Bisonic Mechanical Wing Songs and Complex Kinematics in Aerial Displays of the Subtropical Doradito (Pseudocolopteryx acutipennis).

Loud mechanical sounds with a communication role are called sonations. Male Subtropical Doraditos (Pseudocolopteryx acutipennis) exhibit five conspicuously modified primaries suspected of sonating. Here we (1) describe feather modifications, (2) describe three different territorial/aggressive contexts for these sounds: one-perch aerial displays (1PADs), two-PADs, and Chukrut pursuits, (3) investigate the kinematics of the most common display (1PADs) and the physical mechanisms of sonation using synchronized high-speed video and audio, and (4) assess the roles of modified wing feathers in all contexts by experimental manipulation in four individuals. Primaries p3-p7 were modified in adult males but not in females: p3 was pointed with a reduced distal third to the outer vane; p4 and p5 were slim and falciform with pointed tips curved outwards; p6 was broad, massive, and subtly S-shaped, with a spatulate tip; and p7 was large with the distal third of the outer vane abruptly reduced, and the inner vane with a shallow concave sub-apical emargination. One-PADs consisted of perched short nasal introductory syllables accelerating until the bird performed a super-rapid circular flight of ∽15 cm in diameter from and to the same branch, during which two syringeal syllables and three mechanical syllables were given (chik… chik…. chik-chik frrrottt). The syllables were produced during rapid downstrokes by fluttering feathers and were bisonic, being conformed by two simultaneous main tonal, flat, narrow band sounds: a low-pitched note (f0 ∽1 kHz) and a high-pitched note (f0 ∽3.7 kHz). Primary p7 is the necessary and sufficient sound source of the low-pitched note (removal of p7 caused the sound to disappear) and p3 is the sound source of the high-pitched note, being necessary but perhaps not sufficient (removal of p3 caused the sound to disappear); the other modified feathers seem involved in different roles related to either producing the sonation (p4 and p5 interacting with p3) or allowing it (p6 raising dorsally letting p7 flutter freely; removal of p6 did not affect sound production). The specialized shape of p6 might be compromised to allow sonation of p7 without losing flight functionality. Sonations in Subtropical Doraditos occupy the position of the vocal flourish in the songs of other Pseudocolopteryx suggesting the evolutionary replacement of vocal by mechanical sounds. We propose that wing songs in flying birds may be constrained to occur in temporally broken patterns due to intrinsic features of flapped flight and structurally constrained by the demands of creating an airfoil.
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