Thursday, January 31, 2008

Upcoming events in early 2008

Feb 24 - I'll be speaking at the Northwest Branch of the Corpus Christi Public Library on February 24th at 4 PM to help celebrate the opening of their new Clif Moss Nature Education Center. Some details here. And if you're in the area that weekend check out the Celebration of Whooping Cranes in nearby Port Aransas.

March 21 - I'll be in Irvine, CA for the Annual Dinner of the Sea and Sage Audubon Society, and on the following morning I'll be presenting a workshop on drawing birds. Details here.

March 29 - I'll be among a group of authors at the Newton, MA Public Library for their "Spring Fling" Here's the blurb from the Library's website:
It’s not too early to dream of spring, so mark your calendars for the Newton Free Library’s annual Spring Fling gala event, Saturday, March 29, 2008, 7:00 pm, at the library. This library lovers’ evening will honor regional authors including co-authors Jody Adams and Ken Rivard, Joseph Finder, Rebecca Goldstein, Nathaniel Philbrick, Rishi Reddi, and David Allen Sibley. Enjoy conversations with the authors, a literary-themed silent auction, excellent food, and good music. You will have the opportunity to purchase the honorees’ books and have them personally signed by the author. Tickets go on sale in February. For more info, contact Devra Kiel Simon, Director of Development, 617-796-1407 or
May - I'll be participating in a Fossil-Fuel-Free birdathon supporting Malkolm Bothroyd's Bird Year as part of the Bird Day Challenge, which you can read about here. And I'll also be participating in BirdStudies Canada's Baillie Birdathon. I will fill in more details on these events later. If you'd like to sponsor me in either of these efforts please use the links above for info.

Monday, January 14, 2008

Urging caution when identifying Common Redpolls

Redpoll identification is challenging because Hoary and Common Redpoll seem to show an unbroken continuum of variation from pale to dark, and there are no fully reliable differences. So birders have to rely on a subjective assessment of overall color and struggle to define the threshold for confident identification. Virtually all birders see redpolls only in the winter, where identification is reduced to an utterly one-sided question: "Where can we draw the line so that we are sure the accepted records of Hoary Redpolls are correct?"

Since the goal is high confidence, this leads to a narrow definition of Hoary Redpoll - only the palest (and smallest-billed) birds are named and reported as Hoaries. This leads to two problems (the second more serious). First, we only identify part of the population - some darker Hoary Redpolls are excluded. Second, many of those intermediate birds are simply lumped into our broad definition of Common, and receive no extra attention. There's nothing wrong with excluding some intermediate birds from being called Hoaries, as long as birders understand that some Hoaries are being excluded. This is conservative. But most people give redpolls very uneven treatment by demanding that Hoaries meet high standards, and then calling everything else Common. This is not conservative. We should at least be fair and apply equally strict criteria to our Common Redpoll identifications.

By excluding some darker Hoary Redpolls (and calling them Commons) this uneven treatment prevents us from understanding Hoary Redpoll as a population.

Some might argue that this approach is correct, but it is not supported by Troy's (1985) research. Troy found that all redpolls breeding in taiga/forest habitat were uniformly in the darkest one-third of all redpolls he studied (with no intermediates at taiga sites). Redpolls breeding at tundra sites, on the other hand, were primarily in the paler two-thirds (from intermediate to very pale). If we start with the hypothesis that Common Redpoll is a dark southern population with its core nesting range in the forest and Hoary is a paler tundra-breeding population, then the intermediates, which Troy found only in the tundra habitats, seem most likely to be either hybrids or darker variants of Hoary. Calling these intermediate birds Commons would imply that Common Redpoll is represented by dark birds in the taiga and paler birds in the tundra. This seems less parsimonious and, if true, suggests that we should consider the possibility of two subspecies of Common Redpoll along those lines. I don't know of any research that would help resolve the status of these intermediate birds.

So we need two different approaches to identification. The traditional conservative approach is fine if we acknowledge its limits and we are just trying to be confident about adding Hoary Redpoll to a list. But if we are trying to understand Hoary Redpoll as a species, we need a more inclusive definition that will identify the whole population. In the short term, I think we need to use the "intermediate" label more, calling lots of birds "intermediate" until we get a better understanding of where to draw the line.

I realize that the birds in my redpoll photos are outside of the traditional "safe" zone for Hoary Redpoll, but it's misleading and obscuring to say that these birds are just "pale Commons". I am convinced that the birds I photographed are not just Commons and they presumably come from tundra habitats where Hoary nests. Whether they are good Hoaries or hybrids/intergrades is the next question. But we can't even begin to answer that question if we just dismiss them as non-Hoaries. We need to spend more time studying these intermediate birds, and resist the urge to stick arbitrary labels on them, if we are to have any chance of answering the bigger questions about redpolls.

1) Matt Sharp suggested that the differences between two of the redpolls I photographed and called Hoary-type and Common, are simply age-related. I have very little experience judging tail feather shape so I don't want to attempt ageing, but if there is a difference between these two birds I would reach the opposite conclusion to Matt: it looks to me like the paler bird has more pointed outer tail feathers than the darker bird and is therefore more likely to be a 1st-winter, ... but I'll defer to banders' opinions on feather shape!

Regardless of feather shape, I can't accept that age alone explains the plumage differences. This pale bird was one of two or three similar individuals among 75 to several-hundred Redpolls I saw that day. These pale birds stood out subtly but distinctly from the darker brownish Commons. There was certainly a range of variation in color among the Common Redpolls, but these few birds still stood apart. They were paler overall than any female-type Common, with a "colder" gray tone, had slightly finer and paler streaking on the flanks (index score 3+), mostly white rumps with just a few dusky marks (index score 4+), less distinct streaking on the head and neck, and less brown on the back. And they often gave the impression of a slightly broader (fluffier) neck and fluffier forehead than the typical Common.

2) James Smith has posted photos of some other pale redpolls he saw along with the previously-reported Hoaries on Cape Ann; and Marshall Iliff photographed one obvious and five probable Hoary Redpolls on Cape Cod last week. In both cases these are birds that, like my New York photos, stand out among Common Redpolls but are not quite pale enough to reach everyone's threshold of "safe" Hoary. So let's call them intermediates until we have a better understanding of variation. I think they're closer to Hoary than to Common, so it would be wrong to call them Common with no further discussion. If anyone with more redpoll experience is willing to offer an opinion on any particular bird the discussion could be very helpful.

For comparison, here is a Swedish bird carefully-diagnosed as a 1st winter Hoary (so I'm not the only one calling such dark birds Hoaries).

3) On bill size as a distinguishing feature of redpolls: look at this page of variation in bill size of European Hoary Redpolls (scroll down for Hoary). Here is photographic evidence that bill size is not very useful for identifying Hoary and Common Redpoll, and this answers the question I posted previously about bill shape - Hoary can have a bill just as slender as the typical Common.

4) I agree with Ron Pittaway's point on ID-Frontiers that Redpoll identification is especially challenging in photos, since the differences are so slight and are strongly affected by lighting and image processing. The most useful photographs are often those with multiple birds rather than full-frame portraits, and I would urge photographers to try to include several redpolls in each image so that any Hoary candidates can be compared to adjacent birds under identical conditions.

Tuesday, January 8, 2008

Comments on two redpolls

I'm reposting here some comments I just sent to ID-Frontiers, about a pale redpoll in Illinois and another in Massachusetts:

The comment under these photos on the ILBIRDS forum giving the bird an index score between 11 and 12 is, I would say, overly generous. The undertail coverts that we can see (not much) seem to show a small dark streak on one of the smaller undertail coverts coming up along the side of the tail. So the score for that would be at most 3, and more likely 2. The flank streaking looks solid and broad, with about equal dark and white bands, so I would put that between 2 and 3. Only a tiny fragment of the rump is visible (the index is only meant for the rump, not back color). What we can see is a few gray marks on the lower rump, which puts it at most 4, and possibly 3 or even lower. Note that redpolls tend to have more white on the lower rump than the upper, so it's possible that this bird could move its wings and reveal a heavily streaked upper rump like index 2. So I would say the highest possible score for this bird would be ten, and more likely something like 7.5.

If we can use Troy's numbers with this index, this bird should be called intermediate, and closer to Common. All that being said, the frosty-gray back is another feature suggesting Hoary, so maybe that tips the balance a little more towards Hoary. This is a bird that, if seen in northern Alaska, would probably blend just as easily with a flock of Hoaries there as it does with Commons here. I don't think we should call it a Hoary, but at the same time it would be incorrect to dismiss it as a "pale Common". It is intermediate.

For comparison, check out this photo recently posted in Massachusetts. This one scores higher on the index ( I'd say undertail coverts 4 or higher, flanks 4, rump not visible). Furthermore, the back is pale and grayish, and it looks pale overall. I would feel pretty comfortable calling it a female Hoary.

One other comment:
I see a lot of blog visitors coming from "across the pond", where there is a very interesting ongoing discussion of a pale redpoll on Birdforum and the Surfbirds forum. I'm sure it hardly needs to be said, but I would be cautious about applying ID criteria from one side of the ocean to the other. These birds are regionally variable - and the species differences we are talking about are so slight - that a small shift in the average paleness of one population will confuse the issue. The questions are basically the same, but the precise details of the answer may be different in Europe and North America. Just something to keep in mind.

Monday, January 7, 2008

More Bird-friendly Window Treatments

Just a quick update on my window-collisions research:

I've heard from several more people who have tried the highlighter with pretty good success.

Continuing my own experiments here I've had over a month of 100% success with an array of monofilament stretched between two wooden boards outside the window. The monofilament is suspended a couple of inches in front of the window in vertical lines about three inches apart. It is very inconspicuous from inside, and has worked perfectly under some very busy conditions. I have some other experiments lined up to try, including a more streamlined version of the monofilament.

I also wanted to pass along a very promising suggestion from Diane Salter of Canada, who says:

Attached is a photo of something I've been using on my windows which is very effective. My house has windows totally covering front & back. Most of the windows I netted (as I have wooden frames) but that was impossible on sliding doors & the very high upper windows.

This is the clear (with patterns) wrapping that stores use for gift packages. It sticks by static (on the inside) & can be removed easily by just pulling it off. I found that the light coloured patterns work best. It makes a visible barrier & doesn not block your view out the window altho not as beautiful (or deadly) as clean sparkling windows. It comes in big rolls, around 40" wide & a roll will do lots of windows. My house has 36 major windows + 2 sliding doors & I had plenty in one roll. It needs replacing every couple of years as it starts to lose its grip.

It really does work. I rarely have bird strikes & when I do it's only a glancing blow. Currently I have 80+ redpolls, 100+ juncos & numerous other species at my feeders.

Finally, to report some negative results, about four weeks ago I tried drawing vertical lines on the outside of the glass with a black permanent marker, but that had no effect - still lots of collisions. It might work in some situations, but not here.

Saturday, January 5, 2008

A Character Index for Redpoll identification

[edited 12 and 14 Jan, 2008, adding comments about the index]

I've been thinking about redpolls more, and had another, more careful, look at Troy's 1985 paper, which is filled with interesting observations and some serious "food for thought".

First of all, Troy identified redpoll specimens using a character index, which is a well-established method of objectively sorting out variation when there are no obvious discontinuities. I don't know why I never thought of this before, because the character index is probably the best way to classify redpolls, and it can certainly be used in the field.

I've done a series of sketches (below) showing the range of coloration of undertail coverts, flanks, and rump. I have tried to match Troy's published reference photos. You need to see each feature well enough to match it to a number, then you can add up a total score - a sort of "paleness index". Troy simply divided the range of scores into thirds: the darkest third he called Common, the palest third Hoary, and the middle third he left unidentified. This is arbitrary, but at least it's somewhat objective. Numerically, a male (pink breast) with a score from 14 to 18 was called a Hoary in Troy's paper, a score from 3 to 7 would be Common, and from 8 to 13 intermediate. For females and immatures (no pink) it only took a score of 11 or higher to qualify as a Hoary, since no female-type scored higher than 13, and a score of 3 to 6 indicated a Common; leaving 7 to 10 intermediate (I'm using the straight numbers, Troy adjusted so that the lowest score was 1).

Important: Since I have sketched my own character index, even though it is based on photos in Troy, it will give different results, and the biases of different observers will inevitably skew the results, so we can't expect Troy's numbers to work universally with my index. Also, remember that redpolls become significantly darker as their feathers wear, and Troy's study was done in summer when birds would be at their darkest. In mid-winter we should expect all the redpolls to be a little paler, and it might be necessary to adjust the index a bit.
[12 Jan - Ron Pittaway on ID-Frontiers has emphasized the seasonal changes in redpoll plumage and questioned the value of this index, but I think the darker appearance of redpolls in worn summer plumage would come almost entirely from the upperparts. The streaking features being scored by this index should not change very much. That would be easy to test with specimens. In the meantime I think the index is still a very useful tool for objectively comparing redpolls, but of course we still need to work out what the index numbers mean.

14 Jan - And another caution about using the index: Declan Troy commented that he never intended the index to identify species, only to provide a measure of variation in plumage. Then by dividing the whole range into thirds he could test pale, dark, and intermediate groups for differences in size.]

I hope this index will provide a starting point for discussion, and I encourage people to try it out, comment, and suggest revisions.

1) The most interesting observation in the whole paper, I think, is the fact that all redpolls from study sites in the taiga (boreal forest) were dark (low-scoring) Common-types. No intermediate or Hoary-types were found at those sites. At tundra sites most redpolls were either intermediate or Hoary, with small and variable numbers of dark birds scored as Commons. Troy found no location with exclusively pale Hoary-type birds, although a couple of far-northern sites were close to that (and in those cases it's possible that his method of simply dividing the scores into thirds was too conservative, and maybe all those intermediate birds are good Hoaries). Places like Churchill, Manitoba, with a mixture of forest and tundra habitats, show the entire range of plumage, while places farther away from the forest had a higher proportion of Hoary-types.

So, based on this, one way to look at the redpoll situation would be to name a consistently dark, southern, forest population (Common) and a paler, northern, tundra population (Hoary). In this arrangement, any redpoll paler than a typical Common could be assumed to have a tundra origin and one could make an argument for calling them all Hoary. This may seem like an extreme suggestion, but it is less extreme, and more defensible, than the current status quo, in which the Hoary name is reserved for only the very whitest individuals, and ambiguous pale birds are casually and confidently (and falsely) called "pale Commons".

One of the contradictions I have been trying to resolve in this discussion is my observation that a lot of the Hoary-type redpolls I have seen at places like Barrow and Gambell, Alaska, would not be pale enough to satisfy most observers as Hoary in the lower 48. There do seem to be true intermediate redpolls, so I don't really advocate calling all the pale birds Hoary, but calling them "pale Commons" is even worse, and simply reflects a narrow southern bias.

2) Note: There is the possibility of subtle regional variations within North America, but this has not been studied. Complex regional variations from Iceland across northern Europe makes the situation much more complicated there.

3) The three characters used by Troy - streaking on undertail coverts, rump, and flanks - are all related, so combining all three is probably not much more powerful than using a single one. Adding independent features to the identification process - especially back color and pattern - might significantly increase the separation of two species. I haven't put up an index for back color yet because it is more subjective than the three streaking features, but I'll work on that and other potential features to add. Anyone using the index should also look at back color, back pattern, fluffiness of forehead, paleness of neck sides, and perhaps other things to supplement the streakiness score.

4) Like other studies, Troy found no real difference in bill length. Actually, he did find a significant but very small difference in female bill length (premaxilla length), the difference between males of the pale and dark types was not significant. So there is a slight tendency for pale Hoary-types to have shorter bills, but it should not be emphasized as a field mark (compare the short-billed and long-billed Hoary-types in my previous post). I would compare it to the difference in bill size between Thayer's and Iceland Gulls: we get the impression that there is a difference between the two populations as a whole, but it is so small and there is so much overlap that it has very little value for identifying an individual bird. The fluffiness of feathering around the base of the bill is probably a better feature to focus on, but that is also variable, and can be tricky to assess depending on angle of view and the bird's attitude.

5) Also very interesting... At his only study site east of Hudson Bay - Fort Chimo on the northern coast of Quebec - Troy found consistently dark Common-types, averaging just slightly paler than the birds of interior Alaska but none as pale as Hoary-type. Since this is about as far north as one can get in Quebec, this suggests that pale Hoary-types may be scarce or absent east of Hudson Bay, and that the pale redpolls appearing in the east may come from farther to the northwest. Just north of Quebec, on Baffin Island, the larger Hornemann's (Greenland) Hoary Redpoll nests. So I wonder what the status of Hoary Redpoll is in mainland Canada east of Hudson Bay?

Troy, D. M. 1985. A Phenetic Analysis of the Redpolls Carduelis flammea flammea and C. hornemanni exilipes. Auk 102:82-96. pdf here

Thursday, January 3, 2008

Parula Hybrids?

I have had two inquiries about Tropical Parula in the last 24 hours, prompted by a bird currently in Austin, Texas. You can check out that bird in photos by Andy Balinsky and photos and some background by Martin Reid. It has the throat pattern and extensively yellow underparts of Tropical Parula, but the pale eye-crescents and the extensively green crown of Northern.

In The Sibley Guide to Birds I illustrated a drab Tropical Parula with faint pale eye crescents, which is now the focus of questions. That illustration is based entirely on one specimen which was collected in southern Texas (and is currently at the Academy of Natural Sciences in Philadelphia). I strongly suspected that it was a hybrid, and discussed it with Jon Dunn, who also looked at the specimen and included a special mention of it in his and Kimball Garrett's Peterson Guide to Warblers. We both concluded that it could be a hybrid but could also be just a pale and drab variant of Tropical, since the only thing that looked odd for Tropical was the drabness and the eye crescents.

So when it came time to paint those illustrations and reduce the text to fit, I simply called it a variant of Tropical, didn't mention the hybrid possibility, and left out all the background info.

Since 2000 the evidence has piled up suggesting that Northern and Tropical Parulas do hybridize (see especially Tony Gallucci's blog post from 2005). Martin Reid also has photos of a male Tropical-like Parula with faint white eye crescents from southern Texas in 2005. I have not seen any similar specimens or photos from outside of the geographically limited area in southern Texas where the two species meet, but Tony Gallucci's map suggests tantalizingly that he has found intergrades far to the north and east of Texas. I hope we will learn more details of that soon.

From Louisiana this week, Erik Johnson has posted photos of a pure-looking Tropical Parula, but he includes the comment "many, if not most, Tropical Parula sightings outside the normal range (i.e. in Louisiana) have some evidence of Northern Parula genes or characteristics."

So I would now say that the bird I illustrated in the Guide is a hybrid, and that the Austin bird (which matches my illustration closely) is also most likely a hybrid.

And of course this just brings up more questions:
How widespread is the introgression? or How many US Tropical Parulas are pure?
Is the northeast Mexican population of Tropical Parula just part of a cline between Northerns and true Tropicals to the south? (Tony Gallucci has some interesting comments on this)
Should they be considered all one species?


Wednesday, January 2, 2008

A couple of Redpoll additions

On the MOU website: a Hoary photo Here from Minnesota (where Hoary is expected and the threshold is low) of a pale Hoary-type with a relatively long bill

Some other recent Hoary photos from Michigan - scroll left and right to see about 5 shots

From Ontario this photo and the two following show a very small-billed Hoary

The Redpoll Questions

More thoughts on redpolls after yesterday's post, prompted by my own nagging doubts and a query from Nick Anich: In all of this redpoll ID discussion, I think people ask the general question "How can I identify a Hoary?" when what they mean is the more specific question "Just how pale/small-billed/unstreaked does a redpoll have to be before I can feel good about calling it a Hoary?"

I don't have the answer.

We know that there is a continuum of variation from the palest Hoary to the darkest Common, and that in the middle there are some (many?) intermediate (possibly hybrid) individuals that simply cannot be confidently assigned to either species. The questions are: Where is the middle, and how far from the middle does a bird have to be before it is outside of the range of intermediacy and therefore safely identifiable?

This mirrors the challenge of other similar species-pairs like Thayer's/Iceland Gulls or Western/Glaucous-winged Gulls. One can take a very conservative approach and identify only the extreme ends of the variation, or set the threshold at any point towards the middle.

The key phrase in the original question above, then, is what it will take before I can feel good about calling a Hoary Redpoll. Such subjective decisions require experience and careful consideration, and this is also to some extent a personal decision. Some people can tolerate a "probable" or "very likely" Hoary-type, others demand more certainty. It will also depend a lot on the context of the observation. In Barrow, Alaska, where Common Redpoll is a rarity, a lot of fairly dark birds are confidently labeled "Hoary". If those same birds were seen in the lower 48, where Hoary is a rarity, I'm guessing that they would be called Common. Observers in places where Hoary Redpoll is rarer will naturally set the bar higher for identification.

The birds in my photos are subtle, and I realize that if I had seen those birds in, say, California or North Carolina, I would be reluctant to accept them as Hoaries and would want a more thorough study of all features. In other words, I am not 100% confident that they are true Hoaries. But at that feeder, where I had seen one very pale and obvious Hoary, these few birds that were on the pale side and also somewhat fluffy and small-billed seemed like good candidates for Hoary and I was comfortable sticking that label on them. They are consistent with Hoary Redpoll, and match birds I have seen in northern Alaska. Whether they are pure Hoaries, or hybrids, or maybe even pale Commons, is an open question. I've revised yesterday's post to label them "Hoary-type" redpolls, which more accurately reflects how I think of them.

Having said all that, as I browse the photos of recent claimed Hoary Redpolls in the lower 48, they all look to me like solid Hoaries.
James Smith's photos from Gloucester, MA
James Smith's Redpoll pages from NH, winter 2003-04
Some recent photos from Michigan

I think people are being admirably conservative, and that such pale birds are well towards the pale side of redpoll variation and are safely identified as Hoary. It's understandable to wish for more certainty, but, if you can tolerate the uncertainty, I think there's a lot to be learned by looking for and studying the more subtle and intermediate birds.

Another excellent resource for redpoll info from Ron Pittaway is here

Tuesday, January 1, 2008

My Holiday (Redpoll) Photos

[Edited 2 Jan to add a hint of uncertainty to the identifications]

Over the holidays I was able to spend a few days at my parents' house in upstate New York, and enjoyed the redpolls that were visiting the bird feeder. I saw one clear-cut Hoary and at least two other Hoary-type Redpolls, and at least one "Greater" Common Redpoll among the flocks of regular Commons. There were up to about 60 redpolls at once visiting the feeder, but there must have been far more that passed through during the biggest day (29 Dec) since the unusual redpolls were each only seen for brief periods. I set up my Wingscapes Birdcam and just let it run, and ended up with a pretty good assortment of redpoll pictures, including a couple of Hoaries that I didn't see in real life.

I saw one really pale Hoary that stood out as a very white bird. Unfortunately that one didn't get caught by the birdcam. I did get pictures of several of the more subtle Hoary-types (clicking an any picture below will bring up a higher resolution image that shows more detail).

Here's a typical scene under the feeder:

Notice the brownish backs of the Commons, and the one paler bird towards the right is an adult male (pink showing on the breast) so it's just a Common. Below is a shot from a few minutes later, and there on the left is a paler bird - a female Hoary-type!

Notice the grayish back without the brown scapulars of the Commons, the clean white facial markings and pale neck. But it's certainly not "obvious". Below is the same bird moments later next to a Common:

Again, compare the ground color of the sides of the back (and scapulars). Commons typically have two white stripes down the center of the back, contrasting with brown stripes towards the sides. On Hoary the back is more uniform - white in the center and pale gray on the sides. Towards the rear, where the rear scapulars overlap the wing coverts and tertials, Hoaries have the scapulars edged whitish so the rear edges of the back are paler, where the same feathers on Common are just brown. The pale edges of wing and tail feathers are whiter and broader, the supercilium is unstreaked white, and the sides of the neck are pale and faintly streaked.

Below is another Hoary-type (more subtle) with its back to the camera:

And a Common in similar pose below:
Notice the difference in back color and pattern. The pattern and extent of wing markings is similar (and extremely variable) in both species, but Hoaries often look whiter there. And below is the same Hoary-type with its head up:

Notice the short, stubby bill, pale neck sides, "frosty" back and wings. Below is a rather large-billed Common to compare, but bill size is not always different:

Finally, below is an ambiguous individual:

This bird is heavily-streaked below and on the neck-sides, a bit brownish on the back, etc, and I doubt that anyone would call it anything other than a Common, but it's paler and grayer above than a typical Common, and could be intermediate. [One might say this is on the Common side of intermediate, while the Hoary-type above is on the Hoary side of intermediate. Both may be unidentifiable, or at least many people would want to see a less ambiguous individual before making the call.]

After this experience I suggest that back color is one of the best things to look for as a first-cut. Then try neck-side markings, bill size, streaking on flanks and undertail coverts, and then other features. The paler color of Hoary is expressed as an overall whiteness and "clean" or "frosty" appearance. For example, on birds with a buffy wash on the face and throat this leads to a cleaner yellow-buff color, which on Commons is sullied by thin streaks and dusky smudges. All features overlap between the two species, and it's difficult to find anything objective to latch onto and to support an identification. Therefore it will be difficult to identify the more subtle Hoary Redpolls unless you get a really good look at the whole bird and have looked carefully at many other redpolls, but there is definitely a clean, pale overall appearance that is associated with Hoary.

More on Redpolls

After my previous posts on redpolls here and here, I've had a chance to skim some of the redpoll literature, and it confirms that these are two very similar species (gasp! really?). Two papers in particular by Seutin et al (1992, 1993) offer some interesting observations. They conclude that it is possible to classify specimens into either a small-billed pale population or a large-billed dark population (about 87% of the time), and that Hoary and Common "may be specifically distinct".

One important point is that redpolls molt only once each year - in late summer. So the pale tips and edges of new feathers will gradually wear off during the year, leading to a slight but significant darkening of the plumage month to month throughout the year. This is not a big issue for birders in the field, since we will generally be comparing redpolls to their companions in real time, but museum workers have to be careful to compare only specimens from similar dates. It is suggested that Troy (1985) - who concluded that there are not two distinct populations - did not adequately control for date and for age and sex.

What Seutin et al found is that specimens can be sorted by measurements, with pale (Hoary-types) having shorter bills by about 10% on average; means of 7.67 mm for dark birds and 7.01 for pale. Taking their mean for bill length +/- 2 standard deviations (I'm told this will encompass about 95% of all individuals) the bill measurements don't look so different:
Common 7 - 8.2 mm
Hoary 6.5 - 7.5 mm
In other words, the differences are very slight, and there is extensive overlap, with the largest Hoary matching the average Common and the smallest Common matching the average Hoary.

Importantly, measurements of bill depth do not differ significantly between the species, so a Hoary should have a short and stout bill (if a 7 mm bill can be called stout). In other words, a Common's bill should be longer and more slender, a Hoary's shorter and stubbier. But this is also subject to overlap, and we don't know if the longest-billed Hoaries still have a relatively thick bill or a bill shape just like Common (or if the shortest-billed Commons have slender or stout bills).

Measurements of tarsus and hallux (hind claw) were also significantly smaller on pale birds, but it is not likely that either of these measurements will be helpful to birders. Wing and tail averaged very slightly larger on Hoary-types, but not significantly so.

In their study of plumage, Seutin et al. found that most adult males could be distinguished by darkness and thickness of streaking on flanks, undertail coverts, and rump, but that females and immatures were much less distinct. They also studied forehead color and "poll" color (the red crown patch) and found no significant difference. Unfortunately they did not measure scapular color, wing markings, neck markings, or other plumage features that now seem like they might be useful.

Breast color also distinguished nearly all adult males in summer, with Commons having deeper red color and Hoaries a pale pink. This color is obscured by pale feather tips in fresh plumage in fall, which gradually wear off to reveal the reddish color by summer. Some of the underlying color is usually visible from at least December on, under the fringing whitish feather tips, and may be a helpful clue on adult males. But this should only be used on adult males with unstreaked and extensively colored breasts (see Brooks, 1973) and these birds are generally the most easily identified by overall darkness.
Typical breast color of adult male Common Redpoll (left) and adult male Hoary Redpoll (right)

Brooks, W. S. 1973. A tentative key for sex determination of Common Redpolls (Acanthis flammea flammea) in the northern United States during winter. Journal of Field Ornithology 44:13-21. pdf here

Seutin, G., et al. 1992. Plumage variability in Redpolls from Churchill, Manitoba. Auk 109:771-785. pdf here

Seutin, G., et al. 1993. Morphometric Variability in Redpolls from Churchill, Manitoba. Auk 11o:832-843. pdf here

Troy, D. M. 1985. A Phenetic Analysis of the Redpolls Carduelis flammea flammea and C. hornemanni exilipes. Auk 102:82-96. pdf here