Refers to the feathers of a bird collectively, especially their arrangement or quality. A somewhat literary or ornithological term.
That bird has beautiful plumage.
Variant spelling without okurigana; less common.
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.