There are birds that do not buy a mattress; they pull it straight from the source. Sparrows are among those opportunistic animals that look at a sheep grazing peacefully and think: there is free material.
With their beaks, they take tufts of loose wool, the ones already falling off the animal’s body, and fly straight to their nest. Wool traps heat better than any little twig, so the eggs and chicks are protected as if in a natural duvet.
Meanwhile, the sheep keeps chewing grass, unaware that it has just donated thermal insulation for an entire family of sparrows. In nature, nobody pays rent, but everyone finds a way to charge something.
