Feathers are unique to birds. Today April 9 chick #1 known as Sunday is one day shy of being four weeks old. She (?) has begun to show feathers pushing up through her second coat of down. You can see the black looking specks that are these feathers. In the closeups you can see the feather shafts (quills) that the wing feathers are growing out of. The body feathers (contour feathers) are growing out of the same follicles as their natal down, thus the feathers have white tips. This down will fall off.