The nest protection hypothesis (NPH) is one of multiple hypotheses that seek to explain the behaviour of birds repeatedly introducing green, often aromatic, plant material into the nest after its completion and throughout the incubation and nestling periods. The hypothesis suggests that this behaviour is an evolutionary strategy to ward off or kill ectoparasites that would otherwise cause higher nestling mortality through blood loss and the spread of pathogens. The aromatic species of greenery are often collected from trees and long-living shrubs containing strong aromatic compounds which are expected to either disrupt olfaction in host seeking parasites or kill harmful parasites and pathogens.