showed that laying hens learned to avoid
eating loose feathers when they had been soaked in a
bitter-tasting quinine solution. Additionally, when the
feather cover of adult laying hens was sprayed with the
bitter-tasting quinine, the birds learned that feathers from
conspecifics were not attractive to peck at for a period of
time (Harlander-Matauschek et al., 2009).