Split Calonectris edwardsii from Calonectris diomedea
Proposal (483) to South American Classification Committee
[Note from Remsen: this proposal passed the North American Classification Committee in 2005 and is posted here with the permission of the author. English names: NACC retained Cory’s Shearwater for diomedea and use Cape Verde Shearwater for borealis; I see no reason not to follow this]:
Effect on SACC: This proposal would add a species to our list, Calonectris edwardsii, as a split from C. diomedea (in our case subspecies borealis). [See Stotz in Comments for basis for South America].
Puffinus edwardsii was described by Oustalet as a species in 1883 (and again, as P. marianae, in 1898 by Alexander). Godman (1910) kept edwardsii as a separate species, but Murphy (1924), who had deep reservations, and Peters (1931) lumped it with Calonectris diomedea. This course has been followed by most recent authors (Cramp 1977, Sibley and Monroe 1990, del Hoyo et al. 1992, Dickinson 2003). The AOU (1983) included the Cape Verde Islands in the extralimital distribution of Calonectris diomedea, but the AOU (1998) did not, implying that it had changed to consider edwardsii specifically distinct. However, this change was never brought to vote. Interestingly, though Sibley and Monroe (1990) make no mention of edwardsii (as a species or as a group within diomedea), they failed as well to include the Cape Verde Islands in the distribution of diomedea. It is possible that the lapsus of Sibley and Monroe (1990) may have been carried over to AOU (1998). However, in their supplement, Sibley and Monroe (1993) included edwardsii as a group within diomedea. In his proposal (#2001-B-04) to the AOU-CLC to split C. [d.] borealis from C. diomedea, R. C. Banks (in litt.) included edwardsii with borealis, although he suggested that it too might be specifically distinct.
Bannerman and Bannerman (1968) were the first in recent times to accord edwardsii specific status. They based their decision primarily on morphometric and vocal differences and distribution. Hazevoet (1995) also recognized edwardsii as a species. In detailing the record of edwardsii off North Carolina, Patteson and Armistead (2004) considered edwardsii specifically distinct.
New Information and Analyses
Bannerman and Bannerman's (1968) analyses of Calonectris edwardsii provide little qualitative data to address their consideration of edwardsii as specifically distinct, though their argument is fairly convincing. They had extensive experience on the breeding grounds with both C. d. borealis and edwardsii. They mentioned that edwardsii is much smaller, has a much darker bill (dark base compared to the distinct yellow base of borealis), the crown and nape are darker, the darker back lacks white feather edges, and the flight style and vocalizations differ (though they do not mention how either differs). Porter et al (1997) describe differentiating edwardsii and diomedea in the field. Patteson and Armistead (2004) summarized plumage and morphometric differences between diomedea and edwardsii, and, in nine photographs, show these distinctions quite clearly.
The major differences are:
1) smaller size. Using ample series (50 of each sex) of edwardsii and borealis Murphy (1924) showed no overlap in wing, tail, exposed culmen, and middle toe, and some overlap in tarsus. Patton and Armistead (2004) cited measurements taken by Dick Newell and Tony Marr that show that edwardsii is 10% smaller than the smallest diomedea (eastern breeding nominate populations). Photos in Patteson and Armistead 2004 show that edwardsii is appreciably smaller than the nearby diomedea (presumably the larger borealis).
2) Bill noticeably "thinner" than diomedea, and basally gray or pinkish gray instead of yellow or ivory at base in diomedea (Porter et al 1997); I would say the bill is less deep, and the photos in Patteson and Armistead 2004 show this very well. Patton and Armistead cite measurements taken by Dick Newell and Tony Marr that show that the smallest diomedea (eastern breeding nominate populations) have bill depth of 12.5-13.5 mm, while edwardsii was 9.5-9.9 mm.
3) Upper parts darker and grayer brown in edwardsii, especially on the head and nape, with little contrast between the nape and mantle.
4) 'Upper-tail coverts tend to show consistently more white' Porter et al. 1997. Patton and Armistead found this variable in both species.
5) A clean demarcation between the dark upper and white lower parts of the head (Porter et al. 1997); not noticeably cleaner in the photos in Patton and Armistead (2004).
Wing beat perhaps less deep and the wings held slightly more forward.
Appears longer in the hind-body + tail than diomedea in the field. Visible in some photos in Porter et al. (1997).
The closest breeding population of C. diomedea is C. d. borealis in the Canary Islands; this subspecies also occurs on other north Atlantic islands (Azores, etc.). The smaller nominate subspecies nests on islands in the Mediterranean, and varies clinally in size from large in the west to small in the east. Gene flow may be substantial between borealis and diomedea, as individuals banded on borealis colonies in the Atlantic have been recaptured as breeders in nominate colonies in the Mediterranean (Randi et al. 1989, Martinez-Abrain et al. 2002). The geography of morphometric variation within diomedea, borealis, and edwardsii indicates gene flow between diomedea and borealis, but not between diomedea and edwardsii or between borealis and edwardsii. Morphometric variation within borealis is much less than within nominate diomedea, and there are no indications that populations of borealis that are the closest to edwardsii tend toward edwardsii in either plumage or morphometrics.
Calonectris diomedea and edwardsii differ from one another as much as or even to a greater extent than do several taxa in the closely related genus Puffinus that the AOU-CLC currently considers separate species. For example, Puffinus puffinus, opisthomelas, and P. auricularis (along with the extralimital gavia and huttoni), are all considered separate species, though structurally they are quite uniform, and they differ in plumage only in color of undertail coverts, with subtler differences in the darkness of the upperparts and facial pattern. Likewise, P. assimilis, and P. lherminieri are structurally uniform and differ only subtly in plumage. Puffinus tenuirostris and P. griseus differ consistently in plumage only in the color of underwing coverts, though they are structurally quite different. Breeding ground vocalizations are thought to be important species recognition in procellariids and C. d. borealis and diomedea show consistent differences (Bretagnolle and Lequette 1990). The calls of edwardsii are said to be quite different from borealis (Murphy 1924, Bannerman and Bannerman 1968), but these differences have not been quantified.
Given that no rationale was ever given for lumping C. diomedea and C. edwardsii in the first place, and that consistent differences between the two taxa are at least equal to the differences between taxa that we currently recognize as specifically distinct in a closely related genus, I recommend that we vote yes to split edwardsii from diomedea. Vocal differences lend further support to there being potential reproductive isolation between edwardsii and diomedea. And lastly, the type of gene flow and its effects that can be seen currently between C. d. diomedea and C. d. borealis are not in evidence between C. diomedea and C. edwardsii, even though the breeding population of edwardsii is not that far removed from the nearest populations of C. diomedea.
Alexander, B. 1898. An ornithological expedition in the Cape Verde Islands. Ibis 7: 74-118.
Bannerman, D. A., and W. M. Bannerman. 1968. History of the birds of the Cape Verde Islands, vol. 4, Birds of the Atlantic Islands. Oliver and Boyd, Edinburgh.
Bretagnolle, V., and B. Lequette. 1990. Structural variation in the call of Cory’s Shearwater (Calonectris diomedea, Aves, Procellariidae). Ethology 85: 313-323.
Cramp, S. (ed.). 1977. The birds of the Western Palearctic, vol. 1. Oxford University Press, Oxford, U. K.
del Hoyo, J., A. Elliott, and J. Sargatal. 1992. Handbook of Birds of the World. Vol. 1. Lynx editions, Barcelona.
Dickinson, E. C. 2002. The Howard and Moore checklist of birds of the World.
Godman, F. du C. 1910. A monograph of the petrels. Witherby, London.
Hazevoet, C. 1995. The birds of the Cape Verde Islands: an annotated check-list. BOU check-list no. 13. British Ornithologists’ Union, Tring, UK
Martinez-Abrain, A., A. Sanchez, and D. Oro. 2002. Atlantic Cory’s Shearwaters breeding in a colony of Mediterranean Cory’s Shearwaters. Waterbirds 25: 221-224.
Murphy, R. C. 1924. The marine ornithology of the Cape Verde Islands, with a list of all the birds of the archipelago. Bulletin of the American Museum of Natural History 50: 211-278.
Oustalet, E. 1883. Description díespecies nouvelles díoiseauc provenant des iles du Cap-Vert. Annales des Sciences Naturelles (Zoologie) 16:1-2.
Patteson, J. B., and G. L. Armistead. 2004. First record of Cape Verde Shearwater (Calonectris edwardsii) for North America. North America Birds 58: 468-473.
Porter, R., D. Newell, T. Marr, and R. Joliffe. 1997. Identification of Cape Verde Shearwater. Birding World 10: 222-228.
Randi, E., F. Spina, and B. Massa. 1989. Geographic variability in Cory’s Shearwater (Calonectris diomedea). Auk 106:411-417.
Sibley, C. S., and B. L. Monroe. 1990. Distribution and Taxonomy of the birds of the World. Yale University Press, New Haven.
Sibley, C. S., and B. L. Monroe. 1993. A Supplement to Distribution and Taxonomy of the birds of the World. Yale University Press, New Haven.
Comments from Stiles: “YES. Again, the evidence for the split, although imperfect, is certainly better than that for considering edwardsii a subspecies of diomedea – and more in line with current standards in Procellariidae.”
Comments from Robbins: “YES. The reported morphological and vocal differences coupled with the apparent lack of gene flow between edwardsii and the nearest population of diomedea seems to support the recognition of edwardsii as a species.”
Comments from Nores: “YES. The size and bill differences are very noticeable, as is evident in the photos by Patteson and Armistead.”
Comments from Stotz: “YES. I think the proposal should be amended to mention the basis of this species in South America. The NAB article on it off North Carolina mentions Brazilian records from Petry et al 2000, and Olmos 2002. Lima et al have an article in Ararajuba 10 (2002) regarding this species in Brazil as well.”
Comments from Pacheco: “YES. No Brasil, esse tratamento como espécie à parte foi implementado por influência direta de Olmos, F. (2002). At-sea records of Cape Verde Shearwaters Calonectris edwardsii in Brazil. Atlantic Seabirds 4(2): 77-80.”
Comments from Zimmer: “YES. Described morphological differences are consistent and, comparable to those found between other pairs of shearwater taxa already recognized as being specifically distinct. Vocal differences, although not quantified, appear to differ to an even greater extent, and there is no evidence for gene flow between edwardsii and any population of diomedea. And, once again, we are dealing with another pair of taxa that were lumped without a published justification by Peters.”
Comments from Pérez-Emán: “NO. Besides clear morphological differences, the proposal is not supported by other data from other characters. Vocal data are given for comparison between diomedea and borealis but not for edwardsii. Although evidence seems to suggest these are two species, there is no much support to recognize these taxa as different based on the Biological Species Concept we implement here.”