Song. Referred to as the Dawn Song by Bent (29) and Pieplow (136), the “mating song” (137), or the Regular Repeated Vocalization (RRV) by Smith (17). The song of Eastern Kingbird (Figure 5A) is a fairly stereotyped series with a twitter of variable length followed by a higher pitched, buzzy dzee note, and finished with a two-syllabled, clearer underslurred call that is slightly lower pitched, the whole phrase described as "dzit dzit dzit DZEE-tyurrit" by Pieplow (136). It is sung only by males, and generally only in predawn or (sometimes) evening periods (18). It is distinguished from other Eastern Kingbird vocalizations by continuous alternation of 2 complex phrases t't'tzeer, t't'tzeer, t'tzeetzeetzee (Figure 5A), continuous repetition, long duration, and timing (usually only predawn).
Dzeer. A very high-pitched (7 kHz), short (0.2 s) buzzy note that can be downslurred or evenly-pitched (Figure 5B; repeated vocalization [RV] of 17). Dzeer calls can be given singly or in a series, and tends to be the most commonly heard call of the species. Used in a variety of contexts, including aggression.
Twitter. A high-pitched (5–6 kHz), fast (up to 20 notes/s) series of slightly buzzy notes that can be given by either males or females, sometimes in agitation. Length highly variable, and male versions sometimes end in a dzeer note. Called the “kitter-call” in Stokes (138), and versions ending in a dzeer note the chatter-zeer or compound vocalization [CdV] of (17) (Figure 5C).
Interaction Call. Called the Tick-it Series in Pieplow (136), or the “kitter-call” of Stokes (138), and chatter-zeer or compound vocalization [CdV] of (17) (Figure 5C). The Interaction Call is a high-pitched (5–6 kHz), rapidly repeated 2-part or 3-part call that can last for many seconds, and is given under wide variety of circumstances involving interactions among individuals (see below), often accompanied by wing flutters.
There is very little information on the timing and maturation of vocalizations, or extent of vocal learning, but young begin to give calls like those of adults at about 2 wk of age (17, MTM). Kingbirds are suboscine passerines; song-learning has not been demonstrated in suboscines (but see 139), so it is doubtful that kingbirds learn vocalizations.
Eastern Kingbird vocalizes infrequently during migration. After territories claimed, however, males vocalize frequently while patrolling boundaries. Number of vocalizations more tightly linked to breeding cycle than calendar date. Males begin dawn song soon after arrival on breeding grounds, but not before females arrive (L. J. Redmond, personal communication). Once population begins to sing, failure of a male to sing more common in isolated males and late in the breeding season, but unaffected by nesting stage or fertility of either a male’s mate or availability of fertile female in the population (19). In dense populations, dawn song (RRV) may be sung throughout breeding season (17, 19), and as late as mid August (K. Russell, personal communication).
Males and females call frequently to one another using Twitters as greetings when they have been apart, and to maintain contact. Both birds are generally quiet during incubation, but become more vocal during nestling period (17). After young have been out of nest for 1–2 wk, they also call frequently, and family units can become loud (17, MTM). At all points in breeding cycle, parents respond to predators by vocalizing with dzeer, but responses to other kingbirds decrease late in summer, with probable exception of RRV.
Daily Pattern of Vocalizing
Full description of all vocalizations given by Smith and Smith (18). Dawn song (RRV of 17) delivered in predawn darkness, typically beginning 1 hr before first light, but much variation exists (88–37 min before dawn; 19). Smith (17) stated and Sexton (19) demonstrated that males in low-density populations may not sing this vocalization (17). Once started, dawn song lasts for up to an hour, but usually just over 30 min. Start time of dawn song for individual males repeatable between years and highest quality males are earliest singers (20).
Dawn song (RRV) ends ca. 30 min before dawn, and marked by a change in vocalizations to the Twitters as males patrol territory. Replaced by dzeer within about 30 min. Dzeer gradually becomes more common than Twitters and is given more frequently for remainder of day unless encounters occur among individuals. In general, the midday hours are characterized by few vocalizations. End of day uncommonly marked by a few RRVs.
Places of Vocalization
RRV given from a prominent perch usually located 30–50 m from nest, but occasionally much farther away (> 250 m) in low density portions of population (L. J. Redmond and MTM, personal observations). Other songs given while perched or during flight (17, 138, MTM). Songs and calls, when delivered from perch, usually given from exposed site such as utility wire or top of medium-sized tree or shrub.
Repertoire and Delivery of Songs
Males vocalize more than females. Individual variation in repertoire size unstudied, but being suboscines, likely limited or nonexistent. Dzeer and Twitters vary in length and loudness. Much information transferred through both vocalizations, but interpretation also depends on visual signals and behavior of other individuals. Smith (17) found no evidence of geographic variation in songs or calls. Age-related changes unstudied.
Singing rate (i.e., number of 2 phrase t't'tzeer, t't'tzeer, t'tzeetzeetzee/min) during dawn song (RRV) varies considerably among males and at peak singing, an average of 14 t't'tzeer, t't'tzeer, t'tzeetzeetzee phrases are sung per minute (but can exceed 20/min; 19). Individual differences in song rate repeatable among males between and within years and likely associated with male quality (20).
Social Context and Presumed Functions of Vocalizations
Interpretations of calls based on 17, 18, but interpretation of dawn song (RRV) based on 19 and 20. Twitters are complex vocalizations emitted by both sexes, but mostly by males. Used extensively in encounters between individuals (mates, parents and offspring) and in patrolling territory. Meaning varies with context of encounter, but seems to (1) identify caller as an Eastern Kingbird, and (2) indicate that caller is willing to approach intruder and defend territory. Commonly gives dzeer while guarding nest or if passing raptors have been spotted (but not chased). Indicates general attentiveness, and is often delivered by male when female departs from vicinity of nest. Observations and playback experiments show that territorial Eastern Kingbirds are much more likely to approach Twitters than dzeer (18). Both signals may represent “honest signals” of intent. T-zee used as part of Tumble-Flight Display and when an individual begins to lose an encounter; context suggests “frustration.”
Dawn song (RRV) is given only by the male, and while possibly directed towards other males, within-pair and extra-pair mates are the most likely targets. The earliest breeding females pair with the most rapidly singing males, suggesting that females may use song rate to identify and pair with quality males (20). However, male song rate is reduced during the social partner’s fertile period (19), and a male’s extra-pair mating success varies inversely with dawn song start time; i.e., the earliest singing males most successful (2). The former suggests song does not function as a social stimulus for the social partner, while the latter suggests that extra-pair females identify extra-pair partners using dawn song start time. Continuation of dawn song until almost the end of breeding, well after the end of territory establishment, is also inconsistent with it functioning mainly for intrasexual communication and territory establishment. The ability of 2 mute males to acquire territories and mates also suggests that dawn song is not performed primarily for among male or possibly even within social pair communication (19).