Embryo study #1: Flight Training
Songbirds, hawks, and many birds look naked on hatching. But look closely and they are covered by natal down all over their bodies that they grew when they were in the egg. In the nest about a week or so later, they sport newly grown thermal down. Then, in three or four weeks, juvenile feathers replace the thermal down. These three molts happen before the young birds even leave the nest.
Below is an easy to read article about feather formation from the Raptor Research Project: What are feathers? What is molt?https://www.raptorresource.org/2019/05/19/what-are-feathers-what-is-molt/
Much more detailed information about feather growth is in this article, Development, Regeneration, and Evolution of Feathers
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5662002/