The avian nest protection hypothesis explains the widespread behavior of incorporating fresh green plant material into an otherwise dry nest matrix as an evolutionary adaptation. Species that exhibit this behavior tend to breed in previously inhabited nest sites and experience high ectoparasite and pathogen loads. Metabolites in the green plants control parasite and pathogen populations and decrease the effects of these agents on nestlings. The choice of plants is not arbitrary. In the case of European starlings, a nonrandom subset of available vegetation is selected, and it is these plants that have the largest impact on ectoparasite and pathogen populations. Experimental and observational evidence suggests that starlings select plants on the basis of chemosensory cues.