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Peculiar modifications of certain primary feathers are worthy of mention and are very reminiscent of those noted by Zimmer (1939) for C. subbrunneus (Fig. 1). The twisted orientation of the leading edge of the wing is immediately obvious and very like that noted for C. subbruneus (Zimmer 1939). Primary 10 has a strongly curved leading edge. Primaries 9 through 5 have noticeable emarginations, least exaggerated on primary 9. There is a "bulge" of slightly decomposed barbs on primaries 8, 7, 6, and 5 more basal than the emarginations. These are similar to decomposed barbs on primaries 7, 6, and 5 of C. subbrunneus. In addition to the unique primary modifications, both species of Cnipodectes share a darkened, longitudinally raised "ridge" that is a narrowing of the vane of the inner webs of primaries 8, 7, and 6 (and 5 in C. superrufiis). The shaft structure of these primaries also has the same T-shaped cross-section noted by Zimmer (1939) for C. subbrunneus. Although the evidence is not conclusive, some or all of these feather modifications may be involved in the production of mechanical sound (see below).
Mechanical noises have been noted in both species of Cnipodectes. Hilty and Brown (1986: 483) note that C. subbrunneus "can produce a very audible pr'r'r'r'r'r' in flight with wings." This description is similar to mechanical sounds T.V.H. witnessed, performed by C. superrufus at Kirigueti. In response to playback, Valqui observed the bird approaching aggressively, making an accelerating and decelerating buzzing noise, while flying ca. 4 m off the ground. Although it was not clear what the source of the sound was (and it was not tape recorded), it seems likely that it was mechanical and produced by the wings.