Booming in cassowaries:
Pigeons can detect ultra-low frequency sounds those produced by crashing waves and thunder storms, for example-that are useful in navigation. Indeed, low frequency sounds can travel very long distances with relatively little attenuation. This same property makes low frequency vocalizations ideal for conveying information in densely vegetated habitats. Until recently, however, only one bird, the Eurasian Capercaillie (Tetrao urogallus), was known to produce ultra-low sounds. According to researchers working in Papua New Guinea we should add cassowaries (Genus: Casuarius spp.) to this list.
Sound recordings of Dwarf (CC bennetti) and Southern Cassowaries (C. casuarius) reveal that the booming calls of both species contain sound energy in very low frequencies-23 Hz and 32 Hz, respectively. Given the thick understorey habitat occupied by cassowaries, the authors suggested that these ultra-low frequencies are used as long-distance signals. Furthermore, low-frequency booming is well suited for species that vocalize near ground level (see IBB 3:1).
In light of this new information the authors believe that the large keratinous casque common to all cassowaries (see figure) may function as a listening device. Accordingly, the "casque in living birds is spongy and resilient" and within its deeper regions there is an ample deposit of "darkly pigmented sludge." They speculate that the semi-solid interior of a casque could be responsive to the low-frequency vibrations generated by booming calls. True or not, they argue that existing explanations as to the purpose of the casque - e.g., to attract the opposite sex, to wield as a weapon when fighting, to scrape the leaf litter or to protect the head as a bird crashes through vegetation - can all be easily refuted by watching how cassowaries behave in the wild.
Script: Courtesy of 'Interpretive Birding', A science magazine for birders.