Proposal (#294) to South American Classification Committee

 

Treat Buteogallus subtilis as a subspecies of B. anthracinus

 

This proposal would effectively remove a species from the current SACC list. The status of Buteogallus "subtilis" has long been debated. A detailed historical summary of the varying treatments (as a species vs. a subspecies of B. anthracinus) as well as a thorough morphological and distributional analysis has recently been published by Clark (2007), who concludes that subtilis is best considered a subspecies of anthracinus. I will summarize the main points below.

Thayer and Bangs described subtilis (as a species, following conventions of the time) in 1905 from two specimens from I. Gorgona off the Pacific coast of Colombia (interestingly, an island with only a few very tiny patches of mangroves and no mangrove specialists, although there are abundant mangroves on the adjacent mainland coast). Its characters were differences in size and wing coloration from B. anthracinus. The taxon was first considered a subspecies of anthracinus by Chapman in 1926 and then by Swann in 1930, correcting his earlier erroneous assignment as a subspecies of B. urubitinga; he also described the race bangsi from the Pacific coast of NW Colombia and E Panama (original citations of these and subsequent authors are given in detail by Clark 2007). Peters (1931), on the other hand, treated subtilis as a distinct species, giving its range as the Pacific coast from El Salvador to S Ecuador but without explanation or justification (including for extending its range so far northward). Most subsequent authors disagreed, including Amadon (1961) and Brown and Amadon (1958), until Monroe (1963) resurrected species status for subtilis and described a new race thereof from El Salvador and Honduras, based mainly on discontinuities in habitat and size, as well as the apparent propensity of coastal birds to feed mainly upon crabs (although crabs have often been reported in the diet of inland birds as well). Subsequently Amadon in 1979 reversed his opinion and considered subtilis a species, apparently following Monroe, although he presented no detailed analysis: he gave its range as N to S Mexico, apparently based upon a mislabeled specimen. The AOU treated subtilis as a species in 1983 and 1998, also surely following Monroe, who chaired the checklist committee. By contrast, virtually every author with field experience of these birds including (from south to north) Howell & Webb in Mexico, Land in Guatemala, Stiles & Skutch and Slud in Costa Rica, Wetmore and Ridgely in Panama, Hilty & Brown in Colombia and Ridgely & Greenfield in Ecuador, have failed to find convincing evidence for species status of subtilis. In spite of this, Thiollay treated subtilis as a species in HBW, albeit with reservations, and this was followed by Ferguson-Lees and Christie in a recent book on raptors of the world (reference at SACC Bibilo page).

Finally, Clark (2007) presented a detailed analysis based upon examination of ca. 140 adult specimens including both inland and coastal birds, from Mexico to extreme N Peru, as well as field experience from Mexico to Ecuador and Venezuela. He found that birds with the characters of subtilis occur on the Pacific slope from N Peru to E Panama; intergradation with nominate anthracinus occurs in E Panama and perhaps in NW Colombia, (bangsi, considered barely distinguishable from subtilis by Wetmore, presumably represents such an intergrade). He failed to find a clear-cut difference in size, coloration or vocalizations between coastal and inland birds northward, although noting a tendency towards smaller size in mangrove birds, and recommended treating subtilis as a subspecies of anthracinus. I note that on the Pacific slope of at least Colombia, only subtilis is recorded: there is no "coastal-inland" dichotomy. I might mention that in Costa Rica, Manuel Marín and I collected several birds in mangroves and inland and found no differences in size or vocalizations (some mangrove birds were as large as inland ones, other were smaller). The small size of the mangrove-inhabiting race utilensis off the Caribbean coast of Honduras can best be taken as an indication that a total diet of salt-water crabs stunts one's growth in Black-Hawks, and the larger size of some coastal birds of the Pacific likely indicates that some of their food was small vertebrates, or came from less saline areas.

In sum, considering the detailed analysis of Clark (2007) in particular (which I will admit I was gratified to find was in complete agreement with my own experience), I strongly recommend a YES on this proposal.

 

Reference:

Clark, W. S. 2007. Taxonomic status and distribution of Mangrove Black Hawk Buteogallus (anthracinus) subtilis. Bulletin of the British Ornithologists' Club 127:110-117.

(Other references are cited in full by Clark (2007); a pdf of this paper was recently distributed to the committee by Remsen.)

F. Gary Stiles, July 2007

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Comments from Remsen: "YES. Burden of proof now falls on those continuing to rank subtilis as a species. Mark Robbins told me that some genetic data are forthcoming, but I think that Clark's overview of the evidence is sufficient at this point to sink subtilis as a species-level taxon based on what is in print to-date."

Comments from Stotz: "YES. Sorry to see subtilis go, but  it seems pretty clear that it is now the thing to do."

Comments from Jaramillo: "YES - This seems similar to the Buteo polyosoma situation, although my guess is that a paper to counter this new arrangement will not be produced."

Comments from Robbins: "YES. Based on the Clark's morphological data subtilis appears not to deserve species status. I have collected or facilitated obtaining genetic material of "subtilis" from mangroves in Ecuador, Panama, and most recently El Salvador. Those data soon (hopefully) will be presented in a molecular based phylogeny of Buteogallus (Fleischer, Olson, Nyari and I) that corroborates that subtilis does not merit species status."

Comments from Nores: "YES. Los datos de Clark, los agregados por Stiles y los datos moleculares mencionados por Robbins, no dejan dudas de que se trata de una subespecie. Por distribución (ambas en Central América) parecía poco probable que fuera una subespecie, pero si subtilis está restringida a la zona pacífica donde no existe anthracinus, no existen problemas."

Comments from Pacheco: "YES. Diante das informações disponíveis ­ incluindo o testemunho de Robbins acerca dos dados moleculares ­ eu concordo com a subordinação sugerida."

Comments from Zimmer: "YES. Like Gary, I'm happy that the detailed analysis by Clark (and, apparently, forthcoming molecular analysis) was consistent with my own field experience. Vocalizations of subtilis from mangroves in Pacific Costa Rica are identical (to my ear) to those of North American "Common Black-Hawks" from Arizona/New Mexico/Texas, to the extent that tape playback of mangrove birds elicits an immediate response from our birds. Good to clean this one up."