Feathers usually grow in FEATHER TRACTS with spaces between called APTERIA, where there are only scattered feathers. Penguins and screamers are completely feathered birds. Adult ostriches are completely feathered but their embryos have feather tracts, so perhaps being completely feathered is a secondary characteristic in birds.
Look at the diagram of a feather in a textbook. The vanes zip up like a host of tiny zippers, which is what happens during preening.
Black-crowned Night Heron