On: Feathers are one of evolution's cleverest inventio...

[sampo]

> it seems hard to evolve in a gradual way

There seems to be 4 different hypotheses. So there is no consensus on this question.

https://en.wikipedia.org/wiki/Origin_of_avian_flight#Hypothe...

A video about the "wing-assisted incline running" hypothesis:

"The Origin of Flight--What Use is Half a Wing?" https://www.youtube.com/watch?v=JMuzlEQz3uo