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.”