Proposal (878) to South American Classification Committee

Treat Caracara cheriway as conspecific with Caracara plancus

Background. When SACC began in 2000, it adopted the Howard & Moore baseline list which had already considered cheriway to be distinct from plancus following Dove and Banks (1999). As such SACC has never voted on whether to accept or reject this split.

Available data. The standard treatment of C. p. plancus and C. p. cheriway as subspecies had been supported by both Hellmayr and Conover (1949: 283-284), and Vuilleumier (1970), who agreed that there is intergradation between the two forms in Brazil. Then, on the basis of several plumage features, Dove & Banks (1999) considered C. plancus to comprise three biological species with birds of northern South America assigned to C. cheriway and an extinct species which occurred on Guadalupe Island, Mexico as C. lutosus. This was based primarily on patterns in five characters of contour plumage: 1) Breast; 2) Vent area; 3) Upper back/scapulars; 4) Lower back; and 5) Upper tail coverts. Their study of 23 specimens in the contact zone of cheriway-plancus showed highly mixed characters (Dove & Banks 1999; see their Table 2), even when multiple specimens from the same locality were concerned, with the presence of each defined character indiscriminately mixed in different specimens over a very large area.

The biometrical analysis performed by Dove and Banks (1999), showed that variation was extensive with the largest specimens coming from the extreme south of South America; and that there was a clinal increase in wing chord, bill length and bill depth as each taxon was recorded further away from the equator. They also found that females were larger in the Northern Hemisphere, but not in the Southern Hemisphere.

Table 2 from Dove & Banks (1999)

New information. Fuchs et al. (2012) conducted a molecular phylogenetic analysis on all members of the Polyborinae, finding many interesting relationships in the systematics of the subfamily. Divergence between plancus and cheriway proved to be a recent event estimated at 0.2-0.5 MYA.

Moreover, Fuchs et al. 2012 (p. 529) found the following:

"The two species of the genus Caracara differed by a mitochondrial uncorrected p-distance of 0.5%, which is one of the smallest divergences among Falconidae species based on similar sequence data (e.g. tRNA-Leu to ND2; the smallest being 0.08% between the Saker Falcon Falco cherrug and Gyrfalcon Falco rusticolus, J. Fuchs, J.A. Johnson, D.P. Mindell unpubl. data, as well as between P. albogularis⁄P. megalopterus, see below). Despite the low sequence divergence, the two Caracara taxa have been recognized as distinct species based on plumage characters (Dove & Banks 1999). Our samples of C. cheriway do form a monophyletic group with respect to the single C. plancus individual in the mitochondrial and in some nuclear trees; however, the individuals for the two species used in this study were sampled from the extremes of their geographical distributions. Dove and Banks (1999) suggested that biometric measurements are correlated with latitude for wing chord, bill length and bill depth. Hence, the pattern of molecular differentiation we found here could also be the result of isolation by distance. To enable more robust conclusions to be drawn concerning the taxonomic affinities within Caracara, further sampling is required, including samples from areas close to the Amazon River, where the distributions of C. cheriway and C. plancus overlap and where individuals with mixed plumage characteristics have been collected (Dove & Banks 1999).”

This result is all the more remarkable given that the closest samples to one another, came from geographical extremes; the southernmost cheriway sample was from Nicaragua and the only plancus sample from Paraguay.

Figure 3 from Fuchs et al. (2012)

Discussion. From what is known, cheriway and plancus share a broad number of indiscriminately mixed plumage characters in their area of overlap; a large and broad swathe of Amazonia. Furthermore, their mitochondrial DNA hardly differs with nearest samples coming from a distance of 4300 km that is remarkable (indeed, this would be the expected if there was only isolation by distance, without any proper interbreeding barrier). Therefore, we do not see any supporting evidence for species-level differences and at most a subspecific relationship could be claimed, while we cannot rule out that the two forms are linked through a cline.

Recommendation. We recommend that cheriway and plancus be considered as part of a single species (perhaps best considered as subspecies for the time being). A YES vote would lump C. cheriway with C. plancus and a NO vote would maintain the two as full species. If the merger is approved, C. plancus would once again be known as the Crested Caracara.

References

Dove, C. J. & Banks, R. C. (1999) A taxonomic study of Crested Caracaras (Falconidae). Wilson Bulletin 111: 330–339.

Fuchs, J., Johnson, J.A. & Mindell, D.P. (2012) Molecular systematics of the caracaras and allies (Falconidae: Polyborinae) inferred from mitochondrial and nuclear sequence data. Ibis 154: 520-532.

Hellmayr, C.E. & Conover, B. (1949) Catalogue of birds of the Americas and adjacent islands. Part 1. Number 4. Field Mus. Nat. Hist. Publ. 634. Zool. Series 13: 1-358.

Vuilleumier, F. (1970) Generic relations and speciation patterns in the Caracaras (Aves: Falconidae). Breviora 355: 1– 29. 

Mark Pearman and Juan I. Areta, August 2020

Comments from Stiles: “YES to consider C. cheriway and C. plancus as a single polytypic species given the evidently wide zone of introgression with many individuals of mixed plumage; the minimal genetic difference between birds at the extremes of the wide distributions of both could easily be a distance effect, and reverting to the name Crested Caracara is indicated.”

Comments solicited from William S. Clark: “The plumage differences between Caracara cheriway and c. plancus are minimal and well within the range of variation of subspecies of most raptors. Certainly, these differences are much less than the differences between Harlan’s Hawk and Red-tailed Hawk, which the AOS considers subspecies.

“I have watched Southern Caracaras in Brazil and found few differences in behavior or vocalizations with Crested Caracaras, with which I am very familiar. Both have the unique vocalization of throwing their heads back and calling.

“The data presented in the proposal further support treating them as two subspecies.”

Comments from Claramunt: “YES. The proposed “zone of contact” is actual a vast region with mostly intermediate forms, not a narrow hybrid zone, breaking down the apparent diagnosability and separate identities of northern and southern forms. Back to one species.

Comments from Robbins: “YES for treating cheriway as conspecific with plancus based on the Fuchs et al. genetic data set.”

Comments from Zimmer: “Mark me down as an enthusiastic YES! Having spent a lot of time with cheriway at the northern limits of its distribution, even more time with plancus in the heart of its range, and time in the “contact zone”, not to mention all of the time sorting through museum specimens trying to make sense of all of the plumage variation, I’ve never really been on board with accepting the split. Now, the Fuchs et al. data set makes clear just how little genetic separation there is between cheriway and plancus, even when sampling from near the distributional extremes of the two forms. As Santiago notes, the contact zone between these two is actually a broad swath of intergradation, which squares with my examination of many, many, seemingly intermediate specimens from across Brazil. I would be relieved not to have to take too deep of a dive into the weeds of field separation of these two in the Brazil field guide – as Pearman and Areta suggest in the Proposal, there is enough evidence of clinality here, that one could make a case that cheriway and plancus should not even be recognized as different subspecies!”

Comments from Lane: “An emphatic YES on this one! I have been greatly bothered by how weakly the two "species" are differentiated, particularly given the added issue of which is invading western Amazonia at a rapid rate! From what I can tell, the original paper used historic specimens that didn't reflect the apparent introgression already being observed in Amazonia at the time of its writing, which really weakened the paper's punch for me considerably. I cannot see how such a split could be ratified without someone studying the current expansion by both populations into cleared lands between the two source populations. Given how hard it is to recognize hybrids, any such study would, by necessity, require a molecular aspect to show gene flow or a lack thereof. Until such time, I think the only prudent treatment is to consider them conspecific. How that affects the Guadalupe Caracara (RIP) is unclear, but that's also not SACC's problem to resolve!”

Comments from Bonaccorso: “YES. Evidence of clinal variation, what seems to be a broad contact zone, and small genetic differences support the lumping.”

Comments from Remsen: “YES. Evidence for the split was very weak, and all data suggest no barriers to gene flow. As for English name, broadly defined Caracara plancus was always known as Crested Caracara, as noted in the proposal, and so there is no need for a proposal on English name.”

Comments from Jaramillo: “YES – This has always been a weak one. Unless someone can come up with some vocal or behavioral display differences that could be construed as a barrier to willy-nilly gene flow, I think it is best to lump them. Note that I was puzzled by the comments by Bill Clark. Why would differences in Red-tailed Hawks be a basis for comparison to a Caracara? The two are not in the same group at all taxonomically. We have visually nearly identical forest falcons however!”

Comments from Pacheco: “YES. I consider it very pertinent to treat them as a single species. The ‘contact area’ between the two forms is vast. In addition, the nominal form has been expanding northwards with deforestation in the Amazon in recent decades https://doi.org/10.4013/nbc.2015.103.07.”