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Eastern Bluebird

Sialia sialis

Order:
Passeriformes
Family:
Turdidae
Sections

Behavior

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Adult male Eastern Bluebird, bathing; Ohio, September.

Washington Co. Ohio; male in fresh Basic plumage., Sep 15, 2004; photographer J. ZICKEFOOSE

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Adult male Eastern Bluebird at natural nest cavity, Staunton River SP, Halifax Co., VA, 7 July.

Bluebirds are secondary cavity nesters, meaning they put their nests in natural cavities, or in nest boxes, or other artificial refuges. The male Eastern Bluebird will display at his nest cavity to attract a female. He brings nest material to the hole, goes in and out, and waves his wings while perched above it. Image via Birdshare; R. Bruce.

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Figure 4. Nest Demonstration Display of Eastern Bluebird males.

A. Holding nesting material at entrance to a cavity, with drooped wings and spread tail.B. Looking around. C. Looking in entrance.D. Beginning to rock (see text). E. Showing face with nesting material. F. Wing-waving display. Drawing by J. Zickefoose, redrawn from Krieg 1971.

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Eastern Bluebirds mating, Upper Peninsula, MI, 4 May.

Image via Birdshare: Larry McGahey.

Locomotion

Walking, Hopping, Climbing, Etc.

Hop sideways, moving laterally with short steps (sidling), also while simultaneously turning 90–180°. When fully drenched, bluebirds can and do climb trees just as nestlings that have fledged prematurely, before they are able to fly, can climb up trees (Thomas 1946). Pivot while perching. Hopping develops by age 16 d (Krieg 1971).

Flight

Flight height is generally low in open areas, about 10–12 m off ground. Longer flights are higher.

Swimming And Diving

Never observed.

Self-Maintenance

Preening, Head-Scratching, Stretching, Bathing, Anting, Etc.

Self-maintenance includes “billing” feathers, bathing, scratching, sunning, and wing stretching, most frequently in Mar and Sep (Belser 1981); probably related to molt. Clean remiges and rectrices by drawing them through their bills (Krieg 1971).

Two phases of bathing: 1) individuals lower their heads and breasts into water, while shaking their heads and beating both wings; 2) raise the top half of their bodies out of the water, lower their vents and rumps into the water, spreading their tails and fluttering their wings (Krieg 1971).

Scratch their heads by moving a foot up and over their drooped wing. Nestlings flap their wings when they stretch and flap them vigorously to arrange flight feathers and to shake off dust from emerging feathers, most often in the final days before fledging. Wipe their bills after a successful capture and ingestion of insect prey and after dropping fecal sacs collected from nestlings. Stretch when their wing is drooped while extending their leg on the same side, after which they then raise and half open both wings behind their body.

Sleeping, Roosting, Sunbathing

Often rest communally on high, protected perches. In captivity, will rest close to each other, usually about 0.75 m apart. While resting their legs may be fully flexed, with their abdomens touching the perch. Individuals may tuck a foot into their ventral feathers (Krieg 1971). Young nestlings sleep with their heads drooped or held limply; nestlings 13 d old begin adult sleeping patterns with their heads resting on their scapulars.

Adults sleep in nesting cavities or on protected limbs of trees, sometimes communally. Sunbathing includes responses to concentrated light sources. While sunbathing, individuals raise their crests, and direct their body at right angles to and tilted away from the light source. When sunbathing, their feathers may be ruffled and their wings spread completely so that each primary is exposed, and they may droop the wing feathers, spread their tails, raise their crests, and ruffle their body plumage (Krieg 1971).

Daily Time Budget

In Clemson, SC, individuals spend 50–77% of the day resting, in no apparent activity—which may include sleeping, looking around, alert watching, and pausing between other activities (Belser 1981). Feeding (including capture, preparation, ingestion of prey items or berries) constitutes 2–6% of each day over the annual cycle. There is a notable peak in feeding activity, especially by females, in Sep and Oct. Males spend < 1% of the time capturing and delivering food to nestlings and fledglings; females spend 3.5–7% of time brooding nestlings and capturing and delivering food to nestlings. Incubation occupies 4–27% of a female's 24 h period from Apr through Jul (Belser 1981). See also Food Habits: nutrition and energetics, above.

Agonistic Behavior

For detailed, operational descriptions of motor patterns, see Krieg 1971; for experimental evaluation of functions of breeding-season aggression toward conspecifics, see Gowaty 1981, Gowaty and Wagner 1988, and Gowaty et al. 1989.

Physical Interactions

Include “fights”, “pecks”, “supplanting attacks” or “flybys”, “chases”, and “collisions”. In conspecific “fights”, individuals contact each other with beaks, feet, and wings. During fights, individuals face each other, grappling with their feet, sometimes falling to the ground. Males bite and pull females' head-feathers during occasional male aggression against females during “courtship” and sometimes preceding or following copulation attempts. Against immobile models made of bluebird skins mounted in life-like alert positions, individual bluebirds will land on the back of a model and peck at the head. In supplanting attacks, one bird flies toward another perched individual who vacates its perch. Supplanting attacks are the most frequent form of aggression. In experiments with models (Gowaty 1980, Gowaty and Wagner 1988), the investigators called similar behavior “flybys.” Flybys and supplanting attacks are the same for the aggressing individuals. The difference is that models and decoys are immobile and do not “respond”, while a living conspecific moves out of the way of a highly motivated conspecific flying directly at it. Fights are the usual terminal point of “highly motivated” chases. Once individuals make contact, both combatants may strike one another with their wings, grab feathers with beaks, and grapple with their feet at each others' bodies. Individual bluebirds also head-peck during fights. Head pecking appears highly motivated in that it is often done rapidly as if to a staccato rhythm (PAG per obs.). Fights are often extended, with combatants falling to the ground, paying no attention to their surroundings.

Males are more likely than females to attack potential nest-site competitors (Belser 1981); females seem somewhat more willing than males to attack species with similar foraging requirements. Most interspecific aggressive encounters occur during the breeding season. Agonistic interspecific encounters occur with American Robins, House Sparrows (Passer domesticus), Brown-headed Cowbirds (Molothrus ater), Brown-headed Nuthatches (Sitta pusilla) (see video x), Blue Jays (Cyanocitta cristata), Northern Mockingbirds (Mimus polyglottus), Great Crested Flycatchers (Myiarchus crinitus), European Starlings (Sturnus vulgaris), and Carolina Chickadees (Poecile carolinensis), among others (PAG). During migration, when Tree Swallows (Tachycineta bicolor) pass through southern and middle portions of eastern bluebird breeding ranges, chases and fights break out between Tress Swallows and Bluebirds (PAG). On the breeding ground, early in the breeding season, fights are common.

Communicative Interactions

Threat displays include Facing, Gaping, and Wing-Flicking (see below). Appeasement displays include Turning-Away and Fluffed Posture. In Turning-Away, eastern bluebirds fluff feathers, then turn their heads away from the opponent (opposite-sex members of pair, or combating males). Appeasement displays occur during territorial disputes and agonistic encounters between pair members. When in a Fluffed Posture, individuals retract their heads and fluff their feathers. The Fluffed Posture appears similar to resting, and during nesting cycle, either member of pair may assume the posture. In Facing, one individual turns their head to face their opponent. Facing usually occurs when an individual distance is violated. Gaping is like Facing, except that beak is open. During high-intensity Gaping, sleeks head and neck-feathers and leans toward approaching bird.

In Wing-Flicking (Wing-Waving), bird is perched in oblique position and flicks both wings rapidly out to a plane that is level with body. May fan tail; display accompanied by Warbles and Chatter (see Sounds: vocalizations, above). PAG and JHP have never seen Wing-Flicking used in interspecific displays, though it has been reported in response to presence of European Starling at a nest site (Krieg 1971). Gives Wings-Out Display when facing opponent: Legs fully flexed, body horizontal, plumage sleeked, wings out horizontally to side; never given to conspecifics. Usually followed by Alarm Scream and aerial attack on predator or potential predator. Gives Head-Forward Display while facing opponent: Body horizontal, head retracted, tail slightly lowered, legs flexed, body feathers sleeked; Gaping (see above), bill snap (see Sounds: nonvocal sounds, above), and Rasp (see Sounds: vocalizations, above), often given at same time as Head-Forward Display; this display often precedes supplanting attack.

Two bill-raising postures (Krieg 1971): In Oblique Bill-Up Display, body is oblique with head, tail in plane with body, tail slightly raised. In Horizontal Bill-Up Display, legs extended, neck stretched, head and bill pointed upward; black chin-stripes of both sexes are exposed to opponent's view.

Conspecific Patterns And Functions

Most frequently, males are aggressive to males; females are aggressive to females. Both adults can be aggressive to juveniles of either sex. Instances of male-to-female and female-to-male aggression also occur, but rarer than intrasexual aggression. In a comparative study during 1996, male aggression to females was distinctly more frequent in Athens, GA populations than Clemson, S. Carolina populations (Gowaty, unpubl data), perhaps because during females fertile period, females were foraging off territory more often in Athens, GA than in Clemson, SC. The differences may have been due to differences in arthropod abundance with greater availability in Clemson, S. Carolina than in Athens, GA (PAG unpubl data).

Experimental evaluations (Gowaty 1980) indicate male-male aggression most likely serves to protect threatened paternity, because male-to-male aggression is greatest when females are fertile, whether for first or later nesting attempts. Males are aggressive to other adult males usually in defense of paternity; resident males are most likely to respond aggressively to other males when females are fertile (Gowaty 1981).

Experimental studies have shown that male-to-female aggression occurs infrequently in some populations but not at all in others (Gowaty 1981, Gowaty and Wagner 1988). Male to female aggression is facultative occurring during the breeding season as an optional aspect of pair formation and initiation of breeding cycles. For example, in a systematic observational study controlling for time within the nesting cycle and year of observation and methods, rates of male-to-female aggression were significantly higher in Athens, GA, than in Clemson, SC (PAG unpubl. data). In experimental field tests in Clemson, SC, male aggression against females was infrequent (Gowaty 1981, Gowaty and Wagner 1988). In Pennsylvania, males chased females most during pair formation (Krieg 1964), but actually attacked females most during nest-building, when males may have been attempting to condition females' ranging behavior or perhaps even attempting to dissuade them from recruiting to the cavities the aggressive males were defending. Females are also sometimes aggressive to males. An aspect of pair formation: “. . . the pair bond is not formed, until leaving the nest box, a female Supplants the male. As the courtship period progresses this act of dominance by female becomes more intense” (Krieg 1971: 27).

Experiments revealed that aggression of adult bluebirds to same-sex conspecifics is situation dependent. Aggression is induced and modulated by the time-dependent threat a particular conspecific. Experiments indicate female-female aggression protects nests from conspecific nest parasitism (Gowaty and Wagner 1988) and, in S. Carolina was unlikely to occur in contexts in which paternal contributions to parental care were threatened by interloper females (Gowaty and Wagner 1988). Earliest report (Pettingill 1936) indicates that an intruder female won a fight with an already-crippled resident female, took over nest cavity, and bred with resident male. In New York in a systematic observational study during breeding season, 1 female was supplanted another 41 times within 1 h (Krieg 1964). Severe wounding, maiming, or death of females as a result of female-female fights noted in many populations (Nice 1931, Pettingill 1936, Laskey 1939, Blake 1954, Gowaty and Wagner 1988) and personally observed (PAG).

Anecdotal records suggest that after having over-wintered with fathers with little or no agonism, yearling males are aggressively repelled from breeding territories of fathers and their mates in subsequent breeding season (Pinkowski 1974e). Females are aggressive to juveniles usually when they are feeding nestlings in earlier broods; rarely occurs during latest broods (PAG, JHP unpubl. observ.). Sometimes on wintering grounds 1 or other sex is excluded from access to food or roosting sites by aggressive postures, calls, or motor acts of others. Experimental evaluations of variation in aggression in breeding territories during winter are consistent with year-round defense of nesting cavities (Plissner and Gowaty 1995). Juvenile-juvenile aggression, usually chases and supplants, rarely between siblings, usually between juveniles of different ages, may function in establishment of dominance hierarchies in nonbreeding season flocks (Plissner 1994).

Adult males and females are often aggressive to fledglings and young of the year in juvenile plumage (PAG per ob), but only when adults are feeding nestlings in first or second broods. While feeding nestlings in latest broods of the season, adults are seldom aggressive to birds of either sex in juvenile plumage. Gowaty and Plissner speculated that earlier season aggression of adults to bluebirds in juvenile plumage reflected the cost of foraging competitors on their territories.

Aggression intensity of male bluebirds during females' fertile periods to mounts of birds in juvenile or adult plumage are consistent with the information associated with the orange breasts of adult males (Ligon and Hill 2009).

Dominance Relations

Systematic observations of agonistic encounters between wild pairs of females and males during courtship indicate that neither sex completely dominates the other. In 255 recorded encounters between members of 19 pairs during 2 yr in New York, 54% were “won” by males. In some pairs, females were more aggressive; in others, males. In this migratory population, pairs that were already paired upon arrival showed little aggression (Krieg 1971). Although dominance hierarchies among sib-groups have been documented (Krieg 1971), no clear dominance hierarchies have been documented in wild birds (Plissner 1994).

Spacing

Adult bluebirds defend the area around the nesting cavity; bluebirds use the nesting territory for mating, nesting, and feeding. Thus nesting territories include cavities. Home range sizes, sometimes called territories, 1.1–8.4 ha in Michigan; average 2.1 ha in New York (n = 19 pairs; Krieg 1971); similar values for territories in S. Carolina (PAG). Territory sizes decrease as the nesting season progresses, perhaps in response to changing insect availability or other selection pressures, such as male mate-guarding or provisioning of nestlings, that keep adults nearer cavities (Gowaty and Wagner 1988, Gowaty et al. 1989). Territories in burned or lumbered areas are smaller than territories in old fields or in pastures and orchards, probably because of ease of foraging and availability of perches. Territory size can be substantially reduced by increase in number of available nesting cavities within an area (PAG): Placing nesting boxes 10 m apart along fences experimentally reduced territory size.

Eastern bluebirds establish and maintain their territories by singing loudly, patrolling boundaries (both males and females), and directing aggressive defense at interlopers and potential interlopers. Bluebirds are also aggressive to other cavity nesting species, sometimes directly usurping nests from Brown-headed Nuthatches (see video) and attempt usurpation from Red-cockaded Woodpeckers. When resident bluebirds attack and supplant interlopers, most territorial disputes are settled. Either sex of adult may perform these attacks, which are usually directed at same-sex adult conspecifics. See Agonistic behavior, above.

Bluebirds defend feeding areas during the winter, when in flocks of 2–30; this defense is less intense than during the breeding season. Family groups and overwintering pairs defend areas around nesting cavities in areas where resident all year. Experiments demonstrated that during winter, previously used territories with >1 cavity are more closely monitored and more readily defended than territories with only 1 cavity (Plissner and Gowaty 1995). Anecdotal reports indicate competitive exclusion of females from small roosting sites by several cohabiting males. When roosting at night or resting during the day, the nearest distance between neighboring adults is about 0.75 m; fledglings and juveniles may huddle (PAG, JHP).

Most fights are with other cavity-nesting species. See Agonistic behavior, above.

Sexual Behavior

Mating System

Generally eastern bluebirds are socially monogamous in S. Carolina and Georgia. Greater than 95% of social groups are of 1 female and 1 male (called “social monogamy” to distinguish this situation from genetic monogamy, which is less common), but social polygyny (1 male nesting with >1 female) in separate cavities on same territory or different territory and with joint nesting (2 females in same cavity simultaneously) does occur (Gowaty 1980). Social polyandry (1 female nesting with >1 male) and helping also occur (Verner and Willson 1969, Gowaty 1980). See Breeding: cooperative breeding, below.

During 1977–1979 in S. Carolina, from observation of 177 nesting attempts in which all adults were uniquely color-marked, social monogamy equaled 95.5% of nesting attempts, social polygyny 3%, helping by hatch-year young <1.5%, and social polyandry did not occur (Gowaty 1980). Parental care by males in S. Carolina (Gowaty 1983) and Ontario (Mackay 1985, Meek and Robertson 1994a) is not essential for reproductive success of females, whether measured as asymptotic nestling mass, number of nestlings fledged, or number of offspring returning as adults. Breeding success varies geographically and latitudinally as function of male parental care (PAG). Although some males appear to provide more food to fledglings than adult females do, particularly when females are renesting, there have been no systematic, controlled studies of this phenomenon. Within populations, the value of paternal care for individual females varies, so infrequency of social polygyny and predominance of social monogamy is not explained by a universal requirement by females for male help (Gowaty 1996a).

Adult Sex Ratio

Patterns of replacement of adults that disappear during breeding season suggest that in most years replacement females are as available as replacement males; in some years, however, replacement females do not regularly recruit to breeding vacancies (PAG); observations in S. Carolina suggest sex ratio parity in many years, male bias in others.

Pair Bond

Males broker the access of females to their nesting cavities. Males first perform a nest demonstration display (Figure 4) for prospecting females. Pairs are “bonded” once females enter cavities with males for the first time (Krieg 1971). Males may reject potential mates by removing nesting materials of females who are building (Gowaty per sobs). The duration of pair bonds is associated with previous nesting success. In Michigan, 30–50% of unsuccessful pairs (n = 61) renest together in same season, compared to 70–85% of successful pairs (n = 70; Pinkowski 1977b). Pairs remain together between seasons in migratory populations more frequently than expected on the basis of separate probabilities that individuals will return and return to the same place (Pinkowski 1974d).

Courtship-feeding occurs regularly in some populations, but occurrences and distributions within populations seem highly variable, with females “begging” from males in a posture reminiscent of chick “begging.” Males offer females prey without a visible signal from the female. Feeding of females by males is a regular aspect of breeding biology in migratory populations, but it is often rare or absent in facultatively migrant or nonmigrant populations (Thomas 1946, Hartshorne 1962).

“Mate-guarding” was inferred (Gowaty et al. 1989) because males stayed closer to fertile than to nonfertile females. Males also attended fertile females longer and followed them more often than they did non-fertile females. In S. Carolina, males guard females more strongly in multicavity territories than in single-cavity territories, an effect not found in an Ontario population (Meek and Robertson 1994a), where interspecific interactions with Tree Swallows are common. In S. Carolina, males “guard” females who have most, not least, extra-pair young in their nests, suggesting that males do not guard females with low a priori probabilities of mating with other males. Females who are off territories most during fertile periods have significantly more young from extra-pair sires than females who remain on their territories more often. Male may use cues in female's behavior, such as how often she is off territory, to guide guarding decisions. Male's tendency to be aggressive to other males is greatest when female is fertile (Gowaty 1981), so another behavior that may increase paternity certainty is male-male aggression. In comparative studies of populations differing in food resources, males guard mates more strongly where females must forage more often off their territories for more sparsely distributed food (PAG).

Nest-Guarding By Females

Even though females are more active around nesting cavities when they are building nests, they forage closer to their nests during egg-laying than during nest-building, consistent with the behavior of guarding their nests from conspecific nest parasites; females guard their nests more strongly when there are 2 (rather than just 1) nesting cavities/territory (Gowaty et al. 1989).

Copulatory Behavior

Copulation seldom observed; seems surreptitious. Many copulations occur in the hour after an egg is laid, once egg-laying has begun; birds copulate on perches and sometimes within nesting cavities (Hartshorne 1962). Copulation sometimes preceded by Peeps (see Sounds: vocalizations, above); active female approaches male, and sometimes by stereotyped Solicitation Posture. Female crouches, holding body horizontally so that bill is lined up with body, keeps wings still, but drooping, with tips below base of tail; depresses feathers on crown and back and holds tail about 45° upward from line of body. Male mounts female's back; cloacal contact may or may not occur; copulations completed within 1–5 s. Females initiate copulations by assuming Solicitation Posture. In New York during copulations, females utter low Peeps (see Sounds: vocalizations, above; Hartshorne 1962, Krieg 1971); in S. Carolina and Georgia, females may be silent (PAG). Male attempts copulation with female while she is on nest during incubation, but these attempts are unsuccessful because female makes “no effort to respond” (Hartshorne 1962). In Clemson, SC, within-pair copulations occur as early as 8 d before first egg is laid and up to 6 d after (Gowaty et al. 1989). In Clemson, SC, and in Athens, GA, all extra-pair copulations occurred during egg-laying period (PAG).

Aggressive Copulations

In S. Carolina, aggressive copulation is rare (PAG). One reliable observation was between captive bluebirds that were confined to a small cage with no opportunity for the female to escape (Krieg 1965). Male aggression against females is described as “normal” part of copulatory sequence (Krieg 1971), in which the male pecked the female on the head just before or just after copulatory mounts. Males occasionally peck female's head during copulations in some New York populations (Hartshorne 1962) and in Athens, GA, but rarely in Clemson, SC (PAG). Aggressive copulation is uncommon in eastern bluebirds.

Extra-Pair Copulations

From Gowaty 1996b and Gowaty and Bridges 1991b. Extra-pair copulations are seldom seen, but they produce 20% of nestlings in studies in S. Carolina and Ontario; 25–30% of broods sired by >1 male. Experiments in Clemson, SC, showed that extra-pair paternity is more frequent when neighborhoods are densely settled, consistent with the idea that copulation outside of pair bonds is opportunistic, depending fundamentally on encounter probability with potential mates. Females leaving territories most often during their fertile periods have higher rates of extra-pair young, suggesting that females leave territories for extra-pair fertilizations. Broods on territories of older males are significantly less likely to have extra-pair offspring than are broods on younger males' territories. Older females have more sires in their broods than younger females. Males that guard their mates the most have more, not fewer, extra-pair young in their nests suggesting that “guarding” is conditional, something males do only sometimes, usually after. Breeding synchrony of females, measured by day of first egg, had no effect on frequency of extra-pair paternity. Pair-bond duration is negatively associated with extra-pair paternity: Pairs together for their first nesting attempt had 37.6% extra-pair young (n = 23 pairs, 215 nestings); pairs together for longer had 21.1% (n = 10 pairs, 111 nestings).

In Ontario, resident males exact no sanctions against the experimentally manipulated “extra-pair behavior” of their mates (MacDougall-Shackleton and Robertson 1998). In Clemson, S. Carolina there is no significant correlation between a male's fraction of paternity of offspring and his feeding rate to nestlings (PAG unpubl data). Neither result is consistent with the idea that males withhold care of nestlings not their own in this species.

Hormones and Behavior

Eastern bluebirds are easily studied, so it is no surprise that the species has become a model species for testing hypotheses about the associations between hormones and behavior in the wild.

Hormonal Variation And Natural Behavior. Yolk androgens.  Chemicals, such as hormones or nutrients that mothers contribute to their eggs may affect the behavior of their offspring in lasting ways throughout the life of an offspring. Scientists call such lasting effects of mothers “maternal effects”. In many songbirds, including eastern bluebirds, mothers deposit variable amounts of androgens into the yolk of their eggs, such that investigators predicted that these concentrations are important indicators of how mothers “manipulate” the behavioral or morphological phenotypes of their offspring. However, a recent study showed that maternal circulating androgens vary inversely with the amount of androgens deposited into mothers' eggs, suggesting that the level of yolk androgens are a simple by-product of maternal loads, and not an adaptive strategy of mothers for manipulating the behavior or morphology of their offspring. In an experimental study in which laying females were challenged by an intruder, yolk androgens were higher in the offspring of experimental than control females (Navara, Siefferman et al. 2006). The authors concluded that androgen deposition in eggs is an adaptive method for regulating mothers' androgen levels, which there by may prevent potentially deleterious elevation of concentrations of androgens when females are making, laying and incubating eggs.

Testosterone and structural color variation. Many investigators speculate or even assume that the “male” hormone testosterone is associated with sexually dimorphic coloration. However, experimental and observational studies of eastern bluebirds reported no or only weakly convincing evidence that testosterone in eggs (experimentally controlled), implanted in nestlings (also experimentally controlled) or from naturally circulating testosterone affected variation in structural plumage coloration in male bluebirds (Siefferman et al. 2013).

Coloration And Behavior.  The blue color variation of Eastern Bluebird feathers is due to the precise nanostructural arrangements in the spongy layers of feathers (Shawkey et al. 2003), and females and males also differ in the structural components of feathers affecting UV-violet chroma, brightness, and spectral saturation (Shawkey et al. 2005). Eastern Bluebirds' “chestnut”- colored breast feathers are due to the pigments – eumelanin and phaeomelanin - physiologically deposited in the feathers. Thus, using objective measures of the reflectance patterns of the both chestnut coloration and UV-violet coloration, investigators may now seek correlations between coloration and behavior, as well as perform experiments testing hypotheses of the potential signal functions of color variation within and between the sexes of Eastern Bluebirds.

Characterization of between-individual variation in eastern bluebird colors is now an area of active investigation, with studies of both the proximate and ultimate causes of coloration1. Not only are eastern bluebirds dimorphic in coloration, but they are also dimorphic in the feather structures that proximately control coloration (Shawkey, Estes et al. 2005). Bluebirds, like other bird species, see in the ultraviolet, making it potentially significant to between-male competition and perhaps female choice that the nanostructure of the spongy layer of Eastern Bluebird feather barbs predicts between-male variation in ultraviolet blue plumage color (Shawkey, Estes et al. 2003). Nevertheless, nanostructure did not predict the principle color (hue) of the feathers, and the proximate-mechanistic causes of differences in hue in bluebirds remain unknown.

Among male variation in UV-blue and chestnut breast coloration is associated with the age of males such that brighter males are more likely to be older males (Siefferman, Hill et al. 2005).

Among the most interesting of the coloration and behavior studies is one examining the effects of nutrition on female coloration (Siefferman and Hill 2005). In a laboratory experiment, control females with ad libitum (constant) access to food during the period of the pre-basic molt developed more colorful feathers than the experimental females with a restricted diet. Thus, we might conclude that females with brighter coloration are potentially healthier or luckier, perhaps living in better habitats, than females with duller plumage. Similarly, under experimentally induced variation in parental feeding rates to nestlings, nestling plumage color was also affected: better nourished nestlings had brighter coloration (Siefferman and Hill 2007).

Male coloration also suffers when males face challenging environmental conditions or greater work loads feeding experimentally enlarged broods (Siefferman and Hill 2005).

Additional questions about the role of coloration in the social lives of bluebirds include the following. Does male ornamentation correlate with “mate quality” and reproductive success? If one defines “mate quality” as a composite variable indicating how hard a male works (based on variables such as how early a male breeds and how frequently he feeds incubating females and nestlings), male mate quality does correlate positively to the number and quality (mean mass at 14 days old) of fledglings (Siefferman and Hill 2003). In addition, hard working males are often the ones with more ultraviolet hues and they fledge more offspring than duller males. Compared to drabber males, males with brighter UV/blue coloration provisioned females more often during incubation (Siefferman and Hill 2005), though the association was weak suggesting that other factors may play a significant role in the motivation of males who provision their mates during incubation. Future studies of coloration of bluebirds should consider what induces males to be relatively better or worse feeders of incubating females and nestlings by also taking account of the fact that compared to areas without fire ants, in areas with fire ants males increase their provisioning rates as their mates' kill rate decreased (captured insects/foraging attempts) (PAG unpublished data). In other words, male feeding of mates may be a facultative behavior associated with the availability of resources or conflicting constraints on female behavior.

Some evidence exists that adult care-giving males pay attention to the variation in their mate's coloration when defending fledgling-aged offspring (Barrios-Miller and Siefferman 2013). Adult males were prone to protecting their experimentally brightened sons rather than experimentally dulled sons, and they protected their sons in preference to their daughters when their mates had fancier plumage. In contrast adult females did not discriminate between male and female near-fledgling age young, nor did they discriminate between brighter or duller male nestlings.

Despite studies showing correlations between what bluebirds do and their colors, a controlled laboratory experiment testing whether females choose males on the basis of coloration found no support for the hypothesis that the colors of males are sexually selected via female choice (Liu, Siefferman et al. 2007). Similarly, an experimental field study provided no support for the hypothesis that females choose males on the basis of their coloration (Liu, Siefferman et al. 2009).

Social and Interspecific Behavior

Degree Of Sociality

Semicolonial nester. Tends to settle in areas where bluebirds have already settled. In S. Carolina, closest nests 9 m apart in nesting boxes; when excess cavities available in controlled experimental patterns, prefer neighbors to be closer (36 m) rather than more distant (100 m or 1 km; Gowaty and Bridges 1991b). No data on degree of coloniality in populations using natural cavities. Pairs sometimes nest solitarily (PAG unpubl. data).

In Clemson, SC, flocks of 3–20 composed entirely of birds in Juvenile plumage visit nesting territories during mid- to late summer; during late summer and fall, individuals forage in family groups and give Tu-a-wees (see Sounds: vocalizations, above) in flight and while perched. Winter flocks often contain 10–50 individuals and are often composed of family groups (PAG, JHP). During exceedingly cold weather or severe storms, up to 20 individuals may roost together, tightly compressed within a cavity or other confined space (Pitts 1977b). Inverted-cone configuration (heads together and bodies pointed downward) of communally roosting bluebirds in winter may increase warmth and decrease likelihood of suffocation.

Play

No systematic observations.

Non-Predatory Interspecific Interactions

Eastern bluebirds forage with other species in nonbreeding season; in Clemson, SC, and in Athens, GA, eastern bluebirds form persistent associations with Pine Warblers (Dendroica pinus); associations with American Robins are less predictable (PAG unpubl. observ.). See also Breeding: brood parasitism, below.

Aggressive competition between bluebirds and House Sparrows, chickadees (Poecile spp.), European Starlings, and Tree Swallows are relatively common (PAG, JHP). Bluebirds attempt to take over nesting cavities of Red-cockaded Woodpeckers (Picoides borealis) in Francis Marion National Forest, Berkeley Co., SC (M. Lennartz pers. comm.). Whereas House Sparrows and European Starlings can usually evict bluebirds from nesting sites, bluebirds sometimes win in interactions with Tree Swallows; observations of chickadees evicting bluebirds are less common than of bluebirds usurping cavities previously claimed by chickadees (PAG). In Kentucky, bluebirds begin nesting before starlings and sometimes complete second clutch after starlings have left area—a pattern consistent with temporal partitioning of resources, rather than competition; neither species appeared to have important negative impact on the other (Davis et al. 1986). In Ontario, competitive interactions between Eastern Bluebirds and Tree Swallows affect mate guarding by males (Meek and Robertson 1994a). Eastern Bluebirds destroy nests of House Wrens (Troglodytes aedon; Waring 1912) and chickadees, even removing living chickadee nestlings from nesting boxes and dropping them some distance away (PAG unpubl. data).

Predation

Kinds Of Predators

Eastern chipmunks (Tamias striatus) and flying squirrels (Glaucomys volans) enter nests and destroy or eat eggs or young. House Sparrows, European Starlings, black rat snakes (Elaphe obsoleta obsoleta), black racers (Coluber sp.), fire ants (Solenopsis invicta), domestic cats (Felis domesticus), black bears (Ursus americanus), and raccoons (Procyon lotor) prey on adults, nestlings, and fledglings. American Kestrels (Falco sparverius) prey on adults and recent fledglings. A simple dichotomous key (Pinkowski 1975b) provides useful guidelines for determining most common sources of predation at Eastern Bluebird nests.

Manner Of Predation

House Wrens entering cavities to puncture or remove eggs accounted for about one-third (n = 31) of predation in Michigan (Pinkowski 1977b); other predation due to raccoons and other mammals, including domestic cats. In S. Carolina, male House Sparrows enter nesting boxes, destroy eggs, and kill adults and nestlings (Gowaty 1984) by pecking their heads—a competitive interaction that facilitates later access to nesting sites. European Starlings enter cavities when holes are large enough, killing adults and nestlings by jabs to head (PAG). Black rat snakes patrol fence lines, climb poles, enter cavities, and eat eggs or nestlings, sometimes remaining within cavities for several days. Individual snakes appear to visit nest sites regularly (PAG, JHP). Rat snakes are active at all times of day, preying on bluebird nests in daylight, crepuscular hours, and night (Hensley and Smith 1986). In Tennessee, snake predation accounted for 33% of all predated nests during 1941, 23% (n = 142) during 1942, 40% (n = 174) in 1943, 37% (n = 151) in 1944, 37% (n = 144) in 1945, and 39% (n = 136) in 1946 (Laskey 1946). Exotic fire ants enter nesting boxes, swarm over adults and nestlings, and eat nestlings alive (PAG). Domestic cats and raccoons attack during night, pulling incubating females, eggs, or nestlings through hole entrance or, in case of raccoons, lifting top of nesting boxes if these are movable (PAG, JHP). Cats also grab adults as they emerge from cavities. In N. Carolina, black bears took 32 eggs from 7 nests and killed 29 nestlings in 7 different nests (Tardell and Doerr 1982). Kestrels stage on high perches near nesting boxes, waiting for vulnerable fledglings or adults that are intent on providing food for nestlings.

Response To Predators

Bluebird males give distinctive song type in presence of predators (see Figure 3B). This song resembles typical territorial song, but begins with a chip. Females sometimes give full, loud song in response to predators when males are off territory; which may attract a territorial male back to his territory to defend against interloper males (Morton et al. 1978). On detecting a potential predator, both males and females will engage in Wing-Flicking (see Agonistic behavior, above) and give Warble (see Sounds: vocalizations, above), with increasing intensity as predator approaches nesting cavities. Males and females will dive-bomb humans, black snakes, House Sparrows, and other passerine predators, while snapping their bills loudly. Subjective impression of PAG and JHP is that rattle of bill snapping is especially common when snakes are present.


1Definitions: “hue” indicates the principal color of an object; technically it is the wavelength of peak light reflectance. The total amount of light that an object reflects is called “brightness”, which is technically the sum of reflectances across a sample (such as a feather) of wavelengths of light. “Chroma” is about the spectral purity of color, that is how undiluted it is with white. “Spectral saturation” is a measurement that decouples the effects of chroma and hue, as in an analysis of UV-violet chroma, which is highly correlated with hue. Used in this way spectral saturation is the measurement within a range on either side of a hue as the proportion of light reflected.

Recommended Citation

Gowaty, Patricia A. and Jonathan H. Plissner. 2015. Eastern Bluebird (Sialia sialis), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA. https://doi.org/10.2173/bna.381