Colorado (along with much of the northern US, to judge by eBird maps) is experiencing a massive irruption of redpolls this season, with flocks of dozens or even exceeding 100 being found in a state where usually just finding any is noteworthy.
On 29 December, I was chuffed to see several redpolls on my thistle feeders (in SW Longmont), new for the yard and only the third time I'd seem then in Boulder County. During a sharpie-induced lull I set up a photo blind to get some shots. When the birds returned my attention was pretty quickly drawn to a small-billed, overall pale individual with clean undertail coverts (save one dark streak on the outermost undertail covert, OK for Hoary.)
My initial impression, followed by hasty discussions with knowledgeable friends, led me to suspect the bird was a Hoary Redpoll (though not an example from the "cleanest" end of the spectrum.) The bird returned with about 20 commons the next morning, when it was seen by a handfull of additional observers braving redpoll-friendly single-digit temperatures. During this time we confirmed that the rump was also very clean, a feature Steve Mlodinow captured in a photo. Unfortunately all of the redpolls departed by the afternoon of the 30th. As Colorado has no accepted records of Hoary Redpoll, I wanted to dig deeper for ID confirmation than just field guide plates.
Fortunately David Sibley has done a lot of legwork with redpolls, and I found his compillation of identification notes most helpful. This compendium is well worth reviewing if you are seeing redpolls or to prime yourself for a possible encounter with them. I would also highly endorse reading and reflecting on Kim Eckert's article on redpoll ID from The Loon (the Minnesota Ornithologists' Union journal.) I also thank Steve Mlodinow, Daniel Gibson, Peder Svingen, Tony Leukering, and Kim Eckert for their helpful expert feedback on this bird.