days old?, when they are better able to regulate their own body temperature. Kridler reports eggs are rather dull when first laid, but get slick and shiny when they are close to hatching.
  • Development: When they are first born, they look a bit like hairy shrimp. Both parents feed the young. Nestlings defecate right after being fed - parents often wait for this and then take out fecal sacs, dropping them away from the nest (rarely eating them.) See day by day photos to help with determining age. Dead nestlings weighing less than 10 grams are usually removed from the nest by a parent. See photos.
    • Day 1: pinkish skin bare except for sparse dark blue-gray down on crown, back and upper portion of wings. Eyes closed. The babies heads look huge. Their wings are nubs, and legs are weak and spindly. Legs and bills are pink, corners of mouth yellow, gape orange-yellow. Uncoordinated, raising head weakly and unsteadily, faint vocalizations.
    • Day 5: skin ruddy, bills and legs start to darken. Quills first appear along capital, spinal and posterior portion of ventral tracts. Nestlings vocalize more strongly and intensely in response to arrival of parents or humans.
    • Days 7-8: eyes begin to open.
    • Day 8: feathers begin to erupt, down still attached to ends of feathers. Nestlings are able to close feet around objects, may respond to being picked up with wing-flapping, or crouch in nest during monitoring.
    • Day 10-11, 90% of mass attained, 50% of growth.
    • Day 10: May bill snap in response to alarm.
    • Day 11: Usually cease to respond to humans with begging or vocalizing - may freeze or startle when nest is disturbed.
    • Day 12: can tell males from females by intensity of blue on retrices and remiges - more reliable sexing on Day 13-14. Bills and legs continue to darken, feathers fill in.
    • Day 14: nestlings can balance unsteadily if placed on perch while resting on abdomen, legs not capable of supporting body weight.
    • Day 12-15, young can still die of hypothermia.
    • Day 16: May become very agitated if handled.
    • Day 18: only small bare areas under wings and around cloaca.
    • Day 21: bills and legs very dark gray, most of down attached to feathers lost (last to go is down attached to crown feathers.)
    • As fledging approaches become more restless, moving around inside the cavity, peering out entrance, may become very vocal last few days but ten freeze and fall silent when nest is approached.
  • Fledging (leaving the nest): average age 21.8 days (range from 16-23, Eltzroth 1983). Other sources say 18-24 +/-. May be latest for early broods. If the box is empty in this time frame, the nest is flattened, and there is some fecal material (white) on the walls, it usually means fledging was successful. Occasionally a runt will fledge one or even two days later than the others. Insect availability may speed up or delay fledging. Sometimes all young do not fledge together - it is not uncommon for one to go a day later. I saw one report in the Summer NABS Bluebird Journal that one EABL did not fledge until five days after the others.
  • Dispersal: Once they leave the nest, bluebirds do not return to it. Young will call to parents and beg. They usually hide out for the first 7-10 days. Young are almost completely dependent on their parents for the first 3-5 days. They can feed themselves when they are 2 (?) weeks old after learning foraging behavior from their parents, but will continue to beg until they are about 1 month old. Both parents feed young, although if female starts another nest she may leave male to tend fledglings. When the babies are 28 days old, they can fly well.
    • Young from early season broods may join other juvenile in a flock.
    • Fledglings from later broods may stay on with parents. (BNA). They may also help feed a subsequent brood. See video.
    • The return rate of WEBL males to natal sites (where they were born) is 4 times that of females. About 6.2% of females returned the next year, 26.5% of males. (Koenig and Dickinson 1996, California); 2.3% of females, 24.3% of males in another study returned (AZ, JAG and PAG).
    • 11.1% of recovered birds in one study reused nesting sites. 46% of females and 61.5% of males returned to a breeding site in the subsequent year (JAB and PAG).
    • In Oregon, juvenile site fidelity was 58%, almost 100% for adults (Keyser et al 2004.)
  • Number of broods: Normally 1, sometimes 2 broods per year. 3 have occurred in Oregon (Finley 1907, Eltzroth 1983.) The number of broods probably depends on timing, temperatures, food availability, box availability and the experience or age of the parents. A subsequent brood may be started within days or weeks of fledging the previous brood - typically about 2 weeks. Sometimes the female starts building a new nest and laying eggs in a nearby nestbox before young from the previous brood fledge. It may be in the same box or a different box. The latest initiation for a third clutch was August 16 (EKE and JAB unpub) May reuse nest. See nestbox cleaning.
  • Longevity: Eight years 3 mos. is the record for a banded bird in Oregon. Resident CA WEBLs have been lived up to 8 years (Kleiber 2007a.) Wild banded birds have been recorded at 7 years (male, Eltzroth 1990), a female bred in her 6th year (EKE). More info.

References and More Information:


Its beautiful blue plumage and soft warbling voice make it one of the most attractive and charming of our bird friends.
- Frank G. Ashbrook, The Green Book of Birds of America


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