This is long, so hit delete if not interested. Many have asked; so here is my response to the ABA blog Derek highlighted here:
http://blog.aba.org/2013/01/open-mic-redpolls.html
Blogs are like prairie fires; they burn fast but not deep. So it is with this ABA entry on redpolls, which presents a shallow and misleading summary of the situation. The subtext--are they good species or bad--is mainly meant to generate web traffic, but it diverts attention from a fascinating evolutionary question. What one can "count" is important to many people, but most species (and subspecies) are not so neatly defined. We need to get over the good versus bad dichotomy.
On one level, species names are simply tools of communication--ways to tell others about what we see. We try to delimit those names with firm, reproducible, and diagnosable characters so that we can be sure that we are all talking about the same thing. With redpolls, the biggest hurdle I see is getting people to accurately learn and assess the key characters. I think this difficulty drives people crazy and leads them to want to "lump" the redpolls (and create hilarious parodies, http://youtu.be/jdTTxTGcFQs).
The most important thing to remember is that the pale birds (Hoary) all come from fairly well-known geographic ranges distinct from the dark ones (Commons). Reporting as accurately as we can when and where these form occur, regardless of species boundaries, is valuable. Despite the lament "we can't know a Hoary from a Common," they are possible to differentiate, at least as accurately as anybody does Sharp-shinned and Cooper's Hawks, and from what I've seen, those are misidentifed more often.
Okay I have to address the question most asked: are the redpolls good species. The answer is genetic studies, including the one cited in the ABA blog, don't reject that notion. It is also true that the redpolls could be morphotypes of a single species. For understanding and appreciating the diversity of life, however, I think we ought to operate under "good species" until proven "bad." I say not proven bad.
As background to this and why the ABA blog is old news as well as misleading, the genetics paper under discussion was dealt with in a photo essay on redpolls that I helped write for "North American Birds" (http://aba.org/nab/v65n2redpolls.pdf -- see page 209). We noted, just as Marthinsen et al. did, that rapid evolution (and speciation) would not be detected with their methods; hence low genetic diversity doesn't mean what many interpret it to mean. We also noted that other birds show similar genetic homogeneity, the Darwin's finches for example (recent and rapid evolution) and Golden-crowned and White-crowned Sparrows.
Omitted from the ABA post was that where people have looked for it, no interbreeding between Hoary and Common Redpolls has been documented. There are no solid cases of hybrids or hybrid pairs. The intermediate birds that are reported are not *known* to be hybrids. In a significant number of cases, they are simply poorly diagnosed to age and sex. Rampant mixing is not seen or documented. On the contrary, where observed together, Hoary and Common Redpolls appear to nest assortatively, prefer different nest sites, and build different nests. Could they interbreed? Sure, and that still wouldn't answer the species question since we would need to understand the nature of the hybridization and what direction it is taking (e.g. exilipes Hoary and flammea Common could be merging but rostrata and hornemanni could be separate).
Given the arctic is a "new" habitat that has opened up since the retreat of ice, rapid evolution and shifting ranges are easy to accept as an explantion for fuzzy redpoll genetics. Looking at this again, I found another possible explanation for why the type of genetic study in quesiton might show low genetic diversity among redpolls. The Marthinsen study looked at mitochondrial DNA (mtDNA). This is usually chosen because it is thought to be "neutral," and thus a better gauge of overall differences accumulated over time. Recent studies suggest these mitochondrial genes might be under selection in certain environments, however. If that is true, then the prevalence of these genes across populations may reflect adaptation. Why is that important? If they are under selection and adaptive, then using them for taxonomy would be like looking at webbed feet in birds and lumping species based on the similarity of foot morphology for locomotion in water. The role of mitochondria in oxidative phosphorylation (think ATP and what fuels metabolism) could be under selection then, especially in colder climates where keeping warm is a useful adaptation. Other high latitude birds show similar patterns, which may be the result of selection and what are called introgressive mtDNA sweeps (see Rheindt and Edwards, Auk 128 [4]: 620-632). Again, the genetic markers used in this study of redpolls are interesting from an evolutionary standpoint, but if the result of an introgressive sweep, they are uninformative for taxonomy.
As an addendum for our resident OLCO, Bruce Bartrug, who advanced the idea that these redpolls are some fanciful new split, I wish to direct attention to the young whippersnapper by the name of Elliott Coues, who regarded all the taxa of redpolls as separate species...in 1861. Coues's paper, still a fine and useful reference, covers seventeen pages, and you can read it here (pages 373-390): http://biodiversitylibrary.org/page/1683502. Redpoll taxonomy hasn't changed much since then. The AOU did lump the two Hoary taxa (exilipes and hornemanni) and the two dark taxa (flammea and rostrata) under Common. They did that in 1889, a treatment unchanged through five subsequent editions of the AOU checklist: 1895, 1931, 1957, 1983, and 1998. The AOU did note that rostrata and nominate hornemanni might be separate species themselves, and in 1987 said of redpolls: "further study (especially of breeding biology, pairing, vocalizations and ecology) will be required before the systematics can be determined."
Seems like we've known this was a fuzzy problem for a good, long time.
Louis Bevier
Fairfield
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