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.

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.

2 comments:

Sullivan said...

Hi,

http://www.ilbirds.com/index.php?topic=2123.0

Do you think this bird looks like a "safe" Hoary? Thanks for the thoughts.

David Sibley said...

Yes, I do think this is a "safe" Hoary Redpoll. The back is pale and grayish, and the streaking on rump, flanks, and undertail coverts is all thin and relatively weak - well outside the range of "safe" Common and safely beyond intermediate. The observer's comment that this was an "unmistakable" Hoary also emphasizes how much this bird stood out in life.

I would add something that Marshall Iliff touched on in a post in Massachusetts recently - that just because a redpoll flock stays more or less the same size does not mean it is the same birds day after day. I once visited friends in Texas who were banding Pine Siskins at their feeder. They had what seemed to be a constant flock of between 100 and 200 siskins, but after several weeks they had banded over 2000 individuals! I suspect that most redpoll flocks are like this. While some birds stay to be seen repeatedly, many others are just passing through and we should not just assume that we are seeing the same individuals.