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Welcome back Myrtle Warbler?

March 12, 2010
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UPDATES added

Ready for another arm-chair lifer?  Two recent papers have got the International Ornithological Congress ready to pull the trigger on what I’ve always thought was a long needed split between the subspecies of Dendroica coronata, known well to birders across the continent as the Yellow-rumped Warbler.  The species consists of two migratory subspecies, Myrtle in the east and Audubon’s in the west, of which we are all likely very familiar, as well as two little known non-migratory subspecies in Central America; Goldman’s Warbler in the Chiapas highlands and Guatemala and Black-fronted Warbler of western Mexico.

The superspecies was lumped by the AOU in 1973 based on initial studies of hybridization in the narrow range of overlap between the Audubon’s and Myrtle Warblers, a very thin band in the Canadian Rockies, and has been considered Yellow-rumped Warbler ever since.  I’ve always felt this to be problematic for a couple reasons.  For starters, lumping the birds by virtue of their visual similarity seems to be an easy decision, but the fact of the matter is that visual similarity has never been a completely accurate way to determine specific relations, as studies looking at speciation from genetic and audio cues have learned.  Alternately, species that have similar songs and genes, but who look strikingly different, are nearly never considered to be clinal variations of the same species.  Consider that the fact that hybridization is far more common between Golden-winged Warblers and Blue-winged Warblers than it is among the so-called con-specific Audubon’s and Myrtle Warblers, and yet the former are good species in the eyes of the AOU and the latter merely subspecies.  Such a visual bias is not only evident in the AOU, but probably reflected in the general birding population as well.

While there’s absolutely no reason to continue suggesting that the Central American populations are coronata (the genetic distance is substantial), I doubt there are even birders that would fight the split of the migratory Yellow-rumps should the AOU decide to take it up, and they absolutely should, as the most recent data suggests that while hybridization is regular but rare,  and there’s a genetic selection preventing that hybridization from taking place and genetic similarities are due to incomplete lineage sorting rather than to gene flow (see update!).  In layman’s terms. they’re close, but not that close, and the hybrid zone is limited and consistent. They don’t seek each other out in a manner consistent with more closely related populations (i.e. conventional subspecies), but in a way that we typically see in Baltimore and Bullock’s Orioles or the Solitary Vireo complex, groups we already consider to be distinct species.

The most recent study, published last year in the journal Evolution concludes:

Our findings indicate that the Myrtle and Audubon’s Warblers are stable and genetically distinct forms, that parts of their genomes remain distinct despite extensive hybridization, and that selection maintains differences between the taxa. We therefore suggest that these two taxa may meet the current criterion for full species status between hybridizing North American birds, that the hybrid zone be “narrow and stable” (AOU 1998).

So it goes without saying that there’s some pretty good evidence that the Yellow-rumps should be split.  The question then becomes whether the AOU Check-list committee accepts the self-evident and makes it official, which they have been exceedingly reluctant to do in similar cases involving American and British Herring Gulls and the Red Crossbill group, both of which are long overdue splits with significant evidence supporting action.  The IOC, for all its faults not least of which is the fact that it’s a relative new name to the world lister scene, doesn’t have any of these reservations with regard to splits.  While the IOC’s split-happy attitude occasionally makes me uncomfortable, in this case they’re exactly right.

Yellow-rumped Warbler needs to be split, and the AOU would do well to follow suit, though if their foot-dragging on other species is any indication, we’ll likely see the Myrtle Warbler’s return some time in 2035, soon after the American Herring Gull but before Yellow-shafted Flicker.

–=====–

Update 1: Some clarification on a couple of my comments by Nick Sly of Biological Ramblings

“and there’s a genetic selection preventing that hybridization from taking place”
” and they don’t seek each other out in a manner consistent with more closely related populations”

Neither statement is correct. Rather, this paper continues to document that there is essentially no assortative mating – across the hybrid zone there are no obvious social barriers and there seems to be free mating between the parental forms and hybrids and backcrosses. What this paper documents is several genetic loci that do have fixed differences between the two forms. Basically, while the birds freely mate with anything yellow-rumped, some form of selection against hybrids or genetic incompatibility occurs in the hybrids and backcrosses that prevents gene flow out of the hybrid zone at certain, but not all genes.

The fact that there is hybridization in and of itself doesn’t mean that these are one species, bird hybrids are fairly common as anyone watching gulls in the “anything goes” northwest can attest, but that there is something in the genes that prevents these hybrids from occurring outside of this narrow band of overlap is a good indication that we might be looking at two different species here.   Or at the very least, something more than a subspecies but less than a full species.  Which, in my mind given the differences in plumage and vocalizations that birders are aware of, is good enough for a functional split.  The IOC apparently agrees.

Thanks, Nick!

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21 Comments
  1. March 12, 2010 7:42 am

    I have a few things to say about this post:

    a) A stable hybrid zone is an indicator for a species. If there was free gene flow, the hybrid zone would expand or – as this should have been going on for thousands of years after the last ice age and we’ve only been looking at birds scientifically for a couple’ hundred years, there simply shouldn’t be a hybrid zone anymore in conspecific forms but a steady cline. Of course the split is overdue!

    b)How can the AOU split the Empidonax forms that are inseparable visually from each other on song yet decline the same procedure to forms that are visually different?

    c) Look, young lad, if you want me to buy you some beers when we meet at some point in the future, you’d better call that gull “European” instead of “British”, unless you propose to split Larus argentatus argenteus from L. a. argentatus – in the latter case, you’d only earn yourself the wrath of the French. Or just do what the Brits do with their field guides, call it the “Gull of Britain and Europe”. ;-)

  2. March 12, 2010 7:43 am

    I forgot:

    d) Aren’t you the guy who recently digiscoped a Kinglet?

  3. Nate permalink*
    March 12, 2010 8:25 am

    @Jochen- a)More than overdue, the lump should have never been made in the first place.

    b)Who knows? But I think it has something to do with the AOU’s 19th Century approach to bird taxonomy. They haven’t publicly reported on why they rejected the proposed Red Crossbill split, but from what I hear it’s a serious facepalm moment.

    c) My mistake! It must be my english-language bias. It’d better be a British beer then…

    d) Yes, yes I am.

  4. March 12, 2010 9:17 am

    Definitely needs splitting! And wait, you digiscoped a Kinglet? I must have missed that.

    Is Jochen legally allowed to buy you a British beer? I thought there were some kind of laws in Germany about those sorts of things. Mmm… Franziskaner…

  5. March 12, 2010 9:33 am

    I want some beer too!

    Also, will the birds’ nicknames be split too? If so, which of the two will be Butterbutt? And will the other have to be called Margarinebutt?

  6. March 12, 2010 9:44 am

    While I don’t really disagree with your ultimate conclusion at all, a few points of correction or addition:

    “the most recent data suggests that while hybridization is regular buy rare,”

    Hybridization is not “regular but rare” but is pervasive and standing – look at Figure 1, virtually every site in the 100-km hybrid zone across five different transects is intermediate in genetics and plumage. [Note: I was initially going to say every bird in the hybrid zone was intermediate, but then I realized that Fig 1 represents the score for each sample site in the transect, and I can’t find any data right now on the actual frequency of hybrids versus “pure” across the transect (except the reference to one of the earliest papers on the zone which found nearly all individuals at the center have some hybrid phenotype). I wonder now if a cline pattern like that can be generated by changing frequencies of both pure types and not any intermediates at all – I don’t think that is actually the case here, but it could happen – anyways, I digress and I may have undershot my own point here]

    ” and genetic similarities are due to incomplete lineage sorting rather than to gene flow.”

    I’m not convinced of that, as disentangling gene flow from lineage sorting is almost impossible and most of the time authors just make an inference about it being one or the other.

    “and there’s a genetic selection preventing that hybridization from taking place”
    ” and they don’t seek each other out in a manner consistent with more closely related populations”

    Neither statement is correct. Rather, this paper continues to document that there is essentially no assortative mating – across the hybrid zone there are no obvious social barriers and there seems to be free mating between the parental forms and hybrids and backcrosses. What this paper documents is several genetic loci that do have fixed differences between the two forms. Basically, while the birds freely mate with anything yellow-rumped, some form of selection against hybrids or genetic incompatability occurs in the hybrids and backcrosses that prevents gene flow out of the hybrid zone at certain, but not all genes.

    Up until this most recent paper, I wouldn’t really consider this an overdue split. The two types mate freely with each other with no signs of social (prezygotic) barriers to gene flow. Analysis of the hybrid zone width could provide information on selection between the forms and thus you could infer species differences, but different papers came to different conclusions about this. Even the recent phylogenetics paper (Mila et al 2007) that provides the evidence for strong genetic differentiation of the two Central American members of the complex finds essentially no genetic differentiation at the examined loci between Audubon’s and Myrtle, indicating extremely recent divergence. But, on the other hand, they look different and sound different, so there must have been some radical selection in that very short time period as I don’t think you could get that different with neutral differentiation in that time frame – maybe, maybe not. Was it enough to create species-level differences? It is only relatively recently that we can hit these hybrid zones with a battery of genetic loci and get a feeling for whats happening at the genomic level.

    This most recent paper confirms what was suspected but not really known – that there is selection at the hybrid zone after hybridization at only a few loci. This is really interesting because I think it is a more common pattern for birds to exhibit pre-mating isolation first (they won’t mate with something that looks or sounds different) before getting to the genetic incompatabilities that maintain post-mating isolation. Here, in these recently differentiated forms, a small number of genetic loci are likely important for maintaining the species’ differences. I think only now with this evidence can it really be justified splitting these forms again.

    But wait! There is more and it may make this whole situation more complex. Check out this abstract (from authors of the Evolution paper) at the recent San Diego AOU meeting (which I really wish I could’ve attended):

    Brelsford, A., Zoology Department, University of British Columbia,
    Vancouver, Canada, alanb@zoology.ubc.ca
    Mila, B., Museo Nacional de Ciencias Naturales, Madrid, Spain
    Irwin, D. E., Zoology Department, University of British Columbia,
    Vancouver, Canada

    IS THE AUDUBON’S WARBLER A HYBRID SPECIES?
    Hybrid origins have recently been shown for several animal species, but
    no avian examples have been documented with molecular evidence. We
    investigate whether the Audubon’s warbler, one of four visually distinct
    forms in the yellow-rumped warbler (Dendroica coronata) complex, may
    have originated through hybridization between two other taxa in this group,
    the myrtle warbler and black-fronted warbler. Analysis of over 300 nuclear
    markers (AFLP) shows that Audubon’s warblers carry a mixture of alleles
    otherwise found only in one or the other of their putative parent species.
    Audubon’s warblers also carry two deeply divergent mitochondrial DNA
    lineages, each shared with only one parental form. Our results indicate that
    this taxon likely originated through hybridization between two long-diverged
    forms. Nevertheless, broad clines between the Audubon’s and black-fronted
    warblers in morphology and nuclear genetic markers call into question the
    validity of the Audubon’s warbler as an evolutionarily independent taxon.

    I really, really, can’t wait to see that paper!

    Finally, if you are impatient with the AOU’s seemingly snail pace at taxonomic changes, you can always write the proposal to split these species. They are dependent upon proposals to vote on to make taxonomic changes, if no one writes one it can’t be decided.

    Okay, I’m done rambling. I hope I made at least a little bit of sense.

    • Alan permalink
      March 17, 2010 1:01 pm

      A couple of comments to add to this, as an author of the most recent article…

      ” ‘ and genetic similarities are due to incomplete lineage sorting rather than to gene flow.’
      I’m not convinced of that, as disentangling gene flow from lineage sorting is almost impossible and most of the time authors just make an inference about it being one or the other.”

      You’re right, and I think the IOC misinterpreted what we wrote on that subject in the paper. We didn’t present any evidence to distinguish between those two options for the genetic loci that weren’t strongly differentiated, and raised incomplete lineage sorting only as a possible alternative to ongoing gene flow as an explanation of genetic similarity at most markers.

      “I can’t find any data right now on the actual frequency of hybrids versus “pure” across the transect (except the reference to one of the earliest papers on the zone which found nearly all individuals at the center have some hybrid phenotype).”

      191 out of 200 birds sampled near the hybrid zone centre had some evidence of admixture, which is in line with what Hubbard found 40 years earlier. Many of those 191 “hybrids” were very close in appearance to one or the other species, (e.g. a Myrtle appearance but with a trace of yellow in the throat), but intermediates far outnumbered pure Myrtles or Audubon’s in that local area.

      “Finally, if you are impatient with the AOU’s seemingly snail pace at taxonomic changes, you can always write the proposal to split these species.”

      At the request of somebody on the AOU checklist committee, I wrote a proposal for a split, which I’m told is under consideration right now. My personal opinion is that there’s pretty good evidence now for 3 species (Myrtle, Audubon’s, and Goldman’s), but that splitting Audubon’s from Black-fronted would be premature. Yes, Mila et al. 2007 found big differences between those two in mtDNA, but we have some new information (still working on the publication) showing that the nuclear markers put Audubon’s closer to Black-fronted than to Myrtle (as you’d guess from their apperances), and don’t rule out a lot of ongoing gene flow between them. There may very well be 4 species in the complex, but I think more evidence is needed for one of the splits. Another student in the Irwin lab is just starting work on the Audubon’s/Black-fronted contact zone, so there’s a lot left to find out about these birds, and we’ll have a more complete story in the next few years.

      • Nate permalink*
        March 17, 2010 1:43 pm

        @Alan- Really great stuff. Thanks for commenting!

        How often are these intermediate birds reported outside the contact zone? I assume they get swamped by the far greater numbers of typical Audubon’s and Myrtles in their wintering range, but I rarely hear about these birds showing up among the general population of YRWAs in places where they winter. Do they associate with the subspecies they are most similar too? Or do intermediate birds tend to segregate themselves into their own groups? Are reports of Myrtle Warblers in the west or Audubon’s in the east more or less likely to be intermediates than pure subspecies?

      • March 17, 2010 11:05 pm

        Alan – awesome! Thanks for responding. I appreciate the info the on the levels of admixture in the hybrid zone. I can’t wait to read your newer paper. Oh, and for what its worth – you guys (the Irwin lab) are awesome – I really like all of the work you’ve been doing. I met Darren at the Laramie meeting and was considering his lab a good choice for grad school once upon a time.

      • Alan permalink
        March 18, 2010 12:08 pm

        @Nate- Maybe the best answer to your question is in this Condor paper from 1937: http://bit.ly/98x4mq The author looked at 600 specimens, which seem to have been mostly collected in the winter outside the hybrid zone, and decided that 9 of them had mixed ancestry. In some of the 9 cases, this was just based on the extent of white in the tail, which I think is more variable within each species than he realized. So, 9/600 or 1.5% is probably an overestimate of hybrid frequency in the general population, even if you call anything with a trace of mixed appearance a “hybrid.” To get at it another way, the hybrid zone has an area of about 200,000 km^2, and the species range is about 10,000,000 km^2. If all the birds in the hybrid zone were hybrids and population density were uniform, hybrid frequency would be 2%. I’m pretty surprised how well those two estimates agree, given the problems with each.

        Lots of pure Myrtles take a western migration route; I regularly saw mixed flocks of Myrtles and Audubons passing through Vancouver in March and April. For Audubons that show up in the east, I’d love to know how often they’re actually hybrids. On the one hand, there are orders of magnitude more Audubons than hybrids in the world, but on the other hand, migratory direction has a big genetic component so a hybrid could be more likely to migrate eastward than a pure Audubons. Hybrids can be pretty hard to identify unless you have the bird in the hand, or unless it’s a breeding-plumage male, which probably is a big part of the reason why you don’t have a lot of sightings reported.

  7. Nate permalink*
    March 12, 2010 10:01 am

    @Nick- Thanks for the corrections, I have to plead ignorance on most of them and I appreciate your clarification. Between that and the fact that I couldn’t get past the paywall on most of those journal sites which limited me to only the abstract, I wasn’t even able to get common language explanations on what I knew I was unclear about. There’s apparently a serious lack of this sort of thing, so thanks for that.

    As for the Audubon’s Warbler, from a layman’s perspective it appears as though they’re certainly closer to the Central American subspecies that with Myrtle, which seems to be the odd-bird out. At the very least the subspecies are heading in opposite directions towards further speciation, and given the obvious differences between the Yellow-rump subspecies, that enough for me to consider them good species.

  8. March 12, 2010 10:12 am

    They rejected the Red Crossbill split? I’m assuming you’re referring to Loxia scinesciurus? In that case I’m not surprised they rejected a species that is not diagnosable on morphology or genetics, but only based on call type which is a learned trait with birds having been documented changing their call type.

    @Corey – don’t forget the two Mexican splits, so we’ll need Lardbutt and ICan’tBelieveIt’sNotButterbutt too

  9. Nate permalink*
    March 12, 2010 10:25 am

    @Nick- That’s the one. I’d heard they had, but it wasn’t out yet and that the reasoning was suspect. But this is from a confirmed PSC’er so take it with some salt. I do tend to agree that the the AOU CLC is a bit too conservative for their own good, though. And there’s no way they’d split a species 9 ways all at once.

    From what I understand, their flight calls are distinguishable most of the time. Here’s a cool page for identifying them: http://research.amnh.org/vz/ornithology/crossbills/diagnosis.html

    But again, you’re likely in the know more than I am. I’d still split ‘em though… :)

  10. March 12, 2010 10:35 am

    I don’t think I would get a free lifer out of this split, if it happens, as I still haven’t seen an Audubon’s (or either of the southern forms). A lot of older birders would probably be happy with a split. I know some people who still call the eastern subspecies Myrtle Warbler.

  11. March 12, 2010 3:42 pm

    Taxonomy is slower than Evolution!

  12. Nate permalink*
    March 12, 2010 4:26 pm

    @John- I’ve seen Audubon’s once, in spring in south Texas, so I’ve got fingers crossed on this one. And I much prefer Myrtle Warbler to the alternative.

    @Gunnar- No doubt.

  13. March 15, 2010 10:52 am

    When I was growing up, there were certain lumps that my mother always complained about, because they “got rid of” birds she was fond of from her own youth.

    I’m hopeful that she’ll be able to live out her golden years with Baltimore Orioles, Myrtle Warblers, and Yellow-shafted Flickers all back in their proper places. I don’t think she actually gives a particular damn about American Herring Gulls though. (I do, though, since that’s the only mentioned split that would give me a new lifer.)

  14. March 16, 2010 5:45 am

    @Patrick: ah, you see, Germans will not willingly apply the term “beer” to anything that comes out of the UK. I mean, they do some good stuff with hops, malt and barley, but it’s not what we’d call beer. Franziskaner is a Wheat Beer, and I am more of a Pilsen type… So you see, Germans are very picky about beer.

    @Corey: yeah, I guess after my google earth attack, I owe you some.

    @Carrie; no gods, no masters. It’s your list, and if you don’t like the AOU’s decisions, well heck: count the Euro Gull! Science is on your side anyway, and what more can one ask for?

  15. April 14, 2010 10:06 am

    Jochen hit the nail on the head so I’ll just second what he said. It is your list, do it how you want. Some people count birds they hear, some don’t. Some people count birds they see through bird banding, some don’t. If it is your list then it needs to be personally satisfying to you. I’ve always had both myrtle and Audubon warblers on my life list and don’t really care what the AOU has on their list. I also have both slate-colored juncos and Oregon juncos on my list. I was in a completely different part of the country and saw a very different looking bird. It doesn’t really matter to me if they hybridize. I still wanted to see it and once I saw it it went on my list of birds I’ve seen. The debate is very interesting on the scientific level but once it becomes political I lose interest. The AOU and IOC have their lists, I have mine.

  16. October 4, 2010 8:05 am

    6 months later, I am just reading this post, and feel somehow confused by this discussion about the importance of hybridization for this species. Reading all this interesting stuff, I realize that the frequency and geographical range of the hybrids is an important criteria.

    But what’s about golden-winged and blue-winged warblers? I’ve seen far more hybrids of the two than pure golden-winged warblers in my home state of Michigan. However, they are considered as distinctive species. I realize I am very inexperienced in biology in general, but I just don’t get it.

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