Proposal (776) to South American Classification Committee


Note from Remsen: Below is the proposal submitted to, passed by, and adopted by NACC; see latest NACC Supplement in Auk 2018).  I made a few minor edits for SACC.  For NACC members’ comments on this proposal, see:, proposal 2017-A-8).  Northern Harrier (currently C. cyaneus in our classification) is a wintering (NB) species in the Colombia and Venezuela)


Treat New World Circus (c.) hudsonius as a separate species from Old World Circus cyaneus


Description of the problem: In 2015, NACC considered but narrowly rejected (by seven votes to five) a proposal (2015-C-9) to split Circus cyaneus and C. hudsonius, the latter currently considered a subspecies of the former. (See:, 2015-C-9) The primary basis behind some committee members’ rejection of the split was the weak genetic sampling coupled with the relatively low genetic divergence (1.1 to 1.7%). Another objection was the lack of demonstrated vocal differences.


New information: Etherington and Mobley (2016) compared cyaneus and hudsonius in DNA, plumage, measurements, and ecology, and recommended on these bases that they should be considered separate species. These authors sequenced a few new samples of cyt b (8 frozen tissues of hudsonius and 3 toepads of cyaneus), and used GenBank samples of COI from bar-coding (4 hudsonius, 7 cyaneus) and Oatley et al.’s (2015) ND1 sequences for further analyses.  They found in each of their analyses that cyaneus and hudsonius form monophyletic clades. They also found genetic distances ranging from 1.3 to 1.8% between the two taxa. They noted that in several other cases genetic distance between undisputed species-pairs of raptors is in this range, well below 2%.


The morphological data presented by Etherington and Mobley (2016) confirm that, sex-for-sex, hudsonius is larger than cyaneus. They review the sexual and age-related differences between the taxa, noting that adult males differ by 13 morphological characters, females by about four, and juveniles by three or more.


In the discussion, Etherington and Mobley (2016) noted “numerous differences between cyaneus and hudsonius when it comes to vocalization, habitat, distribution and movements, mate choice and breeding biology”, and yet their vocal analysis is limited to two paragraphs summarizing characteristics of sonograms in the Western Palearctic handbook (Cramp and Simmons 1980) and the BNA account (MacWhirter and Bildstein 1996); no mention is made of online resources nor commercial CDs. They concluded based on this tiny sample that the taxa differ vocally in that cyaneus gives kek calls at a faster rate than does hudsonius, both in male and female distress calls.


Etherington and Mobley (2016) then compared life-history information extracted from the literature. They argue that hudsonius is a bird of wetlands, prairies, dry grasslands, and agricultural areas, whereas cyaneus breeds in heather moorland, sand dunes, young coniferous forest, sedge-rich northern lakes, and woodland (both open- and closed-canopy). (Note that Old World marsh habitat is typically occupied by the larger marsh harriers.) Although cyaneus breeds at least mostly in dry upland habitats, hudsonius typically breeds among reed beds in marshes, even constructing platforms that raise the nest above the water level. Other life-history comparisons given by these authors include that in cyaneus females have been recorded as displaying much more than males, whereas the reverse has been found in hudsonius; and that female hudsonius have been recorded as being much more capable of successfully raising young successfully after desertion by the male than is cyaneus. Despite the statement quoted in the previous paragraph, data are not presented on differences in mate choice, and the differences discussed in distribution and migratory route provide no data relevant to species status.


Subsequent treatments: As far as I am aware, SACC is the only major relevant avian taxonomic entity that has yet to adopt this split.


Effect on SACC: New World C. c. hudsonius (Northern Harrier) would be considered specifically distinct but would not necessarily require an English name change.


Recommendations: Etherington and Mobley (2016) provided further evidence that cyaneus and hudsonius are discrete lineages with differing breeding habitats and possibly with behavioral differences. The vocal differences alluded to therein simply do not hold up, however, I reexamined the recordings from commercial CDs that led to our conclusion in Rasmussen and Anderton (2005) that vocalizations are broadly similar (mainly Roché 1996 and Chappuis 2000) for cyaneus, and compared these and the few for cyaneus on xeno-canto with the now-extensive sample for hudsonius from several sources. This expanded sample shows that there is complete overlap in rate of kek calls, and I can hear no intertaxon differences in quality of these or the other main vocalization type, the more prolonged mewing calls. That is not to say that careful study of homologous display sounds would not turn up differences, but these must be subtle at best. Because many diurnal raptors lack obvious vocal differences, especially among those (like harriers) that tend to be fairly quiet, I do not think this is particularly consequential. (Note the dearth of recordings of cyaneus even on xeno-canto, and a total lack thereof on Macaulay Library and IBC sites, which is surprising for a widely distributed Palearctic species.)


However, these taxa are well-differentiated morphologically, more so than most other Holarctic-distributed species (e.g. subspecies of Golden Eagle, Rough-legged Hawk, Common Raven, Greater Scaup, and Common Goldeneye, for example). I have thought them better treated as separate species for a couple of decades now, ever since preparing materials for our book. In retrospect, I think that my quote from Oberholser in the original proposal may have led some to be swayed by his viewpoint, which was probably based on examination of specimens with folded wings, and thus not a full accounting of the prominent differences, especially in adult males.


Please vote on (A), and if your vote is yes, also vote on (B):


(A) recommend splitting Circus hudsonius from C. cyaneus.

(B) If split, I recommend continuing to use the name Northern Harrier for C. hudsonius (rationale given in:, 2015-C-9).


Literature cited:


Chappuis, C. 2000. African Bird Sounds. Birds of north, west and central Africa. 11 CD set. Soc. d’Etudes Orn. France, Paris.

Cramp, S. and K. E. L. Simmons. 1980. Handbook of the Birds of Europe, the Middle East and North Africa. The Birds of the Western Palearctic. Vol II. Oxford University Press, Oxford.

Etherington, G. J. and J. A. Mobley. 2016. Molecular phylogeny, morphology and life-history comparisons within Circus cyaneus reveal the presence of two distinct evolutionary lineages. Avian Research 7: 17.

MacWhirter,  R. B. and K. L. Bildstein. 1996. Northern Harrier (Circus cyaneus). In: Poole, A. and F. Gill (eds.). The Birds of North America (No. 210).

Rasmussen, P. C. and J. C. Anderton. 2005. Birds of South Asia: the Ripley Guide. Lynx Edicions, Barcelona.

Roché, J. C. 1996. Bird Songs and Calls of Britain and Europe on 4 CDs. WildSounds.


Pam Rasmussen, February 2018





Comments from Claramunt: “YES. Morphometrically there’s a lot of overlap and I don’t think there’s anything “significant” there. Although they don’t show quantitatively that there is discontinuity in the variation, they say that all individuals of hudsonius can be distinguished from all individuals of cyaneus based on multiple plumage traits. The illustrations and photographs that I’ve seen so far corroborate that. The differences are not outstanding, but they seem diagnostic. What is missing from this proposal is the results of Oatley et al. (2015), which showed that hudsonius is more closely related to C. cinereus than to cyaneus based (mostly) on ND1 mtDNA sequences (however, some CO1 sequences that I reanalyzed show cyaneus and hudsonius as sister, so don’t take Oatley et al. 2015 as the final word on this). In any case, I think that the evidence so far indicates that hudsonius is a species-level taxon and evidence of reproductive compatibility with cyaneus is completely absent.”