The Hawaiian subspecies, designated by the subspecies name knudseni, differs from the mainland form in having more extensive black on the sides of its neck and face (see photo of mainland Black Necked Stilt below). The difference seems pretty subtle to me. The Hawaiian name is Ae'o. The subspecies is endangered.
From summitpacific.com: Hawaiian Stilts are endangered due to hunting, loss of habitat, environmental contaminants, and introduced predators such as feral cats, rats, mongoose, and bullfrogs. Other avian predators include owls, herons, and Cattle egrets. Hunting or shooting the Hawaiian Stilt is illegal.
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