Owls have nests, but they do not build nests. Most owls will take over someone else's old, abandoned nest.

Small owls will find something like woodpecker nest or the crook of a large branch, and larger owls will find either something like a hawk or eagle nest or a very large old tree that the center is rotted out. Some like Snowies and Short Eareds just find a nice depression on the ground and call it a day. Burrow Owls rely on old badger dens.

Pygmy Owl in a woodpecker nest inside a cactus

Screech Owl in a funky knot hole

Great Horned Owl in Great Blue Heron nest

Great Greys in a hollow treetop

Shortie in the grass

Snowies in a hole

You can see from a lot of these why leaving old dead trees alone is important to owl survival. New trees don't have all this damage that makes it a good home.

They're able to steal nests successfully as their breeding seasons come before the other birds will return to reuse their nests, so the owls basically squat there while the owner is away.

Next we come to the difference between nestlings and fledglings.

Nestlings just have down and can't really do to much. They just chill in the nest and wait to get fed. They don't really have the grip strength to move around yet.

Fledglings are starting to grown in adult feathers and are up and moving around. They can hop out of the nest and start exploring the branches. They begin to flap their wings to develop their wing muscles. They still don't have full length feather or the musculature to actually fly. They'll hop to branches of even to the ground and be able to pull themselves back up into the nest. They're still getting watched over by their parents though. Even once they start to fly the parents generally keep an eye on them for a few months until they're big enough to go out and seek their own territory.

The one in the main photo, is pretty far along the fledgling stage. It has enough feathers it can fly a little bit or at least be doing some enhanced jumping maneuvers. It's doing its roosting behavior that it will do the rest of its life. They only hang out in a nest when they're babies or taking care of babies. They sleep out in the open on branches. Their feet lock in the closed position so they don't fall off when they are asleep. The babies and young ones sleep with their heads down like this as they haven't built up enough muscle to stay upright. Adults will sleep sitting up.