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"Ceratorhina monocerata. Unicorn Auk. Horn-bill Auk. Adults in summer: Bill orange-yellow. Culmen and base of upper mandible dusky; feet some yellow color, the tarsi behind and the soles blackish; claws black. The sharp feathers of the head white, about an inch long. Entire upper parts glossy blue-black; a line of white along edge of forearm. Sides of head and neck, of body along under the wings, with chin, throat, and fore-breast, clear grayish-ash, or pale bluish-gray; under parts from breast pure white, shading insensibly into the color of the sides and flanks. Inner webs of wing- and tail-feathers grayish-brown, paler toward base, the shafts of the primaries dull whitish at base." Elliot Coues, 1884

Horn-billed Auk in Summer

"Ceratorhina monocerata. Unicorn Auk. Horn-bill Auk. Adults in summer: Bill orange-yellow. Culmen and…

"Ceratorhina monocerata. Unicorn Auk. Horn-bill Auk. In winter: Bill orange-yellow. Culmen and base of upper mandible dusky; feet some yellow color, the tarsi behind and the soles blackish; claws black. The sharp feathers of the head white, about an inch long. Entire upper parts glossy blue-black; a line of white along edge of forearm. Sides of head and neck, of body along under the wings, with chin, throat, and fore-breast, clear grayish-ash, or pale bluish-gray; under parts from breast pure white, shading insensibly into the color of the sides and flanks. Inner webs of wing- and tail-feathers grayish-brown, paler toward base, the shafts of the primaries dull whitish at base." Elliot Coues, 1884

Horn-billed Auk in Winter

"Ceratorhina monocerata. Unicorn Auk. Horn-bill Auk. In winter: Bill orange-yellow. Culmen and base…

"Ceratorhina monocerata. Unicorn Auk. Horn-bill Auk. Young: Bill like that of adults in winter, lacking horn, but every way weaker, hardly more than half as large. Mostly dark-colored. No white feathers on side of head. White under parts overlaid and marbled with dark-gray ends of the feathers; black of upper parts brownish. The first spring the horn grows, the accessory piece develops, and the plumage clears up. Nestlings are covered with smoky-brown down." Elliot Coues, 1884

Young Horn-billed Auk

"Ceratorhina monocerata. Unicorn Auk. Horn-bill Auk. Young: Bill like that of adults in winter, lacking…