noun
Refers to the feathers of a bird collectively, especially their arrangement or quality. A somewhat literary or ornithological term.
その鳥は美しい羽並みをしている。
That bird has beautiful plumage.
羽毛 refers to feathers as a material or covering, while 羽並み emphasizes the arrangement or overall appearance of the feathers.
Compound of 羽 (は, 'feather') and 並み (なみ, 'arrangement, line'). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.