From Old French oisel +‎ -illon.

• IPA^(key): /wa.zi.jɔ̃/

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oisillon m (plural oisillons)

1. young bird; chick, fledgling, baby bird, little bird

   Au pied du hêtre, j’ai trouvé un oisillon qui était tombé de son nid. ― At the foot of the beech tree I found a baby bird which had fallen from its nest.

• oiseau

• “oisillon”, in Trésor de la langue française informatisé [Digitized Treasury of the French Language], 2012