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Length and complexity of nestling begging calls increases with age (S. Halkin and PAG unpubl. data). An abstract indicates that a thorough, experimental study of auditory isolation on expression of calls in individuals including eggs, nestlings, and fledglings up to about 2 mo of age exists: “All of common calls . . . alarm, distress, location, etc., appeared in all age classes sampled” (Hartshorne 1962 b: 62). Tu-a-wee, Peep, Zeee, and Squawk seem to require little learning. Song, however, was given by wild individuals in Juvenile plumage, but not by naive captives, indicating that learning is necessary for expression of song. No systematic observations exist testing vocal learning, sensitive periods for learning, or learning from other species.
See Figure 3. Loud Song, Soft Song, and Tu-a-wee are the most frequently noted vocalizations. Many vocalizations are quiet, with a soft, whispering quality—characteristics that may have led to probable underestimates of vocal repertoire. Studies of individual variation among adult males during the breeding season (Huntsman and Ritchison 2002) revealed remarkable complexity and diversity among the songs of five intensively studied males. The 4,238 songs recorded for these birds had on average 3.20 ±1.7 notes. Males uttered songs at frequencies of between 2 and 4 kHz, and most songs occurred in bouts, with successive songs 1 to 13 s apart.
Figure 3 illustrates the diversity of songs among 3 of 5 sampled males. The mean number of song types among these males ranged from 40 to 81 of which 64% were low volume; 15% moderate volume; and 21.5% high volume. Remarkably, males added new song types throughout the single season of study, and none of the repertoires of any of the males approached an asymptote. Importantly, each male's repertoire was distinctive; two or more of the males shared only 6 of 309 song types, and these males used 64% of song types only during a single day's observations.
The singing rates of the males varied with nesting stage. Males sang most during the pre-pairing stages, and least during incubation, nestling, and post-fledgling stages, although vocal communication was rich even in these periods. Males' singing cadence was fastest during pre-laying, slower in the nestling stage. Males' singing versatility was greatest during pre-pairing, pre-laying, and laying stages, but lower during incubation, nestling, and post-fledging stages. 64% of the recorded songs occurred when the investigators had also recorded the location of the male's mate. 43% of songs were when males were > 10 m from mates; 19% between 5-10 m, and 38% at close range.
These data suggest several hypotheses about male song. (1) Male song is mostly between males and females in that most songs were at times when males would have most interactions with females and fewer songs when their interactions with females were lowest, e.g. during incubation and the nestling stages. (2) The highest singing rates as well as the singing of songs with the most versatility occur during the pre-pairing stages of nest cycles, likely functioning to attract potential mates and influence mate choice. (3) The low volume song of males in communication between mates may help reduce location information of females, eggs, or nestlings to rivals such as conspecific nest parasites or predators. These hypotheses are not mutually exclusive and indicate that further research on the singing of Eastern Bluebirds may reveal further richness of vocal communication in this species.
Loud Song. See Figure 3A. A rich warbling, low in pitch and often rapidly delivered, usually by males. “Three to 8 notes grouped in phrases of 1 to 3 notes, each with very short pauses between them” (Bent 1949: 250). Alliterative rendering sometimes given as “tury, cherwee, cheye-le ” (Krieg 1971: 77), “ayo ala loee—alee ay lalo leeo” (A. A. Saunders in Bent 1949: 250). During singing bouts, males may pivot their body so that singing is sequentially in opposite directions. Sometimes males spread their tail while singing or lift their tail vertically. Males sing Loud Song from conspicuous high perches, sometimes in flight.
Loud Song by males predominates Mar through Jul (Belser 1981), most frequently during nest building and egg-laying; usually males stop singing with the onset of incubation (Pinkowski 1971b). Males sing more frequently on display perches than females do. Females sing Loud Song in the presence of predators when males are off territory (Morton et al. 1978). In NY State, wild, unpaired males vary their rates of Loud Song from 15 to 20/min (Krieg 1971). Males give Loud Song as advertisement of territory establishment and to attract breeding females.
Soft Song. Sometimes called Whisper Song; like Loud Song, but much softer. Given on territory in other situations besides advertisement; e.g., when females are laying eggs, males will remain nearby, occasionally uttering Soft Song. May function to assure female of the presence of the male.
Predator/Alarm Song. See Figure 3B. Like Loud Song, the Predator/Alarm song is preceded by one or more clicks. Either sex may give this song from protected perches in the presence of nest predators. These are also known as Anger Songs. Both sexes or either may give the Anger Song in the presence of mammalian predators, usually when only 1 member of pair is on territory. Females and males use Predator Song when their mate is off territory, even when no predator is present. Some observers mistakenly identify Predator Song as Loud Song. The Predator Song may have been the origin of the idea that females “deceive” males into returning to their territories by uttering advertisement song of territorial males (Morton et al. 1978).
Tu-a-wee. Figure 3C and Figure 3D. The most common vocalization of Eastern Bluebirds; loud and low-pitched, with an abrupt beginning; an easy to locate, discrete call. Tu-a-wees of male (Figure 3C) are usually longer than those of females (medialink), a usually obvious visual difference on sonograms; also obvious in the field to some observers (JHP). Uttered by nestlings, most commonly in flight or just before flight. Nestlings utter them near the time of fledging (S. Halkin and PAG unpubl. data). Tu-a-wees are given in all seasons but predominate in the late nesting season and early fall; function as contact calls given while foraging in family flocks during the nonbreeding season. Tu-a-wees are also given by adults as they approach nests to deliver food to nestlings (S. Halkin and PAG unpubl. data). Fledglings and adults use tu-a-wees to signal their locations to other family members (Pinkowski 1971b).
Other Calls. (1) Screech is a single harsh sound, given when individual distance is violated.
(2) Rasp is a loud nasal sound often given with Screech when an individual is alarmed.
(4) Chip is a low-pitched, very soft sound; also called peeping (Pinkowski 1971b); given by females during courtship when approached by males and sometimes preceding copulation. Chips seem associated with a weak tendency to threaten (Krieg 1971). Chips also appear to act as an appeasement signal (PAG).
(5) Turring is a repeated series of single, high-pitched turr notes, used during agonistic interactions, territorial disputes, or when an individual wants to escape being held in hand.
(6) Chatter is loud, continual chit-chit-chit repeated rapidly, during mild alarm in response to ground predators, novel stimuli, or approach by humans to nesting cavities. Bluebirds chatter while Wing-Flicking (see Behavior: agonistic behavior, below). In some cases, chatter seems to indicate that a male “wants” a female to enter the nest cavity (PAG) and may indicate an attack tendency (Pinkowski 1971b).
(7) Alarm Scream is harsh, loud, emphatic vocalization (Pinkowski 1971b) of captured individuals or individuals narrowly escaping capture. During banding, males, more often than females utter Alarm Screams (PAG unpubl. data).
(8) Warble is soft Chatter, often given in conjunction with Wing-Flicking (see Behavior: agonistic behavior, below) at low intensity.
(9) Peep (Pinkowski 1971b) is an abrupt, short call given by nestlings before eyes open in response to bangs or thumps of parents arriving at the nest (see Nonvocal sounds, below). Peeps seem to be feeding invitations, food begging or brooding demands, since they diminish in frequency with onset of homeothermy in nestlings (Pinkowski 1971b). Adult females utter peeps just before copulation; may precede the female's copulation Solicitation Posture (see Behavior: sexual behavior, below), suggesting that a generalized function is to stimulate bodily contact (PAG).
(10) Zeee (Pinkowski 1971b) is a begging sound given by nestlings; longer and harsher than Peeps.
The remarkable intensive sampling study (Huntsman and Ritchson 2002) of the songs of 5 adult male bluebirds demonstrates the difficulty and rewards of documenting individual variation in vocalizations. More studies, particularly of variation in female songs, calls, and vocalizations, as well as of geographic variation in calls and songs, will fill in needed information for a fuller picture of communication in Eastern Bluebirds. Repertoires, although not always obvious to casual listeners, clearly are relatively large for individual males and may be indicators of status, health, or quality of individual males, and may contain information about the types or danger of predators or rivals. Duetting and countersinging have not been observed.
No systematic observations of the development of nonvocal sounds. Nonvocal sounds include bill snaps, which may be ritualized intention movements to bite, occurring when birds dive-bomb human or other potential predators; these bill snaps are agonistic and seem to startle potential predators. Adults make reliably repeated thumping sounds when entering nesting cavities, a by-product of landing on the face of a hollow nesting cavity. There are no discernible differences between sexes in non-vocal sounds.