Fledging is the stage in a flying animal's life between hatching or birth and becoming capable of flight. This term is most frequently applied to birds, but is also used for bats. For altricial birds, those that spend more time in vulnerable condition in the nest, the nestling and fledging stage can be the same.

What do u mean by fledge?

transitive verb. 1 : to rear until ready for flight or independent activity. 2 : to cover with or as if with feathers or down. 3 : to furnish (something) with feathers feather an arrow.

Why is it called fledging?

To leave the nest: nestlings that are about to fledge. [Probably from obsolete fledge, feathered, from Middle English flegge, from Old English *flycge; see pleu- in Indo-European roots.]