neoptile

From neo- +‎ -ptile, literally "new down".

neoptile (uncountable)

1. The down or light, fluffy feathers on a young bird.
   • 2012, Barn Owl Trust, Barn Owl Conservation Handbook: A Comprehensive Guide for Ecologists, Surveyors, Land Managers and Ornithologists:

     Newly hatched young are naked and pink, but within a few days a short white down known as the neoptile unfolds from the skin (Kniprath & Stier 2008).

   • 2017 June 28, Hanneke Meijer, The Guardian‎^[1]:

     Among the neoptile feathers occur isolated bristle-like filaments, whose origin and function are unclear.

• pleonite