Two days ago I was birding Cherry Creek Reservoir looking for the Yellow-billed Loon that was previously reported. About half way across the southern portion of the reservoir a loon was spotted. Because of heat waves the only impression that registered with me was the head was light brownish and the bill, appearing through shimmering light, was light colored.
I checked with Cornell Lab of Ornithology, Birds of North America series (an excellent resource) to see what the latest word was in distinguishing the difference between the Common and Yellow-billed Loon. Here is a brief summery of what I found
Because of sexual and age variation in size between the two loon species, it would be challenging just using size as a distinguishing characteristic for either loon. Also because of the subtle variation in plumage for juveniles and non-breeding adults for both loons relying on appearance is tricky although could be highly suggestive for the Yellow-billed
"Only characteristic diagnostic in all plumages and ages is color of culmen (ridge of upper mandible). In Common Loon in winter plumage this ridge and about 5 mm on either side is black (in breeding plumage the entire bill is black), but in Yellow-billed Loon at least the distal half and usually distal two-thirds is whitish-yellow" (Common Loon BNA series)
Bob Righter
Denver, CO
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