A few days ago, I posted some Hoary and Common Redpoll pictures here, taken at the feeders behind The Owl's Roost, which is the name of Whitefish Point Bird Observatory's gift shop.
Here are some more photos from the same location, taken today.
The snowy-white bird pictured above is of course a Hoary Redpoll. Note, besides the pale appearance and the short, stubby bill, the fine streaking on the sides and the almost complete lack of streaking on the undertail coverts.
Of course, subtle differences in bill size and shape between Hoary and Common Redpoll are difficult to judge on a single individual. Side by side, however, these differences are quite visible. Note that this is a different individual from the one pictured above. This is an adult male - with a pink wash across the breast - while the top bird may be an adult female or an immature.
Often, a first clue to a bird being a Hoary candidate is the difference in overall color. Note how pale this individual is in comparison to the birds surrounding it. An unstreaked rump and the differences noted above will clinch the identification.
About a year ago, David Sibley posted similar photos on his blog, which can be seen here. Sibley's discussion is a lot more precise and detailed than mine here, and centers on separating various subspecies of Common Redpoll, and how they compare to Hoaries and to each other. He specifically talks about 'Greater' Common Redpolls (Carduelis flammea rostrata), a larger and darker subspecies that breeds on Baffin Island and Greenland, and irregularly winters south to the northeastern US (Sibley) but more commonly in northern Canada (Knox & Lowther 2000).
Today I noticed some size differences between the redpolls in the flock at Whitefish Point. Note for example the obvious size difference between the bird on the left back ridge of the bird bath, and the bird to its left (for us right). This putative 'Greater' Common Redpoll is not only bigger, but also more heavily streaked on the sides.
Here's another photo of a Common Redpoll (center) that appears bigger than the others. This bird, however, is a different individual, because its crown is not as red as the previous bird. It tends more toward orange.
And here's another 'Orange-crowned' Redpoll. This also is a Common Redpoll whose 'poll' isn't red, but orange. According to the Common Redpoll BNA account, the "crown [is] shiny, bright poppy red, sometimes more orange or even gold" (Knox & Lowther 2000). I had never seen it before, but apparently this is not all that rare.
Here is a shot of that bird preening. Also conspicuous on this individual is the relative lack of brown tones in its plumage. Still, several field marks points in the direction of Common Redpoll: fairly heavily streaked undertail coverts and rump; relatively longer bill; relatively slender neck. Observe how the streaking on the flanks is less heavy than on the previous 'orange-crowned' Greater Common Redpoll and that structurally the bird seems much more delicate, less bulky.
Knox, Alan G. and Peter E. Lowther. 2000. Common Redpoll (Carduelis flammea), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/543
Sibley, David A. 2008. Sibley Guides Notebook, online only: http://sibleyguides.blogspot.com/search/label/redpolls