In birds, the smaller secondary shaft or branch of a feather that arises from the main shaft, carrying its own barbs.
Compound of 'after' and 'shaft' (Old English sceaft). This ornithological term has been used since the 1800s to describe feather anatomy.
The aftershaft in birds creates their incredible waterproofing and insulation—it's essentially a hidden feather within a feather, and different bird species have different aftershaft sizes depending on whether they need to stay dry or warm.
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