Welcome back Myrtle Warbler?
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.