Fig. 22.—Feathers illustrating conditions where barbicels are unnecessary and 
are hence reduced or entirely lost, causing downiness. 3/5 natural size. 
(a) Primary of Pigeon—an important flight-feather; hence possessing a stiff’ 
vane. (b) Under wing-covert of a Great Blue Heron; downy portion was over- 
lapped by the adjoining feather. (c) Wing-covert of Owl; the downy edge makes 
possible the all-important noiseless flight of this bird. (d) Feather of Ostrich; 
the power of flight being lost, the feathers are downy throughout the entire vane. 
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