副羽

[ noun ]
a supplementary feather (usually small) on the underside of the base of the shaft of some feathers in some birds
<noun.animal>

Aftershaft \Aft"er*shaft`\, n. (Zo["o]l.)
The {hypoptilum}.

Hypoptilum \Hy*pop"ti*lum\, n.; pl. L. {Hypoptila}, E.
{Hypoptilums}. [NL., fr. Gr. "ypo` beneath + ? down.]
(Zo["o]l.)
An accessory plume arising from the posterior side of the
stem of the contour feathers of many birds; -- called also
{aftershaft}. See Illust. of {Feather}.