'''Fret marks''' (stress lines) appear as clear breaks in the feather colour as a result of interruption in feather growth and malformation of the feather barbs. Nutritional, environmental, disease or drug stresses can result in the production of 'fret marks'. | '''Fret marks''' (stress lines) appear as clear breaks in the feather colour as a result of interruption in feather growth and malformation of the feather barbs. Nutritional, environmental, disease or drug stresses can result in the production of 'fret marks'. |