Here is an animal that has all kinds of signals of being a juvenile, outside its bones, inside its bones, in its joints, and it has long, isolated plumes extending from its already-very-long tail. This is quite different from living birds and tells us that these decorative feathers preceded adulthood in dinosaurs. Of course, perhaps they're using these feathers in a very different way from living birds, too.

Ashley Poust

I don't think we know yet how it used its feathers, it seems likely that they helped with temperature regulation and signaling to other animals, but what this would have looked like and how much these functions mattered remains unclear.

Living birds shoot up to adult size very quickly, mainly as a way of getting strong enough to fly as soon as they can. But they may delay getting their adult feathers for a long time. Gulls, for example, don't look like adults for three or four years even though they learn to fly in only three months.