Sounds

“A veritable Ishmael among the waterfowl, his spirit, both of courage and of mean cowardice . . . ” (Peabody 1896: 2; see also Bent 1921: 235). Extensive description of adult and juvenile vocalizations by Hall (1998) forms the basis for much of the discussion below and no attempt is made here to distinguish calls from songs as characteristics and uses of some vocalizations make them difficult to attribute in this manner. Descriptive names based on proposed function. Comparative information on vocal repertoire of Forster’s Tern in relation to documented repertoires of other terns (subfamily Sterninae) also can be found in Hall 1998 .

Development

Semi-precocial juveniles a few days old voice a general Discomfort vocalization, though the same vocalization also can be heard emanating from embryos as early as the early pip-stage of hatching (McNicholl 1983, Hall 1998). Discomfort vocalization shares structural and use characteristics with the Alarm vocalizations of older juveniles, fledged individuals, and adults (Fig. 2A). As a result, these 4 vocalizations may represent steps in ontogeny of the same vocalization. Begging vocalization sounds, and used in situational contexts (such as food begging), similar to adult Submissive vocalization (Fig. 2B). If Common Tern juvenile begging call (Burger et al. 1988) can be used as a model for Forster’s Tern, then individual figures (sound which produces a single, complete, and distinct impression [Davis 1964]) of juvenile vocalization are of comparable duration but of harsher tone compared to adult Submissive vocalization. In general, tonal (sound energy concentrated within a narrow frequency band) elements seem lacking in Forster’s Tern juvenile vocalizations, whereas both tonal and harsh (sound energy spread out over a wide frequency band) elements contribute to adult vocal repertoire.

Vocal Array

Nine adult and 4 juvenile vocalizations (based on individuals from a Columbia River island colony in s.-central Washington during breeding season ) described by Hall (1998), who also provides time-frequency audiospectrograms for all 9 adult vocalizations and 1 juvenile vocalization. Some adult vocalizations may represent derivations or graded variants of other vocalizations. Geographic and intra- and inter-individual variation in vocalizations not described, except for limited data provided by Hall (1998) on inter-individual variation in the Advertisement vocalization. See Social context and presumed functions of vocalizations (below) for sexual differences. Vocal repertoire most similar to repertoire of Common Tern (Hall 1998). Previous descriptions of Forster’s Tern vocalizations used mostly phonetic descriptions that preclude exact comparisons. Of these earlier works, Bent 1921 and Cramp 1985 most comprehensive. Only other published audiospectrogram is in Cramp 1985, the identity of which was questioned by Hall (1998: 402). All vocalization descriptions below are based on Hall 1998, unless otherwise noted. Dominant frequencies (most intense sound energy band) are identified as the biologically relevant signal rather than fundamental frequencies, which generally are derivable from the information provided.

Advertisement. Delivered as single figure (Fig. 2C), repeated at spaced intervals, or in combination with series of Agonistic vocalizations (Fig. 2D). Either 2 structurally distinct segments (part of figure [Davis 1964]) given without a discernible intervening interval (Fig. 2C) or 1 segment that is structurally similar throughout the figure except for shift in dominant frequency (Fig. 2D). Initial portion of vocalization tonal in frequency structure. Dominant frequency of tone peaks at about 3.0–3.5 kHz and continues thereafter as declining frequency sweep that may end >1 kHz below peak frequency. Brief frequency upslur at beginning of vocalization may be present. Two or 3 additional frequency bands evident with harmonic interval of about 1.5 kHz. Second segment, when present, tends to be harsher and may be trilled (temporal breaks present in frequency trace). Total figure duration variable (about 0.2–0.4 s) and, for 2-segmented vocalizations, first segment typically longer than second.

Recognition. Similar to tonal segment of Advertisement, but trilled and briefer (0.15–0.25 s). Dominant frequency peaks at about 3.5 kHz with frequency drop of up to 1.8 kHz and 2–3 additional frequency bands at harmonic interval of about 1.5 kHz. Each figure contains 8–15 pulses trilled at a rate of 50 pulses/s. Vocalization tends to be repeated with interfigure interval greater than duration of individual figures. Melodious, non-harsh (musical) quality mostly lacking in other vocalizations of this species. Vocalization may be unique to Forster’s Tern among terns.

Contact. Tonal in structure and relatively soft-sounding. Dominant frequency of about 5.0–5.5 kHz with 2–3 additional frequency bands at harmonic interval of about 1.5 kHz. Figure duration is 0.008–0.012 s with silence interval, when figures repeated, of >10 times figure duration.

Pair-Bond. Standard figure structure does not seem to be present as various components are of different duration and frequency spectrums. May sound “soft,” “twangy,” or “buzzy” and seems voiced at low amplitude compared with other vocalizations (as a result, often masked by other vocalizations occurring throughout colony). Structures of different figures that constitute vocalization suggest relationships to both Advertisement and Submissive vocalizations.

Submissive. Tonal in structure with figures repeated in rapid succession and resembling rough chevrons (Fig. 2B). Dominant frequency peak generally occurs near middle of figure at about 3.0 kHz; beginning and ending frequencies occur at about 2.7 and 2.3 kHz, respectively. Two strong and 2 weak additional frequency bands evident at harmonic interval of 1.5 kHz. Figure duration typically about 0.065 s; however, duration may shorten as an individual’s motivation changes from fear to threat (Fig. 2B). Interfigure interval about 0.12 s.

Alarm. Harsh and raspy sounding. Dominant frequency occurs at about 3.5 kHz, but declines by almost 1 kHz toward end of vocalization (Fig. 2A). Definitive additional frequency bands lacking as sound energy dispersed over a wide frequency spectrum. Figure duration variable (about 0.2–0.5 s). Vocalization delivered at rate of about 120 pulses/s, which may contribute to vocalization’s harshness.

Agonistic. Harsh sounding; often delivered in association with Advertisement vocalization, immediately following and repeated (Fig. 2D). Dominant frequency occurs at about 2.7 kHz; frequency drop at end of each figure may be present. Definitive additional frequency bands lacking. Figure duration variable (about 0.08–0.18 s) with interfigure interval of about 0.06 s. Each figure contains 7–20 pulses trilled at a rate of about 100–110 pulses/s.

Threat. Structurally similar to Contact vocalization, but less tonal (that is, energy of dominant frequency and each additional frequency band spread over wider frequency spectrum) and delivered as rapidly repeated string of clicks (individual figures; Fig. 2B). Dominant frequency occurs at about 4.0–4.5 kHz with at least 2 additional frequency bands evident at harmonic interval of 1.5 kHz. Figure duration about 0.01–0.02 s with interfigure interval of about 0.08–0.16 s. Threat and Contact vocalizations may represent graded variants of same basic vocalization type with structural differences imparted by differences in motivational state of caller.

Swoop-and-Soar. Similar in structure to Alarm vocalization, but tends to be longer in duration. Delivered as caller passes through nadir of dive, which may impart frequency change in vocalization due to Doppler effect (McNicholl 1971, Hall 1998).

Discomfort. Relatively short in duration, soft, and high-pitched. Possible representative vocalizations recorded in Manitoba 0.01–0.21 s in duration and often given repeatedly with 0.38–0.81 s between sequences (McNicholl 1971).

Begging (Chick Begging of Hall 1998). Sounds similar to adult Submissive vocalization, but weaker. Possible representative vocalizations recorded in Manitoba were 0.2 s in duration, separated by 0.7 s (McNicholl 1971).

Juvenile Alarm (Chick Alarm of Hall 1998). Harsh, about 0.6 s in duration, and with dominant frequency of about 5.0 kHz.

Post-Flight Juvenile Alarm (Fledgling Alarm of Hall 1998). Similar to adult Alarm vocalization. Possibly the 0.3–0.5 s “loud screams” with 0.2–0.7 s intervals recorded by McNicholl (1971) given by juveniles when picked up and/or longer (0.63 s) vocalization given by flying juvenile that sounded different from those picked up, but looked identical (except for duration) on an audiospectrogram.

Geographic Variation

None described.

Phenology

Only major study (Hall 1998) during breeding season; however, potential progression of vocalization pattern during breeding season not described. Studies of vocalizations needed during nonbreeding season. See Social context and presumed functions of vocalizations (below) for sexual differences.

Daily Pattern

Not described.

Places Of Vocalizing

Vocalization can occur while in flight, on the ground, or even just emerging from the water following a dive, within the colony or on foraging areas, and standing near or sitting on the nest.

Repertoire And Delivery Of Songs

Vocal repertoire descriptions are provided in Vocal array (above) and below.

Social Context And Presumed Functions

Adult vocalizations can be grouped into the following functional categories: approach and recognition (Advertisement and Recognition), caller localization and pair or group maintenance (Contact and Pair-Bond), alarm/appeasement (Submissive), and alarm/threat/aggression (Alarm, Agonistic,Threat, and Swoop-and-Soar). All social context and presumed function information provided below based on Hall 1998, unless otherwise noted.

Advertisement. Often, but not always, associated with food provisioning (usually a fish). Given when approaching colony or nest site with food, during courtship flights (with caller carrying food), when leading young from or to nest site (caller may carry food but often without), and calling to young that are away from immediate vicinity of nest site. Functions principally to advertise arrival of individual from a distance and probably to provide information as to species and individual identity.

Recognition. Given by adults when approaching nest site and when calling to their young. Similar to Advertisement vocalization (above), this call seems to provide information as to caller’s identity; call not associated with food provisioning.

Contact. Given when individual either preparing to change location (e.g., when leaving nest site) or has just completed change of location (e.g., following landing at unoccupied nest site). Also used in association with Advertisement vocalization, as part of vocal communication with young, and by male during courtship. Provides information as to caller’s location without necessarily specifying individual identity; indicates to others caller is nearby. May be accompanied by Bent Posture (see Behavior: sexual behavior, below).

Pair-Bond. Given when individual has just landed at nest site with its mate present, when first meeting its own young or mate in other contexts, during courtship (including in flight), and while sitting on nest without any apparent stimulus. Lowers agonistic tendencies each time contact established between members of mated pair or between parent and its young. When voiced on ground, associated with Bent Posture (see Behavior: sexual behavior, below).

Submissive. Given when gulls (predators of eggs and young) approach colony or individual, when humans approach colony once young present, as general reaction to non-specific disturbance, during food-begging from mate, and as part of pre-copulatory behavior by females. Functions as alarm call in which primary motivation of caller may be fear and as appeasement call used between mated pair to enable solicitation of either food (by both sexes) or copulation (by female). As alarm call, functions either to sound general alarm or to elicit mobbing reaction against particular predator. Forster’s Tern may be unique among terns in use of Submissive vocalization as an alarm call; additionally, in most other terns this vocalization is female-specific. When used as appeasement call that is voiced on the ground and off the nest, associated with Hunched Posture (see Behavior: sexual behavior, below).

Alarm. Given when non-mate conspecifics or individuals of species other than gulls (including humans) approach nest site or colony and as general reaction to non-specific disturbance. Functions as alarm call in which primary motivation of caller may be to express low-intensity threat to conspecific or to intruder of different species; also may function as general alarm call. When voiced on the ground, associated with Oblique Posture (see Behavior: agonistic behavior, below).

Agonistic. Given when conspecific intruder approaches nest site of another pair; also used in combination with Advertisement vocalization when the latter used while approaching colony or nest site with food, a vocalization combination shared only with Arctic and Common terns among terns. Functions as alarm call in which primary motivation of caller may be to express a high-intensity threat to conspecific. Forster’s Tern may be unique among terns in directing this vocalization only toward conspecifics. When voiced on the ground, associated with Erect Posture (see Behavior: agonistic behavior, below).

Threat. Often occurs as vocal transition from Submissive vocalization (Fig. 2B) during interspecific encounters; also perhaps given in association with Advertisement vocalization and occasionally when individual leaves its nest site (2 social contexts in which Contact vocalization is used). Functions as alarm call in which primary motivation of caller may be to express high-intensity threat, usually to individual of different species but also during conspecific agonistic encounters.

Swoop-and-Soar. Used in conjunction with human presence within or near colony; however, presumably can be directed toward any potential ground predator. Functions to signal aggressive behavior on part of caller; may serve to distract potential predators away from eggs and young.

Discomfort. In addition to early pip-stage embryos, downy young, up to 2–3 d posthatch, voice Discomfort vocalization when picked up or when begging (McNicholl 1971).

Begging. Given by young when begging food from parent and in direct response to parent’s Advertisement vocalization. When given by young in marsh, it may help parents locate young hiding in floating vegetation away from nesting area (MKM).

Juvenile Alarm. Given when individual disturbed by intruder, especially when handled by human intruder (Harris 1933, McNicholl 1971, Hall 1998).

Post-Flight Juvenile Alarm. Given in situations similar to when adult Alarm vocalization used.

None known.