Open Mic: Hoary Redpoll; A False Dichotomy?

 

At the Mic: Andy Boyce

Missoula, Montana, resident, Andy Boyce is a graduate student currently working on his PhD at the University of Montana.  His area of interest is the ecology of tropical birds in Borneo.

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    We are currently experiencing  one of the largest southern movements of redpolls in the last 50 years and birders across the country are out and about, scrutinizing flocks of redpolls at Nyjer feeders, looking for that holy grail of the redpoll flock; Hoary Redpoll.

    With the birds, there has been a surge of posts to local list-servs, blogs, etc. One group searching for advice -- “just one dark streak on the undertail coverts, is this a Hoary?”, and others diligently going through all of the traits espoused by field guides and experts that are supposed to allow us to sort through these messy flocks of finches.

    Doing my own research on this topic, I discovered a paper published in 2008 entitled “Low support for separate species within the redpoll complex from analyses of mtDNA and microsatellite markers” (Marthinsen et al. 2008). Understandably I was a little shocked. My first thought being, what the heck are we doing trying to sort these things out into species if there is no evidence that they are even species at all?  After a thorough read I was totally convinced that attempting to sort redpolls into species, or even subspecies is a totally futile exercise. 

Colorado's first Hoary Redpoll? photo by Bill Schmoker

    The crux of the paper is this; based on museum specimens from across the old-world range of the 2-3 redpoll species commonly recognized, the authors found no evidence at all that what we humans are describing as species or subspecies corresponded to any sort of meaningful, genetically differentiable groups.  After reading through this paper, I looked around some more and found out that several previous studies have also failed to find any genetic structure among species or subspecies of Redpolls (Marten & Johnson 1986, Seutin et al. 1995, Ottvall et al. 2002, Hebert et al. 2004, Kerr et al. 2007).

    The phylogenetic work was done using both mitochondrial and nuclear DNA, which evolve/mutate at different rates. Thus, this is a pretty robust study in terms of methodology and is unlikely to be a false negative. Some have argued that they simply missed the appropriate genetic markers to differentiate the species/subspecies, but this is also unlikely given that they were able to show the monophyly of two subspecies of Twite (close relatives of Redpolls) using the same methods.

Another hypothesis suggested by some is that the Redpolls represent incipient species undergoing sympatric speciation. I suppose that it possible, but we would still expect genetic differentiation between groups if this was the case.

This finding leaves us with 2 interesting possibilities;

1)      All redpolls belong to one single panmictic gene pool. There is no Hoary Redpoll, there is no Common Redpoll, there is no exilpes, there is no flammea…etc. They are just Redpolls.

2)      There are monophyletic (genetically distinct) groups somewhere in the redpoll complex, but we humans, in particular museum curators and collectors, cannot identify them based on morphology, plumage, or even range.

    These are two very different hypotheses, but for birders they really mean the same thing. They mean that we cannot ID a Hoary Redpoll. You know why? Because we have absolutely no idea what (or even if) a Hoary Redpoll is! If there is a mysterious genetically distinct group out there, it appears that it isn’t linked to any convenient suite of external traits. That means that Sibley’s wonderful page describing how to score a pale redpoll on a scale from 1-Hoary is more or less meaningless. The paper that the page is based on even admits to having essentially no clue about what a Hoary is, they simply state that they assume that any individual falling in the “hoariest” third of the total distribution of variation is a Hoary. This is shaky to begin with, but when you take into account that there is actually no such thing as a 100% surefire Hoary Redpoll, it’s totally nuts!

Common Redpoll, Quebec, photo from wikipedia

    Ok, so you don’t care about genetics, let’s talk about probability. We know that Redpolls that breed in shrubby habitat in the far north, in areas without trees tend to be larger, smaller-billed and whiter in various nebulous ways. For the sake of argument let’s throw the latest science out the window (if the government can, why can’t we, right?). Let’s call those hulking, frosty seed-killers Hoary Redpolls. Unfortunately, we know that all of these traits that are “good” for Hoary Redpoll are extremely variable and they don’t always occur in concert within an individual.  That is to say, there are birds out there that have extremely white upperparts, reduced “poll”, very little flank streaking, but gosh-darn-it look at those monster undertain covert streaks! Blast!  Given that these traits are fallible, we have the following problem, straight from the Ted Floyd book of rhetoric.

    Based on observed traits we can assign some sort of probability that a bird showing those traits is a Hoary Redpoll. We can also assign some probability of a Hoary Redpoll showing up in Colorado versus a Common Redpoll.  Based on that, we can get a probability of a pale redpoll in Colorado being a Hoary Redpoll.  Here are some numbers:

    Probability of a pale redpoll being a Hoary: 99% (this is incredibly generous)

    Probability of a redpoll in Colorado being a Hoary: .001% (numbers from the east coast during irruptions suggest ratios of around 1000:1.

    Probability of a pale redpoll in Colorado being a Hoary Redpoll: ~1% 

    I should point out that the current ratio of Common Redpoll to Hoary Redpoll on Colorado Ebird checklists for 2013 is more like 100:1. This ratio is tremendously skewed because birders, understandably, chase and report rare birds more than common ones.  Even if we take that ratio as gospel, the probability of any given pale redpoll being a hoary in Colorado is still only 50%.

    So, what am I trying to say with all this? I guess the take home message is that if you believe in the current scientific evidence, you have to acknowledge that we have no idea what or if a Hoary Redpoll is. If you don’t believe in the scientific literature but you do believe in the power of statistics, then you have to acknowledge that even if you see a pale redpoll in Colorado, there is no reason to believe that it is a Hoary. In fact, the statistics tell us that it is almost certainly NOT a Hoary Redpoll.

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Literature Cited:

    Hebert, P.D.N., M.Y. Stoeckle, T.S. Zemlak, C.M. Francis. 2004. Identification of birds through DNA barcodes. Pub. Lib. Sci. Biol. 2; 1657-1663.

    Kerr, K.C.R., M.Y. Stoeckle, C.J. Dove, L.A. Weigt, C.M. Francis, P.D.N. Hebert. 2007. Comprehensive DNA barcode coverage of North American Birds. Molecular Ecology Notes 4: 535-543.

    Marten, J.A., and N.K. Johnson. 1986. Genetic relationships of North American Cardueline finches.  Condor 88: 409-420.

    Marthinsen, G., L. Wennerberg, J.T. Lifjeld. 2008. Low support for separate species within the redpoll complex (Carduelis flammea-hornemanni-caberet) from analyses of mtDNA and microsatellite markers. Mol. Phylo. & Evolution 47: 1005-1017.

    Ottvall, R., S. Bensch, G. Walinder, and J.T. Lifjeld. 2002. No evidence of genetic differentiation between lesser redpolls Carduelis flammea caberet, and common redpolls Carduelis f. flammea. Avian Sci. 2: 237-244.

    Seutin, G., P.T. Boag, and L.M. Ratcliffe. 1995. Mitochondrial DNA homogeneity in the phenotypically diverse redpoll finch complex (aves: Carduelinae: Carduelis flammea-hornemanni). Evolution 49: 692-973.