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Calls described as trumpeting, bugling, rattling, or croaking, but these adjectives do not fully convey the volume or quality of the sound produced by a mature Sandhill Crane. In cranes of the genus Grus, and birds of several other genera (including anthropoidea), the trachea extends (coils) well into the sternum; an anatomical feature that expands the amplitude and alters the pitch of the voice by the addition of harmonies (Niemeier 1979). From a day or so preceding pipping until about 9 to 10 mo of age, young cranes utter a variety of peeps and trilled whistles. The change to the adult voice is preceded by a protracted process of tracheal elongation and intrasternal coiling. Adult voice is acquired at about 10 mo (Tacha 1988).
Complete adult vocal repertoire may exceed a dozen vocalizations. Calls can be grouped into 3 groups: trills and purrs, loud rattles, and non-rattle calls.
Trills and purrs are the first vocalization to develop. In chicks Contact calls “a steady trill lasting 0.2 to 0.4 s…given 1 or 2 times a second” appear before hatching (Voss 1976). Pre-hatching call is slower, with a lower fundamental frequency than those given later; given almost continuously from hatching until adult voice acquired, but of such low amplitude it is rarely heard in the wild. Several other purr-like calls given only by chicks. A short, sharp trill or peep, the Pipping call, is a comparatively low frequency call given only during pipping. Stress call, first heard during pipping, is an ascending, loud, unvaried peep of about 0.2 s duration, delivered 1/s. Chicks also give Food Begging peep delivered about 3/s; first appears ≤ 2 d hatching. Older chicks (> 9 wk of age) participate in Guard calling and Unison calling episodes with their parents (SAN). In chicks, the Guard call is a high-pitched, broken sound first ascending then descending in pitch, lasting about 0.45 s (Voss 1976). In adults, purrs are 4 to 12 muffled click-like sounds delivered at a rate of 12 to 20 notes/s (S. A. Nesbitt and R. A. Bradly, unpubl.). Flight Intention purr is a 2-part call with a rising inflection; Feeding purr is the slowest of the purr calls and the Growl purr is given as part of the Low Bow display (Voss 1976, Nesbitt and Archibald 1981). Pre-copulation calls and nesting calls (Voss 1976) are also purr calls. The loudest, highest-pitched, most rapidly delivered purrs are Flight purrs (S. A. Nesbitt and R. A. Bradly, unpubl.).
Loud Rattle calls: 7 to 20 notes delivered at rate of about 15 notes/s (S. A. Nesbitt and R. A. Bradly, unpubl.). Mean fundamental frequency for 2 such calls (Unison and Guard calls) approximately 0.56 KHz for males and 0.93 KHz for females (Weekley 1985); can be heard up to 4 km away. In flight these have rising or falling inflection, with a higher fundamental frequency than those given on the ground; generally given enroute to and from feeding, loafing, and roosting locations (S. A. Nesbitt and R. A. Bradly, unpubl.). Guard call (Fig. 3, top), a slow even rattle, is given when “the bird is afraid but not afraid enough to flee” (Archibald 1975) or in unison by a pair or family group as an aggressive display. When delivered in flight, as a pair flies over a defended territory, the call has a distinct antiphonal quality. The Unison call (Fig. 3, bottom ), is a sexually stereotypic duet call. The male part is lower pitched and more drawn out than the female’s staccato tuck-a-tuck-a-tuck-a .
Adults also give several non-rattle calls; e.g., during bouts of Bill Sparring, an aggressive behavior (Nesbitt and Archibald 1981), a series of loud rapid slurred notes covering a wide frequency range, and lasting < 5 s (S. A. Nesbitt and R. A. Bradly, unpubl.). A short purrt, 4 notes rising in pitch separated by unequal intervals, alerts others in a flock or is given as birds take flight or land. When in flight or on the ground, yelps consisting of 3 or more high-pitched, broken notes and a sustained following note, may serve as location calls (see Voss 1976) or to maintain or reestablish contact between flock or pair members separated by a greater distance than can be bridged by purr calls (S. A. Nesbitt and R. A. Bradly, unpubl.). Moans and goose-like honks are often interspersed among the Flight calls and are intermediate between the full-voiced call and purrs. During attacks, both among conspecifics or toward a potential predator, the attacking bird delivers a loud hiss. A shrill, diminuendo tremolo with a descending inflection, is given by cranes in distress, as when captured. “Snores” have been reported from cranes on roost (L. H. Walkinshaw pers comm.) and are produced by orally tranquilized cranes (SAN).
Territorial advertisement calling increases with onset of nesting season and declines through incubation. Other social calling increases during late summer and fall, as cranes become more gregarious, and is sustained through the winter, until migration and nesting begin.
Daily Pattern Of Vocalization
Birds most vocal from about 30 min before sunrise through mid-morning, during late afternoon feeding periods, and during arrival at roosts or pre-roost gatherings.
Duet calls (Unison and Guard calls) advertise territorial occupancy and boundaries and serve to avoid conflict among territorial adults. In other aggressive situations such vocalizations are used by the dominant pair or individual to intimidate intruding birds and drive them away. The Guard call is given also when birds (individuals, pairs, or flocks) have located a potential predator and are “mobbing” it (S. A. Nesbitt and R. A. Bradly unpubl. data).