Behavior & MigrationAll about Eastern Bluebirds (Sialia sialis) (EABL)

All about Eastern Bluebirds (Sialia sialis) (EABL)

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

Species: There are three bluebird species: Eastern (Sialia sialis), Mountain (S. currucoides) and Western (S. mexicana.) There are also 8 recognized subspecies of the Eastern Bluebird – S.sialis bermudensis, caribaea, episcopus, fulva, grata, guatemalae, meridionalis, and nidificans. (See more info). The alpha code for the Eastern Bluebird is EABL. It may interbreed with Mountain Bluebirds where their ranges overlap.

Interesting Facts:

  • Unlike some other songbirds like a Meadowlark, male bluebirds do not open their beaks widely when singing. (Tekiela) Males may stop singing during incubation (BNA).
  • When bluebirds turn or cock their heads, it is probably to enable them to increase their field of vision (not for hearing), since their eyes are located on the sides of their heads. (Tekiela)
  • Some studies have shown that 30% of bluebirds return to previous nesting sites the following season.
  • As many as 20% of nestlings may be fathered by a different parent (Gowaty)
  • Can spot their prey 15-20 m, sometimes 40 m away from a lookout. (Preston & McCormick, 1948). May smash large prey against a perch before eating it. ( BNA)
  • There are an estimated 10,000,000 Eastern Bluebirds in North and Central America. (North American Bluebird Society, 1999; Terres, 1980; Tveten, 1993)
  • Flying speeds have been measured at 17 miles per hour (Fimbel, K. 2000, Animal Diversity Web)

Identification: The Eastern Bluebird is smaller than a robin, with a red/orange-brown throat and white belly. Female has grayish-blue back (blue is more obvious in flight) and white eye ring, male has rich blue back. Short, stout black bill. Juveniles look almost like another species, with spots on their chest and a white eye ring (males have more blue in wing and tail.) EABLs are smaller overall, and have a slightly thicker bill than Western Bluebirds.

Song: Pleasing musical series of mellow whistles, sings frequently in sort of a soft whisper. Also a chatter, and bill snapping or clicking scolding call (e.g., when nestbox is approached) sometimes called a predator or anger song. Bluebird landlords may be able to tell one male’s song from another.

Distribution: See Range Maps. East of the Rocky Mountains, spanning from southern Canada to the Gulf states and on into Mexico and Honduras. Populations are found in Cuba, although it is not a native species there. (Terres, 1980) In the fall, some birds, migrate, others do not. Migration may depend on weather and/or food availability (BNA).

Preferred Nesting Habitat: Open or semi-open grassland habitat with an open canopy and little or no understory, with sparse ground cover/low grass. Orchards, mowed meadows, large lawns, cemeteries, orchards, roadsides, and areas with scattered trees and short ground cover. Clear-cut forests, burned tracts of pine woods, swampy habitats near urban areas. Perching spots (fence, telephone lines, medium size trees) preferred for hunting and nest-guarding.

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 fruit -e.g., flowering dogwood, holly, mulberry, wild grape, Virginia creeper, pokeweed, and Viburnum. (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 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 (about 1.5% of time) fledglings from the previous brood will help tend to the subsequent brood. (Gowaty 1980). Females will “beg” from males (possibly to test their foraging skills), especially for migrant birds. Males and females may fight with each other during nesting season. Bluebirds (male and female) have been videotaped removing some nesting material placed there by another bird competing for the same nestbox.

Nestboxes: Build or purchase a nestbox designed specifically for bluebirds. These 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, a round 1.5″ diameter hole (or 1.375″ x 2.250″ oval hole. Mountain Bluebirds need a 1 9/16″ hole), 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. MAY prefer boxes facing NE to South (Devlin and Willner 1982).

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: Prefer areas with short grass, maybe with <5-10% cover. (BNA). Meadows, large lawns, cemeteries, orchards, roadsides, and areas with scattered trees and short ground cover. Areas with fence lines, some medium size trees, or telephone lines provide perches for hunting and nest-guarding. If no native birds use the box for two years, try a different spot. For more information and do’s and don’ts, see Bluebirding Basics. Note: Moving an active nestbox 10-12 feet (e.g., if it is too close to a tree and flying squirrels are an issue) is usually not a problem – just keep the entrance hole facing the same direction and the parents should have no trouble finding it.)

Recommended distance between nestboxes: Boxes should be a minimum of 100 yards apart. 125-150 yards apart may be better. 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. EABLs seem to prefer areas where other bluebirds are within ~118 feet vs. 328 feet (Gowaty & Bridges 1991) However, never say never or always. I have heard reports of bluebirds nesting approximately 70 feet apart within sight of each other.

Various studies show home ranges (for feeding, mating and nesting) vary from 2.7 to 20.8 acres (average ~ 5 acres), getting smaller during nesting season. 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.) (BNA) Home ranges of Eastern Bluebirds may range from 1.1 ha (during the breeding season) to 120.8 ha (during winter). (Gowaty and Plissner, 1998) Each bluebird couple may require about a 12 acre territory (Krieg, 1971.)

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.

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. Also see www.texasbluebirdsociety.orgEastern Bluebird Nestling Daily Growth Series” or Pam Ford’s photos to help determine age. Nervous parents may hover and swoop, clicking 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 late February through mid-May, depending on the location. (In some areas, bluebirds may overwinter.) May begin very early when winters are mild. February to Mid-March: Bluebirds start establshing breeding territory and checking out nesting sites. Male shows nest sites to female; female makes final selection (good but not guaranteed sign 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, and some pairs have multiple broods. 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 by birds that may return the following spring. Extremely rare to make an open cup nest (sometimes people confuse an open cup Robin’s nest with a Bluebird nest due to blue eggs.) Females do not breed until the year after they are born.
  • Nest construction: 2-6 days. (can be built in 1 day, or can be spread out over a six week period especially in early spring), primarily gathering nesting material from the ground. Female does most of work, male may carry in a bit (e.g., an inital “claim straw” or two), 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.” Rarely, males or females may actually remove nesting material – see Linda Moore video. (Note: Bob Cahill of VA reported a bluebird taking sphagum moss out of planters and using it to build a nest.) Nest description: Neat, cup shaped, “woven” (by tucking in stray needles) nest typically 100% fine grass or pine needles. Cup may be in the back of the box. Occasionally bits of fur or a few feathers, or even some hair (e.g., from a horse). Fairly deep, often cylindrical nest cup – usually 3-4″ deep, with the cup portion 2.5″ in diameter and about 2.25″ deep.
  • Egg laying: 5-7 days (shorter in summer broods). See typical first egg dates by State. 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. Eggs are powder blue (no dark spots), sometimes white. Surface is smooth, shape is subelliptical to short subelliptical. See photos. Occasionally lay “runt” eggs.
  • Incubation: 12-14 days (can be 11-19. Tends to be longer for early nests, colder temperatures and higher latitudes). Begins the day the last egg is laid (or sometimes on next-to-last egg – which may result in a runt hatching 24 hours later than the rest of the clutch). Only the female incubates (male bluebirds do not develop a brood patch), spending about 61% of her time on the eggs (Pinkowski 1979.) 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. During incubation and development, the female often practices tremble-thrusting (possibly to shake parasites out of nesting material.) [Note: I have heard one anecdotal report from Marilyn Michalskiof Pennsylvania of a male bluebird “incubating” eggs, sitting on them about 50% of the time (per a nestcam), jiggling his body over the eggs. The male and the female were both seen together in this nestbox at night.]
  • Hatching: May occur over 24-48 (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. Eggs are rather dull when first laid, but get slick and shiny when they are close to hatching (about ~10 days).
  • 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 21- 110 yards from the nest (rarely eating them.)See day by day photos to help with determining age.
    • Day 1: dingy gray down, eyes closed. The babies heads look huge. Their wings are nubs, and legs are weak and spindly. Uncoordinated, raising head weakly and unsteadily, faint vocalizations.
    • Day 2: contour feathers start to develop. Soft gray down is now along the edges of wings, the head and spine. The skin beneath looks blue-black as feathers begin to develop beneath it.
    • Day 3: femoral tract feathers begin emerging.
    • Day 4: wings are dark.
    • Day 5: feathers appear in crural region. Eyes open day 5-6. While sleeping, head held limply in front or curled to side.
    • Day 7-8: able to maintain body temperature.
    • Day 8: secondary wing-coverts break out of sheaths.
    • Day 9: capital feathers, secondaries and retrices are out of sheaths; birds use bill to work all major feather tracts. Nestlings may show fear if handled, can crawl. Yawning first observed. May lay head on scapulars while sleeping.
    • Day 10-11: most capital-tract feathers emerge.
    • Day 11: Feather sheaths start to disintegrate (leaving a white dust behind) and wing feathers begin to emerge. Nestcams indicate nestlings start to stand up at this age. Nestlings start to preen, pulling at the sheaths of emerging feathers.They may flap wings, stretch and hop a little to strengthen muscles.
    • Day 12: almost completely feathered. except for mid-ventral region. Incomplete bill-wiping movements and head scratching first observed.
    • Day 13: Mid-ventral region is feathered. sleep with head on scapulars. Can tell sex by bright blue color of primaries and retrices, and white on retrices. Sleep in typical adult manner.
    • Day 14: no unfeathered areas visible. Wings are longer. Capable of weak, short-distance flight. Bird can right itself and make short shuffling movements backwards and forwards.
    • Day 15: completely feathered. Nestlings huddle together, preen, exercise, stand on edge of nest and look out of nest cavity.
    • Day 16: able to hop well by day 16. During final days in nest, nestlings flap wings vigorously.
  • Fledging (leaving the nest): Typically 17-18. May leave as early as 12 (premature) and stay as long as 19-21 days. 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 bird 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. When the babies are 28 days old, they can fly well. They can feed themselves when they are 3-4 weeks old after learning foraging behavior from their parents. Both parents feed young, although if female starts another nest she may leave male to tend fledglings.
    • 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.
  • Number of broods: One to four broods per year. Two broods are common in most areas. Fourth brood attempts may be made in southern climates. 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. May reuse nest. See nestbox cleaning.
  • Longevity: Wild banded birds have been recorded at 6 and 7 years old, one male at 10 years +. BNA says a female and male that were bought as adults lived 10 years in captivity. More info.

References and More Information:

Take care of the wee blue birds who chose to join your family as best you can. Rejoice when you see babies fledging and splashing in a birdbath. Grieve when a clutch fails or one dies. In time, that will happen no matter how hard you try to protect them. When disaster strikes, take a moment to count the number that you added to the enjoyment of not just yourself but everyone else who sees flashes of blue.
– Tree Greenwood, Bluebird_L, 2006


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