Yellow-billed Loon in winter plumage is lighter coloured (light brown) than Common Loon (dark grey), particularly on the crown/back of the head. The bill of Yellow-billed Loon also often has an upturned look, is yellow at the tip (not entirely yellow in winter) and is usually longer than the Common Loon bill. The Yellow-billed Loon also has a distinct spot behind the eye.
Yellow-billed Loon is extremely rare, the only record in Ontario from e-Bird is in Sault Ste. Marie, New Year 2009/10.
/Thomas