In considering the true status and distribution of Hoary Redpoll, I realized that I had overlooked one significant source of information - the Christmas Bird Count results. While Common Redpoll nests throughout the boreal forests across the continent, Hoary is virtually unknown as a breeder east of Hudson Bay (ignoring the "Hornemann's" Hoaries in Greenland and Baffin Island). Therefore the source of virtually all Hoary Redpolls seen in North America in winter must be northwestern Canada and Alaska, and one might expect that Hoary would be less common eastwards in winter. The CBC results show just such a pattern.
Hoary (right) and Common (left) Redpolls recorded on CBCs in two "invasion" years representative of recent CBCs, 1993-94 and 1999-2000.
From National Audubon Society (2002). The Christmas Bird Count Historical Results [Online]. Available http://www.audubon.org/bird/cbc [4 Mar 2008]
There may still be a lot of identification issues with these data, and the coverage in most of Canada and Alaska is spotty or nonexistent, but one pattern seems clear: The proportion of Hoary to Common redpolls is much higher in the west (Alaska, Alberta, and Saskatchewan) and decreases eastwards.