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Eastern Bluebird

Sialia sialis

Order:
Passeriformes
Family:
Turdidae
Sections

Breeding

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Figure 5. Eastern Bluebird annual cycle.

Annual cycle of breeding, molt, and migration of the Eastern Bluebird. Breeding shown for population in nw. South Carolina. Molt shown for population in Michigan. Thick lines show peak activity; thin lines, off-peak.

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Figure 6. Timing of egg-laying in different populations.

Distribution of first-egg dates in Florida (A), Tennessee (B), Pennsylvania (C), and S. Carolina (D), showing that Eastern Bluebird is double-brooded throughout most of its breeding range, with tendencies toward triple-broodedness in central portions of range (B and D). A, B, and C, from Peakall 1970; D from Gowaty 1980.

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Eastern Bluebird nest, Pennsylvania.

Longwood Gardens, Chester Co. PA. 13 Jul 1978. Ruler is in cm.; photographer Rene Corado

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Eastern Bluebird clutch, Illinois.

Highland Park, Lake Co., IL. 5 May 1913. Ruler is in cm.  White eggs are rare in this species; most are sky blue. ; photographer Rene Corado

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Adult female Eastern Bluebird, feeding young; Ohio, July.

Washington Co. Ohio; plumage of adult is Alternate (worn), that of young is Juvenal., Jul 15, 2004; photographer J. ZICKEFOOSE

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Eastern Bluebird nestlings, age 6 days; Ohio, April.

Washington Co. Ohio; unlikely that all 5 young will fledge, Apr 15, 2004; photographer J. ZICKEFOOSE

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Eastern Bluebird nestlings, age 11 days; Ohio, July.

Washington Co. Ohio; note emerging primaries., Jul 15, 2004; photographer J. ZICKEFOOSE

Phenology

Pair Formation

Figure 5. Pairs form in breeding-season and nonbreeding-season flocks, so many individuals arrive back on breeding grounds already paired. For nonpaired migrants, pairs form about a week on average after territory establishment. In Tennessee, most pairs form from Nov to end of Jan (Laskey 1939). In Michigan, migrants arrive on breeding range late Mar–late Apr; about half of arriving males were paired before arrival (Pinkowski 1977b). In Minnesota, arrive mid-Mar, extreme dates 22 Feb–13 Apr (Rustad 1972a). Somewhat more males than females arrive early (Pinkowski 1977b). Second-year (SY) and older (after-second-year, ASY) males arrive simultaneously, as do SY and ASY females.

Nest-Building

Begins late Feb or early Mar for first brood period on S. Carolina study sites, and may continue for 3 wk (PAG and JHP unpubl. data). During subsequent nesting attempts, nests may be completed in ≤ 1 d. In S. Carolina (n = 1,662 nests), time between initiation to completion of nest-building averaged 5.9 d ± 0.17 SE. For 895 spring broods, time between nest completion and first egg-laying averaged 8.3 d ± 0.14 SE, but for 585 summer broods only 5.8 d ± 0.17 SE, a highly significant difference (PAG).

First/Only Brood Per Season

Peak month of breeding in all areas is Apr (see Figure 5). Extreme dates for first eggs of clutch are late Feb and mid-Sep (PAG). Duration of stages of second and third broods is similar for first broods, except for nest building.

Incubation usually begins on day last egg is laid; lasts typically 14 d. Duration of incubation period for Eastern Bluebirds is more variable than for American Robins and can last 11–19 d (Peakall 1970). In S. Carolina, for nests at which we observed both onset of incubation and hatching dates, incubation averaged 13.8 d ± 1.4 SD (range 9–28, n = 1,651 [1985–1991]); in Georgia, incubation averaged 13.4 d ± 1.1 SD (range 11–17, n = 119; PAG). Incubation period is longer during early breeding seasons, and at higher latitudes (Pinkowski 1974f, Butcher 1988). In S. Carolina, incubation period was significantly longer in spring (mean 14.1 d ± 0.04 SE, n = 1,063), than in summer (mean 13.2 d ± 0.05 SE, n = 588; PAG).

Analysis of Cornell Nest Records Program cards (Peakall 1970) indicated that the shortest breeding season is in extreme northern portions of the range -- in Canada and New England (71–80 d, 22% of annual cycle). Longest breeding season is in middle portions of the range -- northern Gulf states to Pennsylvania (81–102 d, 28% of annual cycle); and shorter again in extreme southern portions of range -- in coastal portions of Gulf states and Florida (average 83 d, 23% of annual cycle). Records from Clemson, SC, indicate mean duration of breeding season 201.6 d ± 7.6 SD (range 190–212, mean 55.1% of annual cycle, n = 7 yr), during which sampling began well before first signs of nest-building and ended well after last fledging. In Athens, GA, mean 185.3 d ± 8.6 SD (range 176–194, mean 50.1% of annual cycle, n = 4 yr). Very early clutch initiation during exceptionally warm winters (PAG), and occasional observation of fall nestings in Sep and Oct often unexplained, but in Clemson, SC, earliest recorded nest-building 23 Feb during mild winter by 2 females that eventually nested jointly; latest recorded fledging 9 Sep (PAG).

Nest Site

Selection Process

See Behavior: sexual, above. Experiments show preferences for nesting boxes containing old nests (Davis et al. 1994b). Individuals raised in natural cavity sites are no less likely to choose nesting-box sites than are individuals raised in artificial cavities (Pinkowski 1979e); thus, no evidence that imprinting of nest-site characteristics beyond the “cavity” occurs. Higher proportion of yearlings than older birds use natural sites. Same individual will use both artificial and natural cavity sites during a single breeding season. During the late 1800s, when bluebirds were unusually abundant on Bermuda, some “forsook” cavity-nesting habits, building and using open-cup nests (Reid 1884) as they do in modern longleaf pine. Occasionally bluebirds build open cup nests on mature oak limbs (Sprunt 1946, and PAG).

Microhabitat And Nesting Boxes

In Michigan, individuals prefer tree cavities in dead rather than living trees or limbs; 71.5% of nests in trees were in pine or oak, and 77.6% were abandoned woodpecker holes (Pinkowski 1976f). Bluebirds do also build nests in shallow burn scars of longleaf pine (Finch et al. 2012). Individuals prefer nesting boxes on wooden posts to those mounted on steel posts; tend to prefer areas of short grass; no preferences for specific nesting-box designs, hole height, presence or absence of predator guards, or hole diameter (Horn and Bacon 1989). Elegant experimental manipulation of nesting-box characteristics demonstrated that Eastern Bluebirds preferred nest hole entrances 4.4 cm in diameter to those 3.5 cm, and 3 times as many pairs used boxes with floor dimensions equaling 10 x 10 cm than those with dimensions of 15.3 x 15.3 cm (Lumsden 1986a). In Tennessee, bluebirds prefer white interior (Pitts 1977a)—a preference not obvious in Ontario (Lumsden 1986a). In S. Carolina, individuals take artificial cavities from 0.5 to 4 m above ground (PAG); in Maryland, mean height of settled boxes is 1.5 m; natural cavities 0.6–9.1 m high (Robbins and Blom 1996a).

Site Characteristics

In Wisconsin, no preferences for distance to water, cover, buildings, roads, entrance faces, density of surrounding grass, or habitat surrounding cavity, as long as not dense forest (Horn and Bacon 1989). In Maryland, preferred boxes are relatively close (about 6 m) to small shrubs, with surrounding low grass and nearby perch space; preferred boxes face northeast to south (Devlin and Willner 1982). In Wisconsin, tended to select boxes in open area: nesting boxes that had <5% cover within 11 m, <10% canopy cover within 30 m, >35 m from nearest forest edge, >200 m from nearest permanent water source (Kruger 1985a). In Manitoba, preferred sites associated with livestock, away from overhead lines, from which kestrels may hunt (Munro and Rounds 1985). See also Conservation and Management, below.

Nest

Construction Process

Female alone builds over several days, after gathering material usually from the ground and returning to the nest site. Reports of male assistance with nest-building probably mistakenly based on observations of males carrying nesting material, most likely for a Nest Demonstration Display (see Figure 4). In S. Carolina and Georgia, males carry 1 straw at a time, as in Nest Demonstration Display. In S. Carolina and Georgia, males also observed carrying nesting material out of cavities, in effect destroying nest under construction by female (Parks 1948).

Structure And Composition Matter

Loosely constructed cup nests in natural cavities or in nesting boxes; open-cup nests are extremely rare (PAG), but they were common in the late 1800s in Bermuda (Reid 1884) and occur in longleaf forests of the SE US. Often entirely of grasses or pine needles; usually lined with fine grasses, occasionally horsehair or turkey feathers.

Maintenance Or Reuse Of Nests, Alternate Nests, Effect Of Box Types

Often bluebirds reuse nests or rebuild nests with nesting material from a previous nesting attempt. In an experimental study in Clemson, SC removing old nest material had no effect on renesting or on the likelihood that bluebirds fledged young (Gowaty and Plissner 1997). In a paired experimental design bluebirds chose boxes containing old nests in 38 of 41 cases in which boxes with old nests were paired with empty ones, perhaps because old nests more reliably contain Nasonia vitrepennis wasp larvae, a parasite of blowfly larvae (Protocalliphora sialis) (Davis et al. 1994). In a series of experimental studies in N. Carolina (Stanback and Dervan 2001, Stanback and Rockwell 2003), bluebirds were given choices (1) between previously used and unused nesting boxes, or (2) a clean, previously successful box and an identically constructed box. The birds chose the unused box more often given the choice in (1), but given the choice in (2) they chose previously successful cavities, if nesting material had been removed. The NC experiments show clearly that bluebirds make contingent decisions about the reuse of nesting boxes, when given choices between a preferred and non-referred nest box material (woodcrete vs wood) that have been cleaned or not. Bluebirds overwhelmingly preferred woodcrete to wooden boxes, even when the woodcrete boxes were soiled and the wooden boxes clean. However, when given a choice between a soiled or clean woodcrete boxes, bluebirds nested in the clean box.

Pairs sometimes defend territories with >1 cavity. In S. Carolina and Georgia, females often build nests, simultaneously or in rapid succession, in each cavity on a territory, but usually lay eggs from a clutch in only one.

Nonbreeding Nests

Nesting boxes insulated by old nests are used for roosting in winter.

Eggs

Shape

Subelliptical to short subelliptical (Harrison 1978a).

Size

In Michigan, 498 eggs measured in field during years 1969–1977 averaged 20.9 mm ± 0.9 SD x 16.4 mm ± 0.6 SD, with no differences in size of eggs by brood period or clutch sizes of 4 and 5 (Pinkowski 1979b). In S. Carolina, 911 eggs measured in the field on the day they were laid during 1985 averaged 20.88 mm ± 0.86 SD (range 18.1–24.3) x 16.5 mm ± 0.53 SD (range 14.7–19.2) wide. In S. Carolina, no statistical differences in length of eggs (n = 211 clutches) produced in spring versus summer, but spring eggs are significantly wider and heavier (see mass, below); nonetheless, no statistical differences in length, width, or weight of eggs by clutch size. In S. Carolina, females that laid 2 clutches in 1985 had smaller mean egg width in summer clutches compared to spring clutches (PAG and JHP).

Eggs in the Western Foundation for Vertebrate Zoology (WFVZ) collection measured by R. Corado for S. s. sialis measured 21.01 (range 18.13–22.95) mm long (n = 100 eggs, 20 clutches); 16.3 (15.3–17.21) mm wide (n = 100 eggs, 20 clutches); and empty shell mass averaged 0.181 (0.143–0.217) g (n = 100 eggs, 20 clutches). For S. s. grata: 21.01 (18.15–23.81) mm long, (n = 69 eggs, 15 clutches; 16.26 (15.2–17.19) mm wide (n = 69 eggs, 15 clutches).

Mass

In Tennessee (Donahue 1939), mean egg mass on day laid was 3.6 g; on last day before hatching, 3.4 g (n = 20). In S. Carolina during 1985, fresh whole eggs averaged 3.07 g ± 0.26 SD (range 2.15–3.85, n = 829), one-tenth of adult female mass; thus, 5-egg clutch equals about 50% of female's mass (PAG, JHP). Egg mass increases with laying sequence: first eggs averaged 3.01 g ± 0.27 SD (n = 202) and last eggs in clutches of 4 and 5 averaging about 3.1 g ± 0.2 SD (n =253). Average egg mass/clutch increases with brood period. From WFVZ collection: empty shell mass averaged 0.179 (0.145–0.206) g (n = 69 eggs, 15 clutches). Runt eggs occur rarely.

Color

Most eggs are pale blue or blue-green colored by the pigment biliverdin (according to T. J. Robinson, as cited in Siefferman 2006). White eggs come from particular females; clutches contain eggs that are either all white or all blue (PAG), although pink eggs sometimes occur in S. Carolina and Georgia. In a detailed study of egg characteristics in Lee Co., Alabama, 98% of females laid blue-green eggs, < 2% laid white eggs and < 1% laid pink eggs (Siefferman et al. 2006). A cross-fostering study in which eggs of different objectively measured coloration were swapped between nests found no support for the hypothesis that egg coloration is associated with host detection of parasitic eggs (Siefferman 2006).

Surface Texture

Smooth.

Eggshell Thickness

In Tennessee, mean eggshell thickness of incubated eggs reported as 0.736 mm; shells from unincubated eggs, 0.835 mm. Puncture tests indicated that strength of shells declines significantly during incubation, but vesicular holes do not change in size or density (Pitts and Martin 1989).

Clutch Size

Bluebirds lay 1 egg/day for a total clutch size of 3 to 6 or 7 eggs; in S. Carolina, modal clutch size 5 in spring, 4 in summer. No apparent increase in clutch size in Canada in comparison with lower-latitude clutch size, though clutch size is positively correlated with duration of breeding-season and population density; tendency for mean clutch sizes to be slightly larger in the center of the range and at the height of the breeding season, usually in Apr (Peakall 1970). On Bermuda, mean is similar to mainland values (Crowell and Rothstein 1981).

A comparative study across the breeding range of continentally-breeding bluebirds during 1998 (Dhondt et al. 2002) showed geographical and seasonal variation in clutch size. In the south part of the range, where bluebirds are mostly year-round residents, clutch sizes increase, reach maximum size mid-season and then decrease. In the northern part of the range, where bluebirds are migratory, clutch sizes decline from the start until the end of the breeding season.

Egg-Laying

One egg/d, in morning; incubation begins with laying of the penultimate or ultimate egg; determinate layer (Nash 1942), so individual eggs in clutches are not replaced. Egg-laying begins as soon as the nest is completed, up to 3 wk later (see Figure 6). See also Brood Parasitism, below.

In Ontario, lays eggs between 06:30 and 09:00; estimated median laying time 07:30, about 2 h after sunrise (n = 26 clutches; Meek and Robertson 1994c). In S. Carolina and Georgia, laying intervals are more variable; about 25 h between each successive egg although the ultimate egg was later than non-ultimates (PAG). Eggs may be laid as early as dawn and as late as 11:00 (Cooper et al. 2009). Investigators computed from video records (Cooper et al. 2009) an estimated mean laying interval and the time of laying relative to sunrise. Non-ultimate eggs were laid at about a 24 h interval, but there was a significant difference between non-ultimate and ultimate eggs, which were laid about 25 h after the last non-ultimate egg (also consistent with field observations in S. Carolina (PAG).

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

Begins with laying of ultimate or penultimate egg of clutch (PAG) (Cooper et al. 2006).

Incubation Patches

One extensive patch found in females, only on ventral portion of belly (PAG).

Incubation Period

Lasts 11–19 d, generally 14; later, summer clutches and those at lower latitudes having shorter incubation periods than those of earlier or higher-latitude clutches (Butcher 1988). In S. Carolina during 1985 (PAG, JHP), average time from laying of eggs until each hatched was 15.7 d ± 1.78 SD (range 12–23, n = 601). In contrast, active incubation lasts about 14 d. Incubation time decreases with increasing temperature and as solar radiation intensity increases; as wind velocity increases, incubation time increases (Buser 1980), consistent with our observations in S. Carolina that overall duration of incubation is less in summer than in spring.

Parental Behavior

By female only; scattered claims of male incubation probably based on observations of male attentiveness to incubating females or to nesting cavities; males in captivity joined incubating females to attempt copulations (Hartshorne 1962). In Michigan, females spend about 61% of their time on eggs; attentive periods (time on eggs) last 15.6 min ± 8.7 SD (n = 233) on average, inattentive periods 10.1 min ± 9.7 SD (n = 233; Pinkowski 1979f); values from Illinois similar (Kendeigh 1952). Female usually remains on nest all-night; away from nest longest in first hour after daybreak (PAG).

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Temperature of eggs during natural incubation in warm weather averages about 33.9°C (Huggins 1941). In comparison to eggs < 1 wk old, mortality of eggs > 1 wk old due to desiccation can occur when ambient air temperature is > 36.5°C (PAG). Unincubated eggs often remain viable through extended periods of low temperatures.

Hatching

Preliminary Events And Vocalizations

None known.

Shell-Breaking And Emergence

The following is based on direct observation of hatching of > 100 clutches in Clemson, SC, during 1978 (PAG). Many eggs hatch in first 2 h after dawn, but they may hatch at any time of day. Duration of hatching/egg varies: 1–6 h from pipping to full emergence. Intervals between hatching of eggs are sometimes remarkably short, and degree of synchrony is sometimes extreme, with all eggs in a clutch hatching within 20–30 min. In most clutches, all eggs hatch within 1 d, although eggs in clutches from late-summer broods may hatch during a 2-d period, probably a result of early development of first-laid eggs exposed to high ambient temperatures before the onset of incubation.

Parental Assistance And Disposal Of Eggshells

Parents do not usually assist, but they remove broken eggshells from hatchlings; females sometimes eat eggshells (PAG).

Hatching Failure

Comparative nest record data from 7,231 nests across the continental geographical range of bluebirds (Cooper et al. 2006) indicate that 7.3% of eggs failed to hatch. Hatching failure was greater in clutches with 3 and 6 eggs than in 4 and 5 egg clutches. Hatching failure occurred later in the breeding season than earlier and in more southerly latitudes, probably owing to thermal stress. The per-egg probability of hatching failure varied across the geographic range from 1.3% in the north to 4.7% in the south.

Young Birds

Condition At Hatching

The following is after Pinkowski 1975b and Donahue 1939, unless otherwise noted. Newly hatched young are altricial, with dingy gray down (see also Appearance: molts and plumages, below). Size at hatching correlates positively with egg size (Pinkowski 1975b). In S. Carolina, mean mass of just-hatched, not-yet-fed females is 2.4 g ± 0.55 SD, 7.4% of adult mass (n = 112); for males, 2.4 g ± 0.57 SD, 7.9% of adult mass (n = 94; PAG). In Michigan, hatchlings not yet fed averaged 2.4 g ± 0.48 SD (range 1.7–3.1, n = 16); mean mass 8% of mean adult weight. Hatchling mass highly positively correlated with egg diameter (r = 0.98, p  < 0.0001; Pinkowski 1975b). Mean hatching mass not significantly different for male and female nestlings.

Growth And Development

Stereotypic Begging Posture (open beaks, calls, and up-stretched necks) is obvious by the end of day 1. Down feathers lengthen during day 1; contour feathers are developing by day 2. Femoral-tract feathers begin emerging by day 3, when light-colored feather shafts begin projecting from caudal and alar tracts. Wings are dark by day 4, and future rectrices are just visible. Feathers in crural region appear on day 5. Eyes open on days 5–6. Feathers grow rapidly in second week; feathers in sheaths cover capital region; humeral feathers emerge then too. Nestlings achieve physiological endothermy (can maintain body temperature) at about 7–8 d of age (Buser 1980).

Secondary wing coverts break out of sheaths on day 8; capital feathers, secondaries, and rectrices are out of their sheaths on day 9; caudal feathers barely project beyond vent. By day 9, nestlings show fear if handled; can crawl. Innermost primaries, all secondaries, and most capital-tract feathers emerge by day 10 or 11. Almost completely feathered by day 12, except for midventral region, which is feathered by day 13.

Can be assigned reliably to sex by day 13 (see Demography and Populations: measures of breeding activity, below), according to distribution of bright blue coloration on primaries and rectrices, as well as amount of white on rectrices (Pinkowski 1974d); errors likely when assignment to sex is based on younger birds (PAG). See also Appearance: molts and plumages, below.

By day 14, unfeathered areas have disappeared, and rate of increase in wing length is greatest. Nestlings lose mass several days before fledging as a result of decreases in feeding rates by adults, which encourages nest departure (PAG, JHP). Nestlings are capable of weak, short-distance flight by day 14 (PAG), when daughters have achieved 81.7% of adult female mass (n = 61) and sons 88.1% of adult male mass (n = 66). In S. Carolina, no differences by sex in growth curves or in field metabolic rates (Droge et al. 1991). Mean mass not significantly different for males and females on days 1–14. On day 15, completely feathered. During third week of life, motor coordination develops. Nestlings huddle together, gape toward adults, preen, exercise wings, stretch, move wings, stand on rim of nest, look out of nest cavity, and poke at grass. Achieve adult dimensions in wing chord and primary lengths around 35–40 d, about 2–3 wk after leaving nest.

Sibling conflicts seldom noted. In S. Carolina, nestling behavior is remarkably free of agonism (Plissner 1994): No nestling fights during hundreds of hours of videotaped interactions inside nesting boxes (PAG, JHP). Size hierarchies among chicks within broods occur infrequently in Clemson, SC; more frequently in Athens, GA (PAG). In contrast to continental populations, in Bermuda nestling sibling competition is intense because of spotty food availability (Swann 1982); size hierarchies sometimes develop within nests. Last nestling in size-graded broods sometimes fails to fledge (PAG).

Parental Care

Brooding

By female only. Brooding time depends on variation in relative humidity, temperature, and solar radiation: As humidity and temperature decrease, brooding increases; as solar radiation increases, brooding decreases (Buser 1980). Females brood from hatching until days 5–7 of nestling life, seldom after day 5 in S. Carolina (PAG). Brooding rhythm not documented.

Feeding

By adult female and male. In S. Carolina, adults begin feeding nestlings in the first hour after hatching, and continue until about 3 wk after leaving the nest. Both parents feed young; on average, males feed nestlings as often as females do, although in higher latitudes males feed young more frequently than females when females are brooding young. Deliver food items into gaping beaks of nestlings (see below). Feeding rates are highest in early morning. For nestlings 9–11 d old, during morning hours parents make on average 2.2 deliveries/nestling/h ± 1.74 SD (n = 74 nests); for nestlings 3–4 d old, 1.46 deliveries/nestling/h ± 1.08 SD (n = 60 nests; PAG).

Energy budget analyses from S. Carolina and New Jersey indicate that even though food delivery rates by male and female are similar, females spend at least twice as much time and energy with nestlings as males do (Belser 1981, Buser 1980), probably because females brood and feed (see Food Habits: nutrition and energetics, above). Males sometimes provide more food to nestlings than females do during the early nestling period because females alternate feeding and brooding. In Ontario, experimental removal of males revealed that early in the season unaided females were unable to raise any offspring to fledging, but late season nests of unaided females were successful (Meek and Robertson 1994).

Foods Fed To Nestlings

In Michigan, butterfly and moth larvae (35–41% of total), spiders (6–31%), short-horned grasshoppers (7–17%), crickets (7–10%), beetles (1–5%), Heterocera adults (3–7%), ants (Formicidae; 1–7%), Lubbricus sp. (1–8%), long-horned grasshoppers and katydids (0.5–3%); fruits of mulberry (Morus spp.), raspberry (Rubus spp.), dogwood (Cornus spp.), cherry (Prunus spp.), and honeysuckle (Lonicera spp.; Pinkowski 1978a). In Tennessee, types did not vary with nestling age; of total food fed throughout nestling period, 22.4% grasshoppers, 14.7% crickets, 16.3% spiders, 25% caterpillars, 2.4% moths; 19.2% of food items were too infrequently fed to be listed individually (including 1 skink [Eumeces sp.]) or were too small to identify (Pitts 1978a). See also Food Habits: diet, above.

Nest Sanitation

Nestlings often defecate after delivery of a food item. Video records from S. Carolina show that adults often wait for nestlings to turn and stereotypically present vents; fecal pellets emerge as balloons in gelatinous packets, and are easily grasped in parental beaks (JHP, PAG). Adult emerges from cavity carrying white fecal pellet; flies 20–100 m before dropping it. Very rarely adult will swallow fecal pellets, which are offered to adults by all but oldest nestlings. Nest sanitation reduced with loss of adult, particularly following disappearance of adult female (PAG, JHP).

Tremble-Thrusting is highly distinctive behavior of breeding females: Female places her bill straight and deep within nesting material and, with trembling motion, shakes nest (Hartshorne 1962). May be performed during incubation or nestling stage. Videos suggest that Tremble-Thrusting shakes parasitic larvae from nesting material (PAG).

Carrying Of Young

Adults seldom carry living young; while perched on outside of cavity, an adult bluebird removed dead nestlings up to about 10 g from nests by grasping wings and pulling them toward the hole (PAG).

Adoption, Infanticide, And Indifference

Adult males that replace previous male breeders at nests containing nestlings sometimes feed young, but not at rates sufficient to make up for loss of the original male. In an evaluation of behavior of replacement males in Ontario (Meek and Robertson 1991), replacement males fed nestlings 5 times less frequently than nonreplacement males. Feeding, or so-called adoption, by replacement males did not increase the likelihood of successful nesting or subsequent nesting by females, though it may increase the likelihood that such replacement males are able to “take over” territories of lost males (Pinkowski 1978b). In contrast, males, even under experimental conditions, have not been observed to kill nestlings (infanticide; Meek and Robertson 1991, PAG and JHP). On the basis of an anecdotal series of observations in S. Carolina (PAG), adult females that replace previous female breeders are facultatively infanticidal, indifferent, or adoptive, depending on the age of nestlings at the time of replacement (Plissner and Gowaty 1988). Nestlings < 7 d old are removed from nests by replacement females either before or after death; nestlings > 14 d old are treated with indifference; intermediate-aged nestlings are fed by replacement females (PAG). Sexually selected infanticide seems more likely a female than a male option in this species.

Cooperative Breeding

Helpers

In contrast to Western Bluebirds, Eastern Bluebirds rarely have helpers. In joint nesting, 2 females lay 2 clutches of eggs synchronously in the same nest, then incubate synchronously and sequentially, and cooperate with a male in feeding nestlings and fledglings. In S. Carolina, 5 of 4,299 nesting attempts in field seasons 1977–1991 were joint nesting ventures; 2 of these 5 attempts were by the same previously known and banded females, so it was reliably inferred that these jointly-nesting females were mother and daughter (PAG, JHP). Juvenile helpers are rare; almost all reports are of juveniles entering nesting cavities when adults are feeding later broods (Pinkowski 1975d)—something documented on video films twice by PAG and JHP. Adult helpers seen occasionally and irregularly (Pinkowski 1976d, PAG), usually second-year son of current breeders. See Behavior: sexual behavior, above.

Results Of Helping

No systematic evaluations. Anecdotal reports suggest “hindering” rather than “helping” in Michigan (Becker 1944) and in S. Carolina (PAG). In Michigan, adult females are intolerant to fledglings around nest sites; and juveniles do not place food far down the throat of nestlings; many items are alive and wriggling (not well prepared) when delivered (Pinkowski 1975b).

Brood Parasitism

Conspecific Brood Parasitism

Low frequency in unmanipulated populations; genetic data indicate that about 1% of nestlings are from parasitically laid eggs (Gowaty and Bridges Gowaty and Bridges 1991a, Gowaty and Bridges 1991b, Meek et al. 1994). Experiments indicate that fertile females without nests parasitize nests of other bluebirds, leading to the conclusion that in this and other species that may be limited by nest sites there should be constant, if low-level, selection favoring females who lay their eggs in nests of conspecifics whenever their own nests are lost to predation or other catastrophe during egg-laying. Countertactics to conspecific brood parasitism include clutch covering and egg removal (Gowaty and Bridges 1991b), and female-female aggression (Gowaty and Wagner 1988). See also Behavior: agonistic behavior, above.

Interspecific Brood Parasitism By Eastern Bluebirds

Anecdotal reports include observations of bluebird eggs in nests of Carolina Wren (Thryothorus ludovicianus; PAG), chickadee (Krueger 1989b), and House Sparrow (PAG).

Interspecific Brood Parasitism On Eastern Bluebirds

Brown-headed Cowbird, European Starling, and House Sparrow parasitize nests of Eastern Bluebird. American Robin occasionally parasitizes bluebird nests, but robin is too large for most nesting-box entrances. Parasitism by Brown-headed Cowbird is relatively rare; in Michigan, only 1 of 200 nests in both natural and artificial cavities was parasitized by cowbirds (Pinkowski 1974c). In Maryland, 17.5% of nests were parasitized by Brown-headed Cowbirds, a rather high rate associated with conspicuous cardboard, half-gallon cartons with openings that averaged 5.4 cm in diameter (Woodward and Woodward 1979a).

Timing Of Laying In Relation To Host'S Laying

No information.

Response To Parasitic Mother, Eggs, Or Nestlings

In S. Carolina, females cover with new nesting material 75% of clutches parasitized by Brown-headed Cowbirds, rebuild nest cup, and often lay new clutch of eggs (PAG).

Effects Of Parasitism On Host

No systematic evaluations. In S. Carolina, when rare cowbird chicks fledge from nest with bluebirds, adults continue to feed bluebird nestlings.

Fledgling Stage

Departure From Nest

At undisturbed nests in Michigan, mean age at nest departure 18.8 d ± 1.47 SD (n = 184); for spring broods 19.4 d, for summer broods 18.6 d (Pinkowski 1975b). At undisturbed nests in Georgia, mean age at fledging 17.6 d ± 1.8 SD (n = 290; PAG). In S. Carolina, mean age at fledging 17.6 d ± 1.2 SD (n = 1,298); for spring broods 17.7 d ± 1.2 SD (n = 843), for summer broods 17.3 d ± 1.1 SD (n = 566)—a statistically significant difference (PAG). Fledging age, like clutch size and incubation time, varies with latitude and resource availability.

As fledglings leave the nest, give Tu-a-wee call (see Sounds: vocalizations, above), and fly to a perch or to cover up to 30–50 m away. When not sitting quietly, fledglings give Tu-a-wees when they are separated from parents or siblings.

Growth

No systematic data on growth of fledglings up to the age of independence.

Association With Parents Or Other Young

When adults approach with food, nestlings beg loudly, fluttering wings in typical Begging Posture (see Young birds, above). Fledglings from early-season broods often leave parental territories on independence; fledglings from later broods and late-summer broods often stay on parental territories, so natal philopatry is far less likely for individuals that fledge from spring broods (Plissner and Gowaty 1996). Fledglings typically remain in or near cover for first 7–10 d after departing nest, becoming more mobile later, while following adults over longer distances.

Ability To Get Around, Feed, And Care For Self

Few systematic data on development after nest-leaving. Fledglings “freeze,” land on top of each other, drink water, bathe, fly toward adults for food, wipe bill, take food items, give contact calls, engage in agonistic interactions, preen, and huddle together. Fledglings can fly and feed themselves; 2 wk after departing from nest, foraging skill (number of successful attempts at predation on ground arthropods) is half that of adults foraging alongside them (adults, 67% of 189 attempts; nestlings, 32% of 154 attempts; Goldman 1975). In Michigan, fledglings do not feed themselves by gleaning small items from a perch or by hopping on ground until average of 25.4 d of age (range 22–28, n = 28); first begin drop-feeding at 28–34 d (Pinkowski 1975b).

Immature Stage

In S. Carolina, fledglings remain in family groups for up to 3 wk. Young from early-season broods often join other juveniles and move in large flocks. Flock size and membership are fluid, although some multifamily associations remain fairly constant. Adults caring for early and midseason broods are often aggressive to juveniles (see Behavior: agonistic behavior, above). Compared to young from early-season broods, young fledged in late summer are more likely to associate with parents over winter; aggression by adults toward juveniles is sparse late in the breeding season, compared to early (Plissner 1994, PAG).

Recommended Citation

Gowaty, P. A. and J. H. Plissner (2015). Eastern Bluebird (Sialia sialis), version 2.0. In The Birds of North America (A. F. Poole, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.381