![]() Previous work by Lieser, Berthold & Manley (2005) did not succeed in finding any of the low-frequency components described by Moss & Lockie (1979) based on the vocalizations of only one male. Grouse (Tetraonidae) have an enlarged oesophagus which inflates like a balloon during mating vocalizations ( Moss & Lockie, 1979) and functions as resonating chamber to amplify their signals ( Ziswiler & Farner, 1972). In addition to the influence of body size on the vocalization of grouse, other morphological adaptations can affect the resulting design of produced signals. The validity of this rule has been found not only in songs ( Azar & Bell, 2016 Brenowitz, 1982 Ryan & Brenowitz, 1985) but in calls as well ( Azar & Bell, 2016 Billings, 2018 Martin et al., 2011). ![]() As a matter of general bio-acoustic principle, larger animals including birds tend to produce calls of lower frequencies than smaller ones ( Fletcher, 2005 Wallschläger, 1980). The appearance of these low-frequency signals was especially observed in non-passerines of larger body size. All these calls are low-frequency calls, yet they do not fall into the real infrasound range (with frequencies below 20 Hz). Various versions of low-frequency signals also are produced by doves, e.g., the song of the tambourine dove reaches a minimum frequency of 280 Hz ( Osiejuk et al., 2019) or in the spotted dove 524 ± 36 Hz (mean ± SD) ( Guo, Bonebrake & Dingle, 2016). ![]() ![]() Corncrakes are known to produce low-frequency soft calls which largely overlap with the frequency spectrum of the background noise ( Rek, 2013 Rek & Osiejuk, 2011). Boom calls of the Great Curassows ( Crax rubra) have frequencies of maximum amplitude between 100 and 150 Hz ( Baldo & Mennill, 2011), booms of the Houbara bustard ( Chlamydotis undulata) with fundamental frequency ranging between 40 Hz and 54 Hz ( Cornec, Hingrat & Rybak, 2014), those of the Eurasian bittern ( Botaurus stellaris) ranging from 86.6 Hz to 248.5 Hz ( Puglisi et al., 2001). Cassowaries produce booming calls with a minimum frequency down to 32 Hz in the Southern Cassowary ( Casuarius casuarius) and 23 Hz in the Dwarf Cassowary ( Casuarius bennetti) ( Mack & Jones, 2003). Only a few bird species are known to produce very low frequencies. Such frequencies (infrasound) were documented in whales ( Berta, Sumich & Kovacs, 2006 Mourlam & Orliac, 2017), elephants ( Larom et al., 1997 McComb et al., 2000 McComb et al., 2003), tigers ( Von Muggenthaler, 2000), rhinoceroses ( Policht et al., 2008 Von Muggenthaler et al., 2003). While the uses of sound occurring in the range of human hearing have been intensively studied, and similarly studied sounds above the upper limit of human hearing (e.g., ultrasounds in bats, toothed whales, rodents, frogs and insects), very little is known about the sound below that range. ![]() Because these low frequency components temporarily overlap with the Whetting phase, they are hardly audible from a distance larger than several meters. The occurrence of such low frequencies is surprising as this grouse is substantially smaller than cassowaries (Southern cassowary Casuarius casuarius and Dwarf cassowary Casuarius bennetti), the species that produces similarly low frequencies. We discuss a possible function of low-frequency components that remains unclear. The resulting model of discrimination analysis classified 67.6% vocalizations (63%, cross-validated result) correctly to the specific individual in comparison to the probability by chance of 12.5%. These low-frequency components temporally overlap with the Whetting phase (96% of its duration) and they significantly contribute to the distinct vocal expression between individuals. We analyzed the display vocalizations of Western Capercaillie males kept in breeding centers and identified harmonically structured signals with a fundamental frequency of 28.7 ± 1.2 Hz (25.6–31.6 Hz). Only a few bird species are known to produce low-frequency vocalizations. ![]()
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