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.

Comments:
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?

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

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.

Notes:
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.

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.

Redpoll age variation and ID

The question of age variation has come up repeatedly in redpoll discussions, so I finally tried to find an answer to the question of just how important age-related variation in plumage might be. And based on published studies the answer is ... not very important.

Seutin et al. (1992) studied redpolls at Churchill, Manitoba using a character index, and found only one statistically significant age-related plumage difference (aside from the obvious red/pink breast of adult males) - flank streaking on adult males was lighter than on any other age/sex class. All other features - undertail covert streaking, rump color, forehead color, poll color, etc. were statistically the same on adult male, immature male, adult female, and immature female. This near-complete absence of age-related variation differs from what Knox (1988) reported - that in European redpolls immatures are darker than adults. I haven't read the Knox paper, so I don't know how different the results actually are, but regardless of what he found in Europe, the paleness of a redpoll in North America does not seem to be linked to the bird's age, and therefore ageing a redpoll is not helpful to the identification process.

The lack of age-related variation does make sense given the lack of differentiation in redpolls at all. Going back to the species question: Seutin et al. analyzed their data on plumage variation by searching each age/sex class for any sign that the distribution of scores was bimodal; e.g., did each age/sex class show any hint of sorting into paler and darker groups on any of the measured plumage characters? Only adult males showed a tendency to sort into two groups, and that was slight. Immature males showed a suggestion of sorting into pale and dark groups only when sophisticated statistical tests were applied to the data. Attempts to sort females of either age class into pale and dark groups failed completely.

Combining this with Troy's similar finding that redpolls form a continuum from pale to dark, and Troy's data showing that intermediate-colored birds were also intermediate in skeletal measurements, implies that observers should expect to find lots of intermediate birds that cannot be confidently assigned to a species.

It's possible that characteristics not measured by Seutin or Troy - like scapular color, or forehead fluffiness, or voice, or something else (?) - might lead to better separation of two groups, but their inability to distinguish two clear groups does not bode well for identification of non-adult-male redpolls.