Proposal (1049) to South American Classification Committee

Merge Bubulcus into Ardea

Note from Remsen: This proposal is relayed from NACC, which passed it 10-3, i.e., NACC now treats this species as Ardea ibis.

The Cattle Egret is now generally placed in the monotypic genus Bubulcus based on its behavioral and morphological distinctiveness. However, it has also been placed in other genera, e.g., Ardeola (Bock 1956), Egretta (Payne and Risley 1976), or Ardea (Kushlan and Hancock 2005). Sheldon’s (1987a) DNA-DNA data (his Fig. 2) showed it to be part of an unresolved clade with six other species all currently placed in Ardea. The cyt-b data of Sheldon et al. (2000), who sampled 15 species, found B. ibis to be sister to the two species of Ardea sampled (alba and herodias). In the mtDNA barcode tree of Huang et al. (2016), B. ibis was sister to A. alba and these species were nested deep within Ardea.

Hruska et al. (2023) recently published a phylogenetic paper on herons based on sequences of UCEs and mitochondrial DNA. They sampled 55 species of heron, although success with both UCEs and mtDNA was variable: 46 species were included in their UCE tree (Fig. 1 below), and 47 species in their mtDNA trees (Figs. 2, 3 below).

In the UCE tree (Fig. 1 below) from Hruska et al. (2023), B. ibis was nested within Ardea as sister to one of the two main clades of this genus, which consisted of herodias, cinerea, purpurea, melanocephala, and goliath, the other main clade consisting of pacifica, alba, and intermedia. This result received 100% bootstrap support.

Fig. 1. Phylogenetic tree based on sequences of UCEs (Hruska et al. 2023). All nodes received 100% bootstrap support except as noted.

Thus, according to the UCE tree, retaining the phenotypically distinctive B. ibis in Bubulcus would necessitate a split of Ardea into two genera. In their mtDNA trees (Figs. 2 and 3, below), however, B. ibis was sister to Ardea, which was separated into two main clades. One clade corresponded to the second clade detailed above, but the other clade, due to the different taxon sampling, consisted of humbloti, insignis, purpurea, melanocephala, and goliath. Although the Ardea + Bubulcus clade received strong support (100% bootstrap and 1.0 p.p.), support for Ardea as a clade if B. ibis was excluded was not as strong (78% bootstrap, 0.99 p.p.).

Fig. 2. Phylogenetic tree based on Maximum Likelihood analysis of sequences of mtDNA (Hruska et al. 2023). All nodes received 100% bootstrap support except as noted.

Fig. 3. Phylogenetic tree based on Bayesian analysis of sequences of mtDNA (Hruska et al. 2023). All nodes received 100% bootstrap support except as noted.

Based on their results, Hruska et al. (2023) recommended that B. ibis be transferred to Ardea. However, an alternative consistent with all their trees would be to maintain Bubulcus for ibis and to split Ardea as currently delineated into two genera, one minimally consisting of pacifica, alba, and intermedia, the other minimally consisting of humbloti, insignis, herodias, cinerea, purpurea, melanocephala, and goliath. The two unsampled species, cocoi and sumatrana, would presumably belong to the latter group, cocoi thought to be closely related to herodias and cinerea (see Huang et al. 2016), and sumatrana thought to be closely related to insignis. This would preserve Bubulcus for the behaviorally distinctive ibis, and would place two other white egrets, alba and intermedia, with pacifica in a separate genus, for which Casmerodius Gloger, 1841, appears to be the oldest available name. To me, the branch lengths in the various phylogenies in Hruska et al. (2023) do not provide strong support for either alternative: the nodes uniting the proposed three genera are deeper than most nodes within genera in the Ardeinae but not as deep as most nodes that unite separate genera. However, the genus Ixobrychus does contain nodes that are deeper than any within Ardea + Bubulcus, even without including exilis and involucris.

For this issue, votes are needed as follows:

1. Revise the taxonomy of Bubulcus ibis and related species, YES or NO. A NO vote would retain B. ibis in Bubulcus and retain all related species in Ardea, which would be paraphyletic according to the UCE phylogeny in Hruska et al. (2023).

If YES on #1, then vote for either 2A or 2B (unless you favor a different option, which should be specified):

2A. Transfer B. ibis to Ardea

2B. Retain B. ibis in Bubulcus and transfer Ardea alba, A. intermedia, and A. pacifica to Casmerodius

Recommendation:

Based primarily on the UCE data of Hruska et al. (2023), I think changes to the taxonomy are warranted, so I recommend a YES vote on Part 1. For Part 2, I recommend that Bubulcus ibis be transferred to Ardea. As with the Ixobrychus issue above, this is a close call over keeping B. ibis in Bubulcus and splitting Ardea into two genera, but I favor transferring B. ibis because (1) it is the least disruptive option; (2) it limits the genus transfers to a widespread species (B. ibis) that occurs in the Americas, rather than transferring, in addition to a widespread species (A. alba) that occurs in the Americas, an Old World species (A. intermedia) accidental to our area (the third species of Casmerodius, A. pacifica, is an Old World species not known from our area; and (3) there is ample precedence for placing ibis in genera other than Bubulcus (i.e., not recognizing it as so distinctive as to require its own genus). Also, if relying on phenotype to continue to recognize Bubulcus, there is the question of what phenotypic characters distinguish Casmerodius from Ardea. As with the bitterns, should a global body such as WGAC endorse the transfer of alba, intermedia, and pacifica to Casmerodius, then point 2 above becomes moot and the balance may be tipped towards this alternative. In that case we could re-vote, but for now I recommend the transfer of B. ibis to Ardea.

References:

Bock, W. J. (1956). A generic review of the family Ardeidae (Aves). American Museum Novitates 1779:1–49.

Huang, Z. H., M. F. Li, and J. W. Qin (2016). DNA barcoding and phylogenetic relationships of Ardeidae (Aves: Ciconiiformes). Genetics and Molecular Research 15:gmr.15038270

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: Ardeidae. Oxford University Press, New York, NY, USA.

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.

Sheldon, F. (1987a). Phylogeny of herons estimated from DNA-DNA hybridization data. The Auk 104:97–108.

Sheldon, F. H., C. E. Jones, and K. G. McCracken (2000). Relative patterns and rates of evolution in heron nuclear and mitochondrial DNA. Molecular Biology and Evolution 17:437–450.

Terry Chesser, May 2025

Note from Remsen on SACC voting structure: Let’s just simplify this. A YES vote on the SACC proposal means you favor placing Bubulcus in Ardea, as Ardea ibis. A NO vote means you favor retaining ibis in monotypic Bubulcus either (1) pending further data or (2) a new proposal to resurrect Casmerodius to maintain a monophyletic Ardea

Voting Chart: https://www.museum.lsu.edu/~Remsen/SACCPropChart1044+.htm

Comments from Remsen (slightly modified from NACC proposal comments): “YES. Painful as it is, I think we have to go with the UCE data, i.e. Bubulcus is embedded in broad Ardea. So, the next decision is arbitrary, especially without a time-calibrated phylogeny. Sink Bubulcus or resurrect Casmerodius? I see good points for both treatments. However, the biggest difference between Cattle Egret and the others is size – if Cattle Egret were the size of a typical Ardea, I suspect we wouldn’t balk at the decision. Size alone is not a criterion for a separate genus (think of Chloroceryle as just one of many examples). Also, I think two Old World taxa, with which we are less familiar, intermedius and purpurea, close the size gap somewhat. Cattle Egret is obviously a highly specialized feeder that has “broken out” of the standard Ardea morphology, and the smaller size likely is a benefit to its agile pursuit of flushed arthropods and small vertebrates. I will point out that Cattle Egret holds its own with the big Ardea in terms of voraciousness in tackling prey items large for its body size: for entertainment for anyone who read this far, here’s my video of one swallowing a snake that is longer than the bird: https://www.youtube.com/watch?v=fMqjKbuDDUY&t=29s. Also, we’ve all seen Great Egret and Great Blue Heron regularly feeding facultatively in terrestrial situations. One final point: how would one diagnose resurrected Casmerodius other than by tree typology?  Distinguishing Great White Heron from Great Egret takes a little practice, for example: https://www.youtube.com/watch?v=p0B4fOUDQWI.”

Comments from Bonaccorso: “YES. Regretfully. Although the mtDNA tree makes more sense, the UCEs evidence is very compelling.”