Proposal (1033) to South American Classification Committee

Treat Basileuterus delattrii as (A) a separate species from Rufous-capped Warbler B. rufifrons, and (B) having the English name Chestnut-capped Warbler

Note from Remsen: This is a proposal submitted to and passed by NACC; the original is relayed here as is, modified slightly for SACC. Treat this proposal in two parts: A. Species limits. B. English name.

Effect on SACC classification: Approval of this proposal would change the species name and English name of the species we currently call, Basileuterus rufifrons, Rufous-capped Warbler, to Basileuterus delattrii, Chestnut-capped Warbler.

Background:

Basileuterus rufifrons is currently considered to consist of eight subspecies distributed primarily in Mexico and Central America, but also including the southwestern US and northern South America (see map below; subspecies actuosus, difficult to locate on the map, occurs on Isla Coiba, off the Pacific coast of Panama).

Subspecies are generally considered to form two groups: the rufifrons group (including caudatus, jouyi, dugesi, and rufifrons, we will hereafter refer to this group as simply “rufifrons” unless we specify otherwise), and the delattrii group(consisting of delattrii, mesochrysus, and actuosus, we will hereafter refer to this group simply as “delattrii”). In addition to plumage differences, these groups also differ in vocalizations (e.g., Howell and Webb 1995, Demko and Mennill 2019). The eighth subspecies, salvini, is intermediate between the two groups in some plumage features, most notably the extent of yellow coloration on the underparts (see depictions above), but it has typically been considered part of the rufifrons group.

The taxonomic status of rufifrons and delattrii has long been debated due to the variation in plumage and other characters. Ridgway (1902) considered the two taxa to be conspecific, but Todd (1929) not only treated rufifrons and delattrii as separate species, but placed them in different genera based on differences in relative wing and tail length (placing rufifrons, including salvini, in Idiotes). Hellmayr (1935) also treated rufifrons and delattrii as separate species; he was uncertain about salvini but stated that it "seems to be a representative form of B. rufifrons." Eisenmann (1955) treated rufifrons and delattrii as separate species as well, although he noted that delattrii may be conspecific with rufifrons.

Monroe (1968) lumped the species based on intergradation in plumage and morphometrics between rufifrons and delattrii, stating that rufifrons salvini intergrades with delattrii over a wide area in eastern Guatemala, El Salvador, and western Honduras. Peters (1968), in a family account co-authored by Monroe, followed this single-species treatment, and the AOU (1983, 1998) also treated them as a single species based on the intergradation noted by Monroe (1968). AOU (1983) considered the species to consist of two groups (rufifrons and delattrii), whereas AOU (1998) treated salvini as a third distinct group within the species. Regardless of taxonomic treatment, the distribution of neither rufifrons nor salvini was listed as extending south of Guatemala (AOU 1983, 1998), casting doubt on Monroe’s assertions of intergradation with delattrii in El Salvador and Honduras (which were nevertheless cited). In their field guide to birds of Mexico and northern Central America, Howell and Webb (1995) suggested that rufifrons and delattrii may be separate species, based on differences in plumage, morphology, and vocalizations, and noted that they are sympatric in southeastern Chiapas and western Guatemala. Nevertheless, most current references (IOC, Clements, Bird of the World) treat rufifrons and delattrii as a single species.

The status of delattrii has thus gone from species to subspecies of B. rufifrons due to perceived intermediate specimens from northern Central America. At the extremes of the distribution of the B. rufifrons complex, DFL can attest to the very different appearance and voices of the two main groups (rufifrons and delattrii), but the situation within the Chiapas/Guatemalan portion of the distribution has been the crux of the issue, with the supposed intermediate taxon salvini suggesting that these two groups are linked and interbreeding.

New Information:

Demko and co-authors, in a series of studies culminating in Demko et al. (2020), have studied the complex through investigations of various aspects of the biology of rufifrons and delattrii. The stated purpose of Demko et al. (2020) was to use voice, morphometrics, and plumage characters to assess variation within B. rufifrons sensu lato, with a particular focus on determining whether salvini is intermediate to rufifrons and delattrii, or more similar to one or the other group. They took measurements of morphometrics and plumage from more than 400 specimens, focusing on the region of contact in southern Mexico and Guatemala, and also measured more than 400 songs. One interesting finding, evident in the map above, is that the ranges of salvini and delattrii do not meet, meaning that salvini is unlikely to be an intergrade between rufifrons and delattrii.

In addition to their focus on salvini, Demko et al. also studied song and other behavior at a site in Chiapas, Mexico, where the rufifrons and delattrii groups are sympatric.

Morphometric analyses of wing and tail length showed that only wing length differed significantly among rufifrons, salvini, and delattrii, with wings of delattrii significantly longer than those of salvini, and rufifrons in between. Tail length was similar among groups, and wing-tail ratio averaged positive for delattrii but negative for salvini and rufifrons, as previously noted by Todd (1929). In plumage, salvini was more similar to one or the other group depending on the plumage character being analyzed.

The most persuasive and consistent distinctions were found in voice. Here’s Fig. 5 from Demko et al. (2020), showing representative male songs of (A) rufifrons where allopatric to delattrii, (B, D) delattrii where allopatric to rufifrons, (C) salvini, and (E) rufifrons and (F) delattrii where the two groups are sympatric in southern Chiapas:

Salient points from the figure are that (1) songs of salvini do not differ appreciably from those of rufifrons but are very different from those of delattrii, (2) songs of delattrii appear to be relatively consistent throughout its range, including in the zone of sympatry with rufifrons, and (3) songs of rufifrons in the zone of sympatry are very similar to those of salvini and rufifrons from outside of the zone of sympatry. These similarities and differences are further illustrated in their Fig. 6, a PCA of male song, in which songs of salvini and both allopatric and sympatric rufifrons group together apart from those of allopatric and sympatric delattrii:

Demko et al. (2020) concluded that these lines of evidence, particularly the vocal evidence, are sufficient to treat the two groups rufifrons (including salvini) and delattrii as separate species.

English names:

A vote to recognize two species in this complex will raise the question of English names for the daughter species. Although rufifrons probably occupies a larger range than delattrii, the relative range size is similar enough that the default course of action would be to create new names for both daughter species. However, NACC's English name guidelines recognize that range size is a proxy for degree of association of the name with one daughter or the other, and that relative degrees of association, disruption, and confusion are actually the key factors in deciding whether to continue to use a parental name for one of the daughter species in cases of a species split. The relevant part of our guidelines states:

Strong association of names with particular daughter species may provide exceptions to the above policy [of providing new names for both daughter species]. In these situations, a change to the English name of one daughter species would cause much more disruption than a change to that of the other daughter species. In these cases, the potential confusion of retaining the parental name for the daughter species strongly associated with the name is weighed against the potential disruption of changing the name. Overall, the goal is to maximize stability and minimize disruption to the extent possible.

Dating back to Ridgway (1902), rufifrons has always been known as Rufous-capped Warbler, sometimes sensu stricto and sometimes sensu lato, whereas delattrii has been known as Delattre's Warbler (Hellmayr 1935), Chestnut-capped Warbler (Eisenmann 1955, Hilty and Brown 1986, Howell and Webb 1995), and Rufous-capped Warbler (when considered conspecific, as by Ridgway 1902 and Monroe 1968). Of particular note are the names provided to intraspecific groups in recent editions of the checklist (AOU 1983, 1998), which in this case are Rufous-capped Warbler for the rufifrons group and Chestnut-capped Warbler for the delattrii group. It's a bit of a gray area for NACC because of the strong association of Rufous-capped with the northern form (rufifrons sensu stricto) and the retention of Rufous-capped Warbler as one of the AOU group names, contrasted with the fact that this name has also sometimes been used for the southern form (when lumped with rufifrons). 

Our recommendation, following consultation with global references such as the IOC World Bird List and the Clements Checklist, is that NACC retain Rufous-capped Warbler as the English name for B. rufifrons and adopt Chestnut-capped Warbler for B. delattrii. We consider that these English names for these taxa are well-established in the literature, including Eisenmann (1955) and Howell and Webb (1995) in addition to their usage as AOU group names, and that little confusion will result from the past practice of sometimes including the southern form delattrii within Rufous-capped Warbler. If a new English name were to be proposed for rufifrons, we would recommend the name Rusty-capped Warbler, which is close enough to the original name as to be instantly recognizable, but different enough to distinguish it from the umbrella name for the parent species.

Recommendation:

We believe the elegant study of Demko et al. (2020) makes a strong case that rufifrons and delattrii are best considered valid biological species. We recommend a YES vote on this split. If this split is supported, then we recommend the use of Chestnut-capped Warbler as the English name for B. delattrii and Rufous-capped Warbler for the newly restricted B. rufifrons.

Literature cited:

Demko, A. D., and D. J. Mennill. 2019. Rufous-capped Warblers Basileuterus rufifrons show seasonal, temporal and annual variation in song use. Ibis 161:481–494.

Demko, A. D, J. R. Sosa-López, R. K. Simpson, S. M. Doucet, and D. J. Mennill. 2020. Divergence in plumage, voice, and morphology indicates speciation in Rufous-capped Warblers (Basileuterus rufifrons). Auk 137:1-20.

Eisenmann, E. 1955. The Species of Middle American Birds. Transactions of the Linnaean Society of New York, Vol. 7.

Hellmayr, C. E. 1935. Birds of the Americas. Field Museum of Natural History Zoological Series, Vol. 13, Part 8.

Hilty, S. L., and W. L. Brown. 1986. A guide to the birds of Colombia. Princeton University Press, Princeton, NJ.

Howell, S. N. G., and S. Webb. 1995. A guide to the birds of Mexico and northern Central America. Oxford University Press, New York.

Monroe, B. L. 1968. A Distributional Survey of the Birds of Honduras. Ornithological Monographs No. 7. American Ornithologists’ Union, Washington, D.C.

Ridgway, R. 1902. The birds of North and Middle America. Bulletin of the United States National Museum No. 50, Part 2.

Todd, W. E. C. 1929. A revision of the wood-warbler genus Basileuterus and its allies. Proceedings of the United States National Museum 74 (2752): 1-95.

Terry Chesser and Daniel F. Lane

September 2024

The comments of NACC members on this proposal, which passed 8-2, are here:

https://americanornithology.org/about/committees/nacc/current-prior-proposals/2021-proposals/comments-2021-b/

_________________________________________________________________________________________________

Vote tracking chart: https://www.museum.lsu.edu/~Remsen/SACCPropChart968-1043.htm

Comments from Remsen: YES to both A and B. As outlined in the proposal, the evidence for treatment as separate species seems solid, based largely on strong vocal differences. Regardless, if rufifrons and delattrii are sympatric in southern Chiapas, then they have to be treated as two species by anyone’s definition, and there is no need for further discussion. As for English names, retention of Rufous-capped for rufifrons can be justified under our guidelines, because that was the same name used for it when treated previously as a separate species. Chestnut-capped is also well-justified as the name for that species through historical usage and appropriateness. I like the “-capped” common theme, also.”

Comments from David Donsker (voting for Bonaccorso): “B. YES. My votes are to use the well-established names Rufous-capped Warbler for Basileuterus rufifrons and Chestnut-capped Warbler for Basileuterus delattrii as discussed by Terry and Dan in their proposal.”

Comments from Areta:

“A. YES. The sympatry of rufifrons and delattrii in Chiapas without morphological intermediacy and without breakdown of their distinctive songs is very strong evidence supporting their separate species status. Demko et al (2019b*) found that sympatric delattrii responded more to own vocalizations than to rufifrons, while allopatric delattrii and rufifrons, as well as sympatric rufifrons responded equally to homo and hetero playbacks. This adds some evidence on a, perhaps, asymmetric discrimination, as well as possibly character displacement in sympatry.

“B. YES to Rufous-capped Warbler for Basileuterus rufifrons and Chestnut-capped Warbler for Basileuterus delattrii. I don´t see any need to quit using the Rufous-capped for the nominotypic rufifrons.

“*Demko, A. D., J. R. Sosa-López, and D. J. Mennill (2019b). Subspecies discrimination on the basis of acoustic signals: A playback experiment in a Neotropical songbird. Animal Behaviour 157:77–85.”

Comments from Rasmussen (voting for Robbins): “YES for Rufous-capped and Chestnut-capped warblers, as for NACC.”

Comments from Bonaccorso: “A. NO, for two reasons. First, we have no genetic data to back up this decision. What will happen if there is no substantial genetic differentiation between these two “species”? Will we lump them back again and be puzzled by the differentiation in song and morphology? Second, this decision will affect SACC´s nomenclature but is not based on any South American data (vocal, morphological or otherwise). What if B. r. mesochrysus (or at least the Colombian part of it) is another species? We will have to change the names again to reflect the reality of this taxon. For the sake of stability, why not wait until the whole complex is well sampled in terms of both distribution and diversity of characters to make a change?

Comments from Robbins: “A. Based on the vocal data, I vote YES for recognizing delattrii as a species.”

Comments from Zimmer: “YES, to both A and B. As Van noted, the sympatry of rufifrons and delattrii in southern Chiapas alone dictates that they be treated as distinct species. However, I am particularly impressed by the results of the vocal analysis by Demko et al. (2020). Their spectrograms and PCA figure make clear that the songs of the rufifrons-group and the songs of the delattrii-group, both from areas of allopatry and sympatry, are distinct, and confirm that salvini belongs with rufifrons and not with delattrii, as supported by morphometrics, but potentially confused by intermediacy of some plumage characters. My personal initial field experience with these taxa was with members of the rufifrons-group in Mexico, and when I subsequently encountered delattrii in Costa Rica and Panama, I was struck by how different the songs were. It’s nice to see the visual representation of these differences! I appreciate Elisa’s concerns regarding the lack of genetic data in Demko et al. (2020), as well as the lack of data points from the isolated Colombian range of mesochrysus, but to me, genetic distance is just one more character when considering species-limits – it doesn’t carry more weight than any other character set. If we can demonstrate diagnostic morphometric, plumage and vocal characters, and show that the two forms exist in sympatry, with assortative mating and lack of obvious hybrids or intergrades, then it shouldn’t really matter if they haven’t diverged genetically that much. As for what happens if and when Colombian mesochrysus proves to be worthy of elevation to species rank, that’s a simple fix too. I think it is extremely unlikely that Colombian mesochrysus would cluster with rufifrons, either vocally or genetically, given that more proximate populations of delattrii (including sympatric ones) are so clearly different from rufifrons. So, when Colombian mesochrysus is sampled, the results will either confirm that it belongs with delattrii, or, that it should be split from delattrii, which would retain its English name, and the isolated Colombian mesochrysus would require a new name. Waiting to correct obvious taxonomic problems due to potential complications that could arise from currently unsampled populations is a sure-fire recipe for stagnation in my opinion. If the unsampled populations are geographically intermediate with respect to the range of the group as a whole (making it impossible to discount clinal variation), or, if the unsampled population(s) are from actual or potential contact zones, then, I would agree that discretion and waiting for additional sampling is warranted. But, in this case, we’re talking about a peripheral, isolated population at the southern extreme of the overall group range, and I don’t think the lack of clarification regarding its status should keep us from advancing the ball downfield. As for the question of English names, I think the proposed one of retaining “Rufous-capped” for rufifrons, and resurrecting the previously established name of “Chestnut-capped” for delattrii is, by far, the most sensible and least disruptive option.”

Comments from Stiles. “A. YES. B. YES for Rufous-capped and Chestnut-capped, as recommended by NACC.”

Comments from Kimball Garrett (voting for Del Rio):

“A. YES -- Concern about the absence of genetic data is understandable, but given the fuzzy relationship between genetic distance and species-level status it seems unlikely that any such analysis would yield a strong argument either way regarding these taxa. To the many arguments for recognizing two species put forth by NACC members and within the SACC I would add (or underscore) the significance of the difference in call notes (especially the simple ‘chip’ call) between rufifrons and delattrii. Recordings available in the Macaulay archive not only provide evidence for clear differences in songs, but also in the chip calls. This difference was commented upon by Howell and Webb (1995); Howell described the call of delattrii as “…suggesting a Hooded Warbler (quite distinct from Rufous-capped Warbler).” Call note differences in oscine passerines may be as informative taxonomically as songs, if not more so.

“B. YES -- It is completely reasonable to use the long-established names “Rufous-capped” and ”Chestnut-capped” for the two species involved, thus aligning with NACC and many other publications. [Though admittedly the names could be reversed and be equally descriptive.]”

Comments from Claramunt: “YES. The congruence of plumage and vocal variation indicates the presence of two separate lineages with no sign of intergradation despite sympatry.”