Complex sentence: Breeding adults have a plumage that includes a broad black head and neck with a greenish, purplish, or bluish sheen, blackish or blackish-grey upperparts, and pure white underparts except some black on the undertail coverts and vent.
Split into simpler sentences: Breeding adults have a plumage that includes a broad black head and neck with a greenish, purplish, or bluish sheen. It has blackish or blackish-grey upperparts, and pure white underparts except some black on the undertail coverts and vent.