Proposal (831) to South American Classification Committee


Recognize two genera in Stercorariidae


Note from Remsen (May 2019): Kevin Winker submitted this proposal to NACC, which followed his recommendation to stay with a single genus (Stercorarius), as we currently also do.






Recognize two genera in Stercorariidae


Effect on NACC:


This proposal would resurrect the genus Catharacta for all species of Stercorarius except S. parasiticus and S. longicaudus.




Despite considerable attention over the past two decades, the phylogeny of the Stercorariidae (skuas and jaegers) has not yet been convincingly resolved. Most authoritative works presently consider the skuas and jaegers to be in a single genus, Stercorarius (e.g., AOU 2000). Nevertheless, there is support in existing data for S. pomarinus being more closely related to the traditional Catharacta species (skuas) than to the other jaegers (e.g., Braun & Brumfield 1998, Carlos 2016).


New information:


Carlos (2016) re-examined existing data and concluded based on chewing lice, behavior (displays and calls), and mtDNA that S. longicaudus and S. parasiticus form a clade sister to the traditional Catharacta + S. pomarinus, and they proposed splitting the group into two genera accordingly: Stercorarius (spp. parasiticus and longicaudus) and Catharacta (spp. pomarina, skua, maccormicki, lonnbergi, hamiltoni, chilensis, and antarctica). (Those in our checklist area in bold.)


However, this conclusion rests entirely on cladistic reasoning (“a cladistic-based classification by sequencing”, p.193), and there remains considerable uncertainty about relationships in the group. There is no suggestion that all members are not part of a monophyletic clade, and using a single genus for this clade, Stercorarius, is what we chose to do when last visiting this issue (AOU 2000). We are also presently seeing some noteworthy failures of mtDNA to accurately reconstruct intra-generic relationships (e.g., Harris et al. 2018, Drovetski et al. 2018). This becomes relevant here in two contexts: a) it would be good to get final confirmation of this intrageneric split, and b) we need clarification of the relationship of pomarinus with respect to the Catharacta species to know whether there is support for it being considered in its own, monotypic genus (Coprotheres, Braun & Brumfield 1998, Carlos 2016). Given historic uncertainties in this group’s systematics and the interest in it expressed among diverse researchers worldwide, I think we can expect a convincing resolution of these issues in the next few years (although I have no inside knowledge of such an effort). That would enable us to make any further necessary changes just once.




No. Retain the single genus Stercorarius for all species in Stercorariidae at this time.


Literature cited:


American Ornithologists’ Union. 2000. Forty-second supplement to the American Ornithologists' Union Checklist of North American Birds. Auk 117:847–858.

Braun, M. J., and R. T. Brumfield. 1998. Enigmatic phylogeny of skuas: an alternative hypothesis. Proceedings of the Royal Society of London B 265:995-999.

Carlos, C. J. 2016. How many genera of Stercorariidae are there? Revista Brasileira de Ornitologia 24(2):191-195.

Drovetski, S. V., A. B. Reeves, Y. A. Red’kin, I. V. Fadeev, E. A. Koblik, V. N. Sotnikov, and G. Voelker. 2018. Multi-locus reassessment of a striking discord between mtDNA gene trees and taxonomy across two congeneric species complexes. Molecular Phylogenetics and Evolution in press.

Harris, R. B., P. Alström, A. Ödeen, and Adam Leaché. 2018. Discordance between genomic divergence and phenotypic variation in a rapidly evolving avian genus (Motacilla). Molecular Phylogenetics and Evolution in press.



Submitted by: Kevin Winker


Date of Proposal: 31 December 2017



Comments from Stiles: “NO. Especially given the uncertainty regarding what to do with pomarinus, keeping all the species in Stercorarius is currently the best option.”


Comments from Robbins: “ “NO, Clearly more data are needed before making any generic changes.”


Comments from Pacheco: “NO. Given that the two phylogenies published in 2018 maintain uncertainties about intra-generic relationships, I share the view that further studies are advised to ensure a reversal of current treatment.”


Comments from Zimmer: “NO”.  Given that the one undisputed relationship that everyone can agree on is that, as currently constituted, Stercorarius represents a monophyletic grouping, then, I see no reason to tinker just for the sake of achieving more narrowly defined monophyletic groupings, particularly when there is ambiguity regarding the position of pomarinus relative to the two other proposed groupings.  The credo “First, do no harm” would seem to apply here – waiting for better data (allowing for more granular resolution) does no harm, and prevents us from making a destabilizing change now, only to possibly have to reverse course when that better data is actually published.”


Comments from Jaramillo: “YES – This is a case of not seeing the forest for the trees. Sometimes you have to just go with what is obvious and makes sense!! There are skuas (Catharacta) and jaegers (Stercorarius), you could show photos of various species in the group to a bunch of kids and ask them to sort them out into whatever number of groups they wanted, and they would surely come up with two. Jaegers and Skuas. This is obvious, clear cut, and simple. Now if the DNA do not quite fit this dichotomy, it is because there was some weirdness in the past history of the group. I do not doubt that Pomarine may have some genes from skuas from some distant hybridization, or perhaps even a more complex scenario than this. I have Neanderthal genes, but I am not a Neanderthal (at least I try not to behave like one). So whatever funky situation may have gone on in the past that is coded in the genes, it does not take away from the obvious. There are jaegers and there are skuas, and Pomarine is clearly in the jaeger group. You do not need to create a special genus for it. I think that we have to sometimes just look at what the birds are telling us and have that help us guide some understanding of the genes, but not always listen to the genes and ignore what the birds tell us.

“Jaegers (Stercorarius) are not only joined by the tail streamers but a multitude of characters: 1) solid dark caps as adults; 2) yellow coloration on neck and face; 3) solid and unstreaked upperparts as adults; 4) tendency for dark breast band, Long-tailed has this as a subadult and loses it as an adult; 5) presence of dark morphs in the population; Long-tailed shows dark morph as immatures, not adults which is odd; note that South Polar Skua varies in plumage coloration, but it does not have a true dark morph; 6) Long-distance migration in all three species, migration is quite variable in Catharacta; 7) strictly Northern Hemisphere breeders; 8) Distinct immature plumages, two cycles of immaturity before the adult, skuas are much more difficult to age; 9) youngsters have distinct barred pattern on underwing coverts, unlike skuas; 10) adults have clear breeding/non-breeding (alternate/basic) plumages unlike skuas….  How much more does one need to decide that they are a clear and obvious group! I am sure that eventually other genetic work will prove that this is the reality, that Stercorarius and Catharacta are different, because it seems obvious to me just from knowing the birds.”


Additional comments from Remsen: Concerning Alvaro’s comments above, I agree in every detail that the “sorting hat” would place each species cleanly in one of the two groups.  However, I also caution that a major lesson from DNA-based phylogenetic studies is that clean phenotypic groups are not necessarily monophyletic.  Here, for example, the ancestral condition could be the jaeger phenotype, but if all skuas are derived from a split between pomarinus and proto-skua, then pomarinus and skuas are sisters with respect to the longicaudus-parasiticus group.”


Comments from Areta: “NO, for the time being. I understand Alvaro´s reasoning and long list of features, but there are considerable uncertainties on the exact phylogenetic placement of pomarinus, and until this is sorted out convincingly, I prefer to adopt the cautious decision of keeping all skuas and jaegers in Stercorarius.”


Comments from Claramunt: “NO. Given the levels of diversity, I prefer to maintain a single genus that doesn’t make much sense rather than two genera that do not make much sense.”


Comments from Bonaccorso: “NO. I also agree with Álvaro regarding the importance of the long list of biological characters that separate skuas and jaegers; the separation does make sense. However, it would be premature to split them without more genetic evidence (some of those characters could arise by convergence).”