summer plumage; nuptial plumage
Ornithological term for the breeding-season plumage of birds, often brighter or more colorful than winter plumage.
When they get their summer plumage, the males' feather colors become vivid.
A characteristic of this bird's nuptial plumage is that its head turns black.
Compound of 夏 (summer) and 羽 (feather/plumage). The reading なつばね is a straightforward kun'yomi combination.