Rock Pigeons possess a modified region on their outermost primary feathers (P10) that flutters to produce tones during high-powered downstrokes. This morphology may be evolved to communicate alarm to flockmates during take-off.
In spite of being the narrowest of all the Rock Pigeon's flight feathers, barbs in a particular region of P10 are the longer than those found on any other feather. Here, barb lengths are compared between P10 (left) and P9 (right) at different points along the length of the feather (20%-80% of its total length). Around 60% of its length (circled and magnified), barbs are extra long and curved at their tips (magnified circle; indicated in red). This creates a region of lower stiffness that readily flutters above certain velocities during downstroke (see supplemental video 1 below). This flutter is required for normal sound production in take-off.
Supplemental Video 1
To evaluate an individual feather's ability to sufficiently replicate the tones produced during flight, we spun P9 and P10 feathers at average and peak down-stroke velocities, emulating flapping flight. P10 feathers do not flutter or produce tones at the average angular velocity of downstroke (38.1 rad s-1). (Filmed at 3000 frames per second; audio played-back real time; video on loop). P10 feathers begin to flutter and produce tones at or slightly above the peak angular velocity of downstroke (53.9-60.3 rad s-1). (Filmed at 3000 frames per second; audio played-back real time; tone frequency of 500Hz; video on loop). P9 feathers never produced tones.
Supplemental Video 2
P10 in a wind tunnel where flow velocities mimic the average wing-tip velocity of downstroke (11.4 m s-1) and peak wing-tip velocity of downstroke (16.2 m s-1) both before and after the application of an aerosol fixative. Flutter occurs primarily in the region of P10 identified in our morphological investigations (see text) between 50 and 70% of the inner vane length. The feather tip is just out of view in the bottom left corner of each video. Video recorded at 6000 fps in four conditions: 11.4 m s-1 without hairspray (no flutter; no tones); 16.2 m s-1 without hairspray (flutter and tones!); 11.4 m s-1 with hairspray (no flutter; no tones); 16.2 m s-1 with hairspray (no flutter; no tones).
Supplemental Audio 1 (Audio corresponds to Figure 5 of the text). Red highlighted region notes the presence of tonal elements in wing sounds prior to manipulation and the absence of tonal elements after manipulation. Tonal elements in A have a fundamental frequency around 0.65 kHz and up to five harmonics. Vertical, broadband sounds are wing claps in both A and B. In B, the 4.5 kHz tonal sound between 22.7 and 22.9s is a House Sparrow (Passer domesticus) call, not a tonal element of Rock Pigeon wing sounds. Hann window size = 1300 samples; 2048-sample FFT frequency grid.