Most birds are unlikely to be physically capable of producing low frequency sounds, because of their small size and lightweight. Few exceptions concern large species like capercaillies and capercaillies that have been shown to produce sounds with frequency components close to the infrasound domain. As very low frequencies propagate over long distances with little attenuation, they are optimal for long-range communication, such as occurring in species displaying in leks in the reproductive context. One example of such species is the Houbara Bustard (Gruiform, Otidae), a large size bird, in which males perform elaborated courtship displays including booming calls, in peculiar leks sites spaced by at least 500 m in semi-desert open areas. It is a polytypic species, separated on the basis of molecular, morphological, and behavioural criteria: Chlamydotis macqueenii and Chlamydotis undulata. The populations of these 2 species are now separated and were in the past likely to be sympatric in North Africa. In this context, the present study aims to: 1/ characterize the sounds produced by males booming and to quantify inter- and intra-individual variability in C. undulata and C. macqueenii, 2/ to estimate parameters potentially involved in the coding of species specificity and individuality. Our results of acoustic analysis in the temporal and the frequency domain of calls of different males (15 C. undulata and 12 C. macqueenii) show that the 2 species produce species-specific booming calls with components at very low frequency, using a modifying and enlarged trachea. The inter-individual and inter-species differences are discussed in the context of intra-sexual competition and inter-sexual attraction at long range.
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