Feathers grow from follicles on the skin, emerging from within a protective sheath before the vane we know and love expands. You can see that most of the feathers on these nestlings are only just starting to emerge from these tube-like sheaths.
But you can see that the feathers don’t grow everywhere. They grow in distinct tracts over a bird’s body called pterylae (pteron for “feather,” and hulé for “forest”). So while a bird appears completely covered, the feathers are actually growing from specific areas on the body to cover all the gaps. At the base of the feathers are muscles that allow birds to raise them—fluffing up to trap air and stay warm or release heat trapped under the feathers when it’s hot—or lower them to maintain aerodynamics and protect the body.
Because these pterylae leave bare skin in between (apteria, “without feathers”), this has its advantages for research. Birds have such thin skin that when we catch one and hold it carefully with its belly up, we can blow lightly on its belly to part the feathers and see the muscle and fat stores below the skin! This is one way for us to gauge the condition of birds non-invasively, particularly on migration when they’re fattening up in preparation for flight or depleted after one. Take a look at the second video to see this in action as I check out the fat and muscle of a post-flight Blackpoll Warbler!