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plural tertials
(ornithology) Any flight feather attached to the humerus (upper arm) of a bird. quotations
He continues — "There was the same rear up, with the head bent down, followed by an upjerk of the hind parts; the long sickle-shaped tertials, so noticeable in this species, seemed little if at all expanded, […]
1923, Arthur Cleveland Bent, Life Histories of North American Birds: Wild Fowl, page 76