pterencephalon
It’s all with respect to humans. Humans aren’t making the bird nests, so they’re natural, not man-made. Our houses don’t over naturally, we build them.
From the bird’s perspective, sure, nests might be bird-made and humans are part of nature. But at humans, we’ve also done a ton to shape the world and separate ourselves from nature. If your house were a fire-heated lean-to in the woods, there might be less a distinction between it and “out in nature,” but if you’re living in a city or town, your immediate surroundings probably have been heavily constructed and modified by humans.
It looks like corroded metal, maybe?
When we were looking to buy a house, I basically crossed off the list anywhere that I couldn’t walk to at least some essentials, like basic groceries, pharmacy, a couple restaurants. Our new neighborhood isn’t nearly as walkable as where we used to rent, but everyone else heae seemed to have the same thoughts, and it’s too expensive to buy a house there.
… Paint?
Well, they didn’t put carpet on the bathroom floor. But they’ve arguably found something far worse.