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Message: <blockquote data-quote="chris butterworth" data-source="post: 3278610" data-attributes="member: 64315"><p>The rather "jagged" border between the light and dark areas, the obviously darker flanks making the white undertail coverts stand out separately from the rest of the underpart,  the appearance of a thickish, short bill and the suggestion of a pale base to the lower mandible in #5 of your second post look good for Cassin's - or juvenile Rhinoceros Auklet. There is something about the shape of the bill ( giving the appearance of a curved, not straight culmen ) that suggests Rhinoceros, to me. Both Murres would lack the depth of the dark breast and #3 + #4 show the dark breast to be "U" shaped, not a shallow inverted "U" (Common) or inverted "V" (Thick-billed), neither would show dark flanks ( unless it was one of the very rare dark morph Commons found in CA ) and they wouldn't seem so stocky. Moulting Marbled Murrelet would be showing much more white on the underparts,  plus distinctive pale scapulars if the belly was already white.</p></blockquote><p></p>