Do birds ever sit down?

yeah

The closest they get is being REALLLLLY fluffed up, and covering their feet with their fluffy feathers. But still, they are merely perching while keeping their feet warm. They don't sit, as it wouldn't be comfortable for them to do so. They sometimes will rest one leg at a time, though.

Birds nesting is similar to them sitting, but their feet (especially pet birds from what I know) naturally grasp onto whatever they are on, meaning that they sleep in the same position they are usually in while awake

Look up Ostrich pictures when they're resting.

Parrots will only truly 'sit' when nesting, but even then, it's less sitting and more crouching. Parrots are always on their feet and spend their days on branches.

Yes. My chickens love to sit in the yard and take dust-baths.

they do

yes, like these two

they nestle down... like standing, but you cant see there legs

sometimes when my parrot sleeps he hides he's one leg inside he's fur and the other to stand idk its probably comfortable but they do sit i think