Proposal (62) to South American Classification Committee
Split Dendroica petechia into 2+ species
Effect on South American CL: This proposal would split the Yellow Warbler into two or more species: D. aestiva for migrant populations from resident South American taxa. The first vote will be on whether to split at all -- if that passes, then we can vote on how many resident species to recognize.
Background: Our current "Yellow Warbler" consists of two "groups" based on plumage and morphology: (A) the all-yellow-headed, migratory, pointed-winged aestiva group of North America, and (B) the tropical, sedentary, rounded-winged group of primarily coastal Middle America, northern South America, and the West Indies. This latter group has been further divided into two plumage types (1) the chestnut-capped petechia group of the Caribbean, the Pacific Coast of northwestern South America, and the Galapagos; and (2) the chestnut-hooded erithachorides group of coastal Middle America and northern South America (and also Martinique).
Ridgway (1902) recognized 8 species of "yellow warblers," with virtually every plumage type treated as a separate species. All North American breeders were treated as D. aestiva -- the resident tropical taxa were treated as 7 species. The AOU (1931) continued to recognize aestiva as a separate species from Neotropical resident taxa (by implication, at least two species). Hellmayr (1935) collapsed Ridgway's species into 2, aestiva for migratory North American birds and petechia for resident Neotropical birds, with the chief distinguishing character being the more rounded wing with shorter primary extension for resident races (i.e., the morphological symptom of being sedentary vs. migratory).
Aldrich (1942) presented qualitative evidence that the aestiva and petechia groups should be treated as one species. He pointed out that the broad differences are bridged by intermediate individuals or populations in every general character, i.e., habitat, crown color, and wing shape and mensural differences. For example, as might be expected, the southernmost, least migratory subspecies, sonorana and dugesi, have intermediate wing shapes. Aldrich did not discuss the erithachorides group except to state that he was "not yet convinced" that they should also be treated as conspecific. Yet it is clear that subsequent authors considered all three conspecific, following Hellmayr, because (1) treating petechia as conspecific with aestiva while maintaining erithachorides as a separate species seemed inconsistent and asymmetrical, and (2) the presence of an erithachorides phenotype geographically embedded in petechia country naturally made such an arrangement "unsatisfactory." The AOU (1957) recognized one and only one Yellow Warbler, a broad D. petechia. And until recently, virtually all references, including Ridgely & Tudor (1989), AOU (1998), and even Sibley & Monroe (1990), have continued to recognize a single species, D. petechia.
Nedra Klein did her doctoral dissertation at U. Michigan on this complex, with intrepid fieldwork all over the Caribbean and elsewhere, and extensive genetic analyses. Unfortunately, as you all know, Nedra died recently. The portion of her work that has been published (Klein and Brown 1994) analyzed haplotype distributions among the three groups. The findings relevant to species limits might be summarized as follows:
(1) there is a deep split between the North American aestiva group and the tropical group. However, a Baja population that is phenotypically "erithachorides" clusters with aestiva.
(2) the Central American samples, phenotypically erithachorides, were not monophyletic with respect to haplotype distribution.
(3) the West Indian samples, phenotypically petechia, were also not monophyletic with respect to haplotype distribution.
(4) even Venezuelan samples, phenotypically erithachorides, were not monophyletic.
Noting the above and other details (especially evidence that long-distance dispersal has influenced haplotype distribution), Klein and Brown (1994) treated all taxa as conspecific. Nevertheless, Ridgely & Greenfield (2001), citing Klein and Brown for support despite the above, and stating that "behavioral and plumage differences are also quite marked", treated the migratory aestiva group as a separate species from petechia. The behavioral differences were not specified. Hilty (2003) further treated erithachorides as a separate species, but did not provide rationale. Qualitative differences in songs and calls have been mentioned in the literature (e.g., Dunn & Garrett warbler guide, Hilty 2003) but are notably absent, for instance, from Howell & Webb's Mexico guide.
Analysis: In my opinion, published support for any of the three treatments is weak.
One could take the angle that there was never any good reason to change from the 2 or 3-species taxonomy of Hellmayr, etc. However, I think Aldrich's points are basically valid, and the lack of "clean" genetic structure among the three groups only fortifies them. (I have lots of problems with use of mtDNA haplotypes to assess species limits, but that's another story -- at this point the haplotype distribution is all we have to go on.) The reported vocal differences are of interest but completely expected giving the vast geographic range of the Yellow Warbler, and they lack quantification, which would be absolutely required for any species in Dendroica, a genus notorious for multiple distinct song types within individuals.
Recommendation: A "NO" vote means retain current single-species treatment, A "YES" vote only means to split into 2 or more -- if the proposal passes, then I'll do a subsequent one for 2 vs. 3 (vs. 8?).
I'm going to vote NO on this one because I don't think any real progress has been made since Aldrich, other than the Klein-Brown paper, which largely supports Aldrich, at least in a general way. This complex needs some thorough work. That being said, I would be surprised if this "species" did not contain multiple species, but I want to see some real data before drawing the boundaries.
ALDRICH, J. W. 1942. Specific relationships of the Golden and Yellow warblers. Auk 59: 447-449.
AMERICAN ORNITHOLOGISTS' UNION [A.O.U.]. 1957. Check-list of North American birds, 5th ed. Lord Baltimore Press, Baltimore, Maryland.
HELLMAYR, C. E. 1935. Catalogue of birds of the Americas. Field Mus. Nat. Hist. Publ., Zool. Ser., vol. 13., pt. 8.
HILTY, S. L. 2003. Birds of Venezuela. Princeton University Press, Princeton, New Jersey.
KLEIN, N. K. AND W. M. BROWN. 1994. Intraspecific molecular phylogeny in the Yellow Warbler (Dendroica petechia) and implications for avian biogeography in the West Indies. Evolution 48: 1914-1932.
RIDGELY , R. S., AND P. J. GREENFIELD. 2001. The birds of Ecuador. Cornell University Press, Ithaca, New York.
RIDGELY, R. S., AND G. TUDOR. 1989. The birds of South America, vol. 1. Univ. Texas Press, Austin.
RIDGWAY, R. 1902. The birds of North and Middle America. Bull. U. S. Nat. Mus., no. 50, pt. 2.
SIBLEY, C. G., AND B. L. MONROE, JR. 1990. Distribution and taxonomy of birds of the World. Yale University Press, New Haven, Connecticut.
Van Remsen, October 2003
Comments from Stotz: "NO. Although I think that there are 3 species in petechia, I think there are still problems in assigning subspecies correctly between the two tropical groups."
Comments from Robbins: "[NO] Clearly, much more information is needed before we start splitting petechia into multiple species."
Comments from Jaramillo: "YES -- I am getting a feeling I might be the only one heading down this path and that is fine. This is really a difficult one, very little data has been published recently on this, and certainly nothing clarifies the situation. However, my field experience with a bunch of these forms, including the Lesser Antillean taxa and Galapagos birds leaves me thinking that this must be more than one species. I don't know how many and where the split is, but given that the proposal is for one or more I certainly am more comfortable with the more than one scenario. Not only do these groups differ in wing morphology, male plumage, overall size in some cases, female and immature plumages, songs, habitat, migratory tendency and call, the Klein and Brown data does show a genetic rift between the northern and tropical groups. I think there is a need to adequately figure out how all of these forms sort out, but for now splitting the tropical and northern populations out as two seems reasonable."
Comments from Zimmer: "I vote "NO". This is a tough one. I do think that more than one species is involved. But I don't think the limits of the component species are clear, and we are certainly lacking for a thorough, published analysis. Alvaro's point that we could start by recognizing a northern vs. tropical split is well-taken, but I think I'd prefer to wait until more information is available, especially given Nedra's data."
Comments from Stiles: "NO pending more published evidence of various sorts. If one subscribes to a biological species concept, the use of genetic evidence as a main criterion is inappropriate (this problem has led the AOU into a number of what I consider unjustified splits). What genetic evidence gives us is an estimate of divergence time - but this is far from a precise estimator of species status. The requisite mutations or recombinations or whatever that produce reproductive isolation may occur very quickly or very slowly depending upon selection pressures, ecological differences, population sizes, pure chance ETC. Over a long enough time, one can calculate fairly precisely the chance of lightning striking a given place - all things "average out" given enough time - but the shorter the time period the less precise will be the prediction, and this applies in spades to the time period during which much (most or all?) speciation occurs in birds!"
Comments from Nores: "NO. A pesar de que pienso de que hay más de una especie involucrada en D. petechia, por ejemplo la población residente que vive en manglares en el norte de Colombia y Panamá (erithachorides). Creo que hasta que no haya un trabajo que trate al grupo desde varios aspectos, es prudente no innovar sobre el tema."