Cleanliness loses in the hollow: feathers, droppings, and sour air turn a hoopoe nest into a chemical chamber beneath a bird's crownlike crest. The smell is deliberate. During breeding, females and chicks can produce pungent uropygial-gland secretions, and volatile compounds from those fluids help explain why a cavity that looks neglected can function as a defended nursery where airborne cues ling...More