Proposal (994) to South
American Classification Committee
(Note from Remsen: This proposal was submitted to the North American
Classification Committee and passed, and it is submitted here with permission
from Terry Chesser. This is an
extralimital split, but we have to deal with it because it affects the English
name we use.))
Treat Cattle Egret Bubulcus ibis as two species
Background:
Most global lists (e.g., eBird/Clements, IOC,
Howard & Moore) have traditionally considered Bubulcus ibis to be a single species with two subspecies: ibis of southern Europe, Africa, Asia
Minor as far east as Iran, and the Americas; and coromandus of South Asia and southeastern Asia south to Australia
and New Zealand. These two subspecies are separated by a gap in distribution in
Pakistan and Afghanistan. A third subspecies, seychellarum of the western Indian Ocean, is sometimes recognized
(e.g., by Birdlife); otherwise, these populations are considered part of ibis.
The IOC list recently elevated coromandus to species status,
recognizing two species in this complex. Their note on this change is as
follows: “Bubulcus coromandus is
split from B. ibis (Payne &
Risley 1976; McAllan & Bruce 1989; Rasmussen & Anderton 2005). Status
under discussion (Christidis & Boles 2008; Ahmed 2011; HBW).”
The relevant passage from Payne and Risley
(1976), who placed this species in Egretta,
is here:
“Cattle
Egrets of Africa (E. i. ibis) and
India (E. i. coromanda) have very
different breeding plumages and might better be regarded as two species or at
least two allospecies of a superspecies. African birds have orangish-buff
display feathers coloring the entire head, neck, and upper breast; long plumes
of similar color cover the lower back and rump. Indian birds have pinkish-buff
plumes and these are restricted to the crest, the upper breast, and the lower
back; the neck and throat are white. The bill is shorter and stouter in ibis. The extent of feathering on the
tarsus above the distal tarsometatarsal joint is greater in ibis (about 12 mm bare tarsus) than in coromanda (about 24 mm bare tarsus), but
some overlap occurs between specimens of the two groups. Wing lengths differ on
the average (Ali and Kipley, 1968; Mackworth-Praed and Grant, 1970) but the
ranges of wing lengths overlap. The two forms are geographically separated from
each other. Cattle Egrets of the Seychelle Islands have been regarded as intermediate
between the Indian and African birds, but only one specimen in breeding plumage
is known, and it has not been possible to test further the idea that Seychelle
birds (described as a subspecies "seychellarum")
are hybrid results of independent invasions and establishments on the islands
from Africa and India (Benson and Penny, 1971). It is possible that the
differences in breeding plumage would act as behavioral isolating mechanisms
between the two forms of Cattle Egrets, and it would be of interest to
complement the study of behavior of African birds (Blaker, 1969a) with a study
of behavior of birds in India or Australia. Examination of skeletons in the
present study showed no differences in the coded character states in the two
forms, though the interorbital foramen was slightly more rounded anteriorly in
the African specimens.”
McAllan and Bruce (1989), referenced in the IOC
note, is a working list of the birds of New South Wales, Australia. They
presumably recognized B. coromandus as
separate from B. ibis, whereas
Christidis and Boles (2008) presumably treated them as conspecific in their
list of Australian birds.
Volume 1 (Field Guide) of Rasmussen and Anderton
(2005) stated that Western Cattle Egret B.
ibis is “similar but stockier [than B.
coromandus], in breeding plumage with orange-buff mainly on crown, breast,
and mantle.” In Volume 2 (Attributes and Status), they expand on this as
follows:
“[Western Cattle Egret is] Like Eastern but smaller and stockier, with shorter
bill, neck and legs (latter often paler yellowish, olive or grey, but
never black), less bare facial skin and puffier ‘jowls’. Breeding adult
shows a shaggier, paler peach-colored crest only on top of
head, finer, more hair-like peach breast-plumes, and brighter red legs. In
flight, less leg extension than for Eastern….Size Length 330-380 [340-370 in coromandus];
head 90-100 [97-110 in coromandus];
tail 80-90 [81-93 in coromandus];
bare leg 168-180 [205-225 in coromandus]….
Habits Much as for Eastern. Voice Calls noticeably
higher-pitched, more nasal and less gravelly than Eastern’s.?
The Birdlife rationale for continuing to
recognize only a single species was as follows:
“Race
coromandus
The only formal study of phenotypes appears to
be that of Ahmed (2011), a paper published in Dutch Birding and directed
towards identification of potential vagrant coromandus
in the Western Palearctic. He concluded that
“the following features are useful in separating ibis and coromandus: 1 extent
and coloration of adult summer plumage; 2 bill length; 3 tarsus length; 4 tail
length; and 5 bill depth at both nostril and feathering (only in separation
of ‘Indian Ocean specimens’ from ibis and
coromandus). In addition,
vocalisations are of use according to Rasmussen & Anderton (2005) but data
on these were not collected and they require further work. Data to confirm the
validity of the taxon ‘seychellarum’
and its separation from ibis and coromandus are lacking.”
However, although breeding plumage is readily
diagnostic, the morphometric characters listed above, as noted in the Birdlife
spiel as well as by Payne and Risley (1976) and Ahmed (2011), show a fair
amount of overlap (see Table 1 and Fig. 3 from Ahmed 2011 below).
Ahmed (2011) also questioned the vocal
differences discussed in Rasmussen and Anderton (2005), noting that Sangster
(in litt.) could find no differences in vocalizations and that Kushlan and
Hancock (2005) mentioned up to 11 call types. He suggested that the calls
compared by Rasmussen and Anderton (2005) may not have been homologous. I’m not
aware of any further discussion of the vocalizations.
Fig. 3 from Ahmed (2011), plotting bill length
versus tarsus length in the two subspecies of B. ibis, with Indian Ocean populations (“seychellarum”) also separated.
Somewhat surprisingly, no genetic studies have
included both subspecies of B. ibis.
Hruska et al. (2023), for example, included only a sample of subspecies ibis from Louisiana.
New Information:
As part of an effort to consolidate global bird
lists, the IOU’s Working Group on Avian Checklists (WGAC) recently considered
whether to separate B. ibis into two
species. WGAC voted to recognize B.
coromandus as a separate species from B.
ibis. This change has already been adopted in the most recent Clements
update and as noted above, was previously adopted by the IOC list.
Members of WGAC who voted for the split
emphasized the differences in breeding plumage, which involve not only the
extent of the buff coloration but also the color and texture of the plumes.
Also mentioned were differences in shape and proportions, although the
morphometric data do show overlap. The lack of clinality in the plumage
differences was also viewed as significant: breeding plumages of the
westernmost individuals of coromandus and
easternmost individuals of ibis were
noted to be the same as those elsewhere in their respective ranges. Those
voting against the split were not convinced that the differences between coromandus and ibis are more than subspecies-level distinctions, and preferred to
wait for additional data bearing on species status.
Recommendation:
Although I voted against the split, this is
primarily an Old World issue and I recommend that we adopt the new global
taxonomy for this complex, following our standard policy. Most “Old World”
representatives on the WGAC voted for the two-species arrangement. Despite
evidence that may fall short of our usual standards, I would recommend adopting
the new global taxonomy of recognizing B.
coromandus as a species separate from B.
ibis.
English Names:
Both the IOC and eBird/Clements lists are using
Western Cattle Egret for B. ibis and
Eastern Cattle Egret for B. coromandus.
I would recommend that we also use these names, although our guidelines
indicate that the group name should be Cattle-Egret, to indicate their status
as sister species, rather than Cattle Egret.
References:
Ahmed, R. 2011. Subspecific identification and status of Cattle Egret.
Dutch Birding 33: 294-304.
Christidis, L., and W. Boles. 2008. Systematics and Taxonomy of
Australian Birds. CSIRO Publishing, Collingwood, Victoria, Australia.
Hruska, J. P., J. Holmes, C. Oliveros, S. Shakya, P. Lavretsky, K. G.
McCracken, F. H. Sheldon, and R. G. Moyle. 2023. Ultraconserved elements
resolve the phylogeny and corroborate patterns of molecular rate variation in
herons (Aves: Ardeidae). Ornithology 140:
ukad005 doi.org/10.1093/ornithology/ukad005
Kushlan, J. A., and J. A. Hancock. 2005. The Herons. Oxford University
Press, Oxford, U.K.
McAllan, I. A. W., and M. D. Bruce. 1989. The Birds of New South Wales, a Working List.
Biocon Research Group, Turramurra, NSW, Australia.
Payne, R. B., and C. J. Risley. 1976. Systematics and evolutionary
relationships among the herons (Ardeidae). Miscellaneous Publications of Museum
of Zoology 150: 1–115.
Rasmussen, P. C., and J. C. Anderton 2005. Birds of South Asia: The
Ripley Guide. Smithsonian Institution Press/Lynx Edicions, Washington, DC.
Terry Chesser,
March 2024
Comments
from Jaramillo:
“YES - I vote to split. We have an issue with herons
and egrets, in that differences and sometimes striking differences, are not taken into account as strongly in birds like these that are
uniform in plumage. White birds, or we could go to Corvus for the
opposite, or Quiscalus. These groups have multiple seemingly
species-level taxa that are cryptic. I am not sure voice is really all that
important in these colonial egrets and herons, in the way it is in bitterns who
use voice as a primary means of attraction and display. We have the
Intermediate Egret system, the Great Egrets, and even the Great White Heron
situation that include some major differences, in display coloration of soft
parts, ecology etc that suggests some barriers to interbreeding. But I guess
our eye goes to the largely white plumage and we stay there. In this case we
have a big difference with their striking difference in breeding plumage, with
extent of color, strength of color, and as noted some of the structure of the
feathers. I think this is important. Similar to the head coloration in Cathartes
vultures, the birds are using these features to sort themselves out. So, I am
fine with a split on the Cattle Egrets.”
Comments from Del-Rio:
“YES.
I think plumage differences in breeding forms are pronounced enough for the
separation. Size differences are not as compelling though.”