Engineering Acoustics/Resonating feathers of Manakins

=Introduction=

During lekking, Male Club-winged Manakins, Machaeropterus deliciosus (Aves: Pipridae) alter their secondary feathers by hypertrophy. The oscillation of the secondary feathers caused by the cause them to collide and vibrate in order to produce sustained harmonic tones with a fundamental frequency of 1500 Hz. The male manakin produces a totally unique sound in order to attract the attention of the female. Instead of the conventional way of using voice for sound production, he uses his wings as a musical instrument.

=Mechanism of Sonation=



The male manakin modifies the sixth and seventh secondary feathers, which act as a pair of coupled resonators. The five other secondary feathers oscillate in phase with them to result in sonation, which sounds like a ringing Tick-Tick-Ting [1]. The series of motions involves two brief mechanical ticks, when the wings are flicked and a sustained mechanical ting, when they are flipped above the back [2]. These two sounds are not acoustically different except for the longer duration of the ting. Each of them is composed of a fundamental frequency of 1.49 kHz and its higher frequency harmonics.

Tick-Ting video and audio demonstration

Unlike typical secondaries, secondary feathers 1–5 exhibit increasingly wider rachi and an increasingly pronounced transition from continuously tapering to an abrupt taper around the distal two-third, three-fourth and then four-fifth for the third, fourth and fifth secondaries, respectively. Beyond the sudden taper on the fifth secondary, the rachi bend medially. This ‘kink’ in the rachis causes it to overlap and contact the rachis of the adjacent sixth secondary feather, while at rest[3]. The sixth and seventh secondaries exhibit the following distinct modification: the rachi is thick at the base, and at approximately one-half of its length, their width doubles and they twist along their long axis so that the dorsal feather surface is oriented medially. The sixth secondary feather has ridges, while the fifth feather has a curved tip [4]. The innermost pair of the modified feathers (seventh secondaries) collide across the back. Immediately following this collision, the wings shiver laterally and medially, pulling them just millimeters apart. Approximately 8ms later, they are adducted to produce another collision. The sonation tone is produced continuously throughout this process and the feathers generate vibrations at just the right frequency [3].

=Acoustic analysis=

The male Manakin leans forward and flicks his wings together at a frequency which peaks at 1500 Hz and has unusually high Q-values. This frequency is faster than that at which a humming bird beats its wings. The quality factor Q is a measure of the rate at which a system reaches maximum amplitude. Using the spectral method, Q can be determined as

$$Q=\frac{f_0}{BW - 3 dB, SPL}$$

where f0 is the natural frequency of the system and BW – 3 dB,SPL is the bandwidth at 3 dB SPL below the peak. From experiments, Q factor was found to be above 10 for all the feathers and was as high as 27. This implies that the structures can be good biological resonators [3].

=Wing modification=

The extra support required for such a high speed wing movement comes from the super-sized wing bones. The ulna of the male manakin is modified such that it has bumps and grooves to support the wings and its width is fourfold the usual width. Another surprising modification is that the manakin has solid wing bones, as against most birds which have hollow bones which allow them to fly. The wings thus modified are able to create the perfect pitch.

=References=

[1]	K. S. Bostwick and R. O. Prum, "Courting Bird Sings with Stridulating Wing Feathers," Science, vol. 309, pp. 736-, July 29, 2005 2005.

[2]	K. S. Bostwick, "Display Behaviors, Mechanical Sounds, and Evolutionary Relationships of the Club-Winged Manakin (Machaeropterus deliciosus)," The Auk, vol. 117, pp. 465–478, 2000.

[3]	K. S. Bostwick, et al., "Resonating feathers produce courtship song," Proceedings of the Royal Society B: Biological Sciences, vol. 277, pp. 835–841, March 22, 2010 2010.

[4]	http://www.pbs.org/wnet/nature/episodes/what-males-will-do/photo-gallery-manakin-anatomy/955/

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