The 46 Orders














CICONIIFORMES Bonaparte, 1854

Click for Ciconiidae tree
Click for Ciconiidae
species tree

Although Hackett et al. (2008) found that the storks were basal in the remaining Ardeae (after the penguins and seabirds), Gibb et al. (2013) placed them next to the herons. However, Jarvis et al. (2014) came up with a different arrangement of the taxa, casting doubt on the Gibb et al. treatment. The latest analyses are from Prum et al. (2015) and Kuramoto et al. (2015), who both found the storks to be basal in this group.

Slikas (1997) did not come to a definitive conclusion on how to arranged the genera of the Ciconiidae. I've adopted her maximum likelihood tree. However, it may not be correct, and there were indications that Ciconia itself may not be monophyletic.

Ciconiidae: Storks Sundevall, 1836

6 genera, 19 species HBW-1

SULIFORMES Sharpe 1891

AOU officially adopted the term Suliformes in the 51st supplement. Sharpe had previously used Sulae as a suborder. For that matter, he also had Fregatae and Phalacrocoraces as suborders. The Suliformes had previously been considered part of the Pelecaniformes, a tradition that dates back to their naming by Sharpe.

The Suliformes were traditionally considered part of the Pelecaniformes (as were the tropicbirds). After all, how likely was it that such unusual features as a totipalmate foot and gular pouch would arise independently? They also share the location of the salt-excreting gland and all lack an incubation patch. These similarities lead Linneaus to put all but the tropicbirds (which lack the gular pouch) in the same genus.

Fregatidae: Frigatebirds Degland & Gerbe, 1867 (1840)

1 genus, 5 species HBW-1

The frigatebird taxonomy follows Kennedy and Spencer (2004).

Sulidae: Gannets, Boobies Reichenbach, 1849 (1836)

3 genera, 10 species HBW-1

Click for Sulidae tree
Click for Sulidae tree

Sulid taxonomy follows Patterson et al. (2011), which is similar to Friesen et al. (2002), except that Papasula is considered basal. The extinct Tasman Booby, often considered a separate species, is here considered a subspecies of the Masked Booby following Christidis and Boles (2008). More recently, Steeves et al. (2010) provides strong evidence for this treatment. They further argue that Sula dactylatra tasmani is identical with the still extant subspecies S. d. fullagari, in which case both should be referred to as S. d. tasmani.

Anhingidae: Anhingas Reichenbach, 1849 (1815)

1 genus, 4 species HBW-1

The arrangement follows Kennedy et al. (2019), which also provides genetic support for the four species treatment used here.

Phalacrocoracidae: Cormorants Reichenbach, 1849-50 (1836)

3 genera, 42 species HBW-1

The arrangement below is now based primarily on Kennedy and Spencer (2014). Previously, I had used the DNA analyses of Kennedy et al. (2000, 2001, 2009) and the osteological analysis of Siegel-Causey (1988), following Kennedy et al. in case of disagreement. You can click on the tree diagram for the phylogeny. The species in black were included in Kennedy and Spencer, while no DNA data is available for species marked in blue on the tree. In those cases, I've followed Sigel-Causey when possible. The paper by Kennedy et al. (2009) resolved the long-controversial status of the Flightless Cormorant. They found it is sister to the Neotropic and Double-crested Cormorants.

On the tree, I've included some of the available genus names that could be used to subdivide Phalacrocorax. Unless they are generally adopted, they are perhaps mostly best thought of as subgenera. However, the Red-legged Cormorant is so genetically distant and so distinct that I have moved it to Poikilocarbo.

Although work has been done on the phylogeny of the blue-eyed shag complex, the correct species limits remain murky. There are eight Phalacrocorax taxa involved: albiventer, atriceps, georgianus, melanogenis, bransfieldensis, verrucosus, purpurascens, and nivalis. Kennedy and Spencer (2014) found three clades in the group: (1) albiventer, atriceps, and georgianus; (2) melanogenis and bransfieldensis; (3) verrucosus, purpurascens, and nivalis, with clades (2) and (3) closer to each other than to clade (1). The genetic distances are close enough that these allopatric taxa could be considered one species.

Following SACC, the continental representatives of King Cormorant, Phalacrocorax albiventer are considered a color morph of the Imperial Cormorant, Phalacrocorax atriceps (aka Blue-eyed Shag). Rasmussen (1991) makes a strong case for this. The key points are in the abstract: frequent hybridization and non-assortative mating in the contact zones. The genetic distance as measured using allozymes also seems very small.

Kennedy and Spencer (2014) found that the King Cormorants from the Falklands are different from continental `albiventer' (labelled atriceps in the paper, and presumed a color morph). They don't seem to include any of the white-cheeked color morph of atriceps. They found atriceps to be sister to georgianus and the Falklands sample basal to both. Note that Rasmussen's (1991) arguments don't necessarily pertain to the Falklands birds. Because of this I treat the visually distinct King Cormorant of the Falklands as a separate species, Falkland Cormorant, Phalacrocorax albiventer (the type of albiventer is from the Falklands).

Antarctic Shag, Phalacrocorax bransfieldensis, and South Georgia Shag, Phalacrocorax georgianus, are split off as separate species (Siegel-Causey and Lefevre, 1989). They present evidence that the breeding range of the Antarctic Shag formerly included the area around Tierra del Fuego, part of the breeding range of P. atriceps. They argue that there is no sign of interbreeding, indicating they are separate biological species. Kennedy and Spencer (2014) found that the Antarctic Shag is more closely related to the Crozet Shag than the Imperial Cormorant. The South Georgia Shag seems distinct from the Imperial Cormorant, and is arguably also a separate species.

That still leaves 3 taxa to deal with. Unfortunately, there seems to be little solid information to work with. Christidis and Boles (2008) note all this, but consider these three taxa to be subspecies of P. atriceps. There is one piece of evidence. The genetic distance between purpurascens and albiventer is small enough for them to be a single species. However, it's also large enough to be different species. In HBW-1, Orta (1992) takes the opposite tack and splits them.

In version 2.17 I followed Christidis and Boles concerning melanogenis, nivalis, purpurascens. This left me with the same four species as Sibely and Monroe (1990). I gather I'm not the only one uncomfortable with that solution. It just doesn't make biogeographic sense to have birds breeding on the other side of the world lumped into atriceps when the physically closer taxa are considered separate species. In the absence of definitive information, this version follows Orta (1992) in considering them as three species.


Although some analyses have indicated that the herons and ibises are sister clades, support has been weak. Gibb et al. (2013) studied the complete mitochondrial genome of 4 herons, 4 ibises, and related taxa. They found that the herons and ibises do not form a clade, and estimated that they have been separate lineages since the early Paleocene. Since there is uncertainty about their closest relatives, and each represents a truly ancient lineage, I treat them both in their own orders.

Kuramoto et al. (2015) have found evidence of early hybridization between ancient ibises and herons following the split of between the heron and pelican lineages. That would explain the conflicting results in earlier analyses.

Threskiornithidae: Ibises, Spoonbills Poche, 1904

13 genera, 35 species HBW-1

Threskiornithidae tree The traditional treatment of the ibises and spoonbills as sister subfamilies is just wrong. The spoonbills are not the sister group of the ibises. Rather, they are most closely related to Threskiornis and perhaps Pseudibis. Krattinger's MA thesis (2010) shows this fact clearly. Chesser et al. (2010) is consistent with this idea, and it already appeared in Sibley and Ahlquist (1990; esp. Fig. 367). Oddly, Sibley and Ahlquist did not comment on it. Perhaps they found it too unbelievable.

Interestingly, there had been other hints that the spoonbills should not be treated as a subfamily. Matheu and del Hoyo (1992=HBW-1) mention that the Eurasian Spoonbill has been known to hybridize with Black-headed Ibis,Threskiornis melanocephalus. Unfortunately, they did not make the connection with Sibley and Ahlquist's results.

Krattinger (2010) also estimated divergence times. His results suggest that the spoonbill clade originated about 15 million years ago (with large error bars). That time span is more than sufficient to evolve even such a distinctive bill. The Hawaiian Honeycreepers evolved theirs in half that time (Lerner et al, 2011).

Krattinger did find a deep division in Threskiornithidae, but it was between the exclusively New World genera (Eudociminae) and the rest (Threskiornithinae), not between the ibises and spoonbills. The treatment as subfamilies emphasizes this radical change in taxonomy of the ibises and spoonbills.

Krattinger (2010) examined DNA from just over half of Threskiornithidae. The exact position of some of the Old World genera was not conclusively resolved (Bostrychia, Lophotibis, Nipponia), but this tree is a reasonable interpretation of what Krattinger found. The resulting tree is also consistent with Chesser et al. (2010) and Sibley and Ahlquist (1990). Question marks indicate genera that were not sampled. The order within the spoonbills is based on Chesser et al. (2010), which included all of the spoonbills.

Current thinking is that the extinct Reunion Solitaire was actually an ibis! Moreover, it seems to have been closely related to the sacred-ibises (see Mourer-Chauviré et al., 1995). Accordingly, it appears at the head of Threskiornis.

You may think it odd that the family is called Threskiornithidae when Eudociminae is a much older name. The family was once referred to as Ibididae (based on Ibis Cuvier 1816), but the oldest use of the genus Ibis actually refers to the Mycteria storks. Ibididae had to be replaced, and everyone ultimately settled on basing it on Threskiornis, which replaced Cuvier's version of Ibis. Ultimately, the ICZN ruled on this (Opinion 1674) and the family is called Threskiornithidae.

Eudociminae Bonaparte, 1854

Threskiornithinae Poche, 1904


The status of two monotypic families, the Shoebill and the Hamerkop, has been a perennial issue. The analyses of Ericson et al. (2006a) and Hackett et al. (2008) indicate that both are relatives of the pelicans. Indeed, they could all be lumped into the same family. We keep them separate not only because of their uniqueness, but also because the division between them seems to be ancient. Gibb et al. (2013) estimate that the pelican-shoebill split occurred in the early Eocene. According to Prum et al., the Shoebill is more closely related to the Pelicans and the Hamerkop is basal in the Pelicaniformes.

Scopidae: Hamerkop Bonaparte, 1849

1 genus, 1 species HBW-1

Balaenicipitidae: Shoebill Bonaparte, 1853

1 genus, 1 species HBW-1

Pelecanidae: Pelicans Rafinesque 1815

Pelican tree
Pelican species tree

The pelicans have been studied by Kennedy et al. (2013). The arrangement on the tree and order below reflects the relationships they found. Note how the New World Pelicans and Old World Pelicans form sister clades. They also found that the Pink-backed, Dalmatian, and Spot-billed Pelicans are quite closely related.

1 genus, 8 species HBW-1

ARDEIFORMES Wagler, 1830

Ardeidae: Herons, Egrets, Bitterns Leach, 1820

22 genera, 77 species HBW-1

Ardeidae tree The Boat-billed Heron was previously considered to be the only member of the Cochlearidae, but is really just another heron. The list here is pieced together from the limited DNA evidence available (Chang et al., 2003; Huang et al. (2016), Päckert et al. (2014), Sheldon et al., 2000; Zhou et al., 2014, 2016) as well as the barcoding tree Raty posted to BirdForum in 2014. The traditional morphological evidence of McCracken and Sheldon (1998) was also consulted.

Much of the DNA evidence is limited to cyt-b and/or barcodes. Exceptions are Chang (2003), which uses 12S rRNA, and Zhou et al. (2014, 2016), which both use the complete mitochondrial genome. We lack a complete tree with good coverage. As a result, the placement of some taxa remains quite tentative.

Limited DNA evidence puts the Tigriornithinae (tiger-herons) first, followed by the Cochlearinae (Boat-billed Heron). Next are the Botaurinae (bitterns) including Zebrilus, Botaurus and Ixobrychus. Chang et al. (2003), Päckert et al. (2014), and Zhou et al. (2014, 2016) found the Black Bittern embedded in Ixobrychus. It's sometimes put in a monotypic genus Dupetor, which is here considered part of Ixobrychus.

All but three of the Botaurinae were considered by Päckert et al. (2014). The linear order here is based on their results, except that they found the Least Bittern, Ixobrychus exilis, closer to Botaurus than to the other Ixobrychus. They looked at the barcoding region and part of cytochrome-b. The barcoding results were a bit strange. The cyt-b + barcoding phylogeny (Figure 3) was much more reasonable. Both suggest moving the Least Bittern to Botaurus, which I have done. Even though they look similar, it's not surprising that the Least Bittern is not sister to the Little Bittern group (I. minutus–novaezelandiae). The osteological evidence in McCracken and Sheldon (1998) had long ago indicated it is not as close to the Little Bittern group as one might think.

It's not at all clear what happens with the night-herons. There's some evidence that they are a clade, but that is incomplete and has only weak genetic support. The cytochrome-b analysis of Sheldon et al. (2000) was ambiguous about whether Nycticorax and Nyctanassa form a clade, but Chang (2003) found them in a clade using 12S rRNA. Zhou et al. (2016) grouped Nycticorax with Gorsachius. Zhou et al. also found that White-eared Night-Heron was closer to Egretta. I've transferred it to the monotypic genus Oroanassa (Peters 1930). There is also uncertainty about the affinities of the White-backed Night-Heron. It has been variously placed in Nycticorax and Gorsachius, or in its own genus Calherodius (sometimes with the White-eared Night-Heron). McCracken and Sheldon (1998) did not find them to be sisters. For now, I'm putting the White-backed Night-Heron in a monotypic Calherodius (Bonaparte 1855) and leaving it in the night-heron subfamily, Nycticoracinae.

The remaining genera seem to be more closely related to each other than to anything else, and are placed in subfamily Ardeinae. They appear to fall into two main clades.

The first clade includes the endangered White-eared Night-Heron, Oroanassa magnifica, together with Pilherodius, Syrigma, and Egretta. I've ordered Egretta to conform with available genetic data.

Based on the Zhou et al. (2016), which uses the complete mitochondrial genome, the other clade consists of two parts. The first includes Butorides and Ardeola. Barcoding data suggests that Agamia is sister to Ardeola. Interestingly, barcoding data also suggests that the South American and Old World Striated Herons are sister taxa, with the Green Heron a more distant relative.

The remaining herons and egrets are in the Bubulcus-Ardea clade.

We consider the Ardea clade first. The Great Egrets are put in a separate genus, Casmerodius. The 12S rRNA tree of Chang et al. puts them sister to the Intermediate Egret, and both sister to Ardea (or Ardea+Bubulcus). Sheldon et al. (2000) didn't include the Intermediate Egrets (Mesophoyx), but also found Casmerodius sister to Ardea. These were formerly placed in Egretta, but the DNA says no on this. Some authors put them all in Ardea.

The placement of Bubulcus follows Chang et al., (2003) Sheldon et al. (2000), and Zhou et al. (2014, 2016). The last two are based on the complete mitochondrial genome. Zhou et al. actually suggest merging Bubulcus, Casmerodius, Mesophoyx, into Ardea. However, these are both distinctive and genetically distant from the main Ardea group and better maintained as separate genera.

Splits and Potential Splits

The Cattle Egret, Bubulcus ibis, has been split into Western Cattle Egret, Bubulcus ibis, and Eastern Cattle Egret, Bubulcus coromandus based on differences in plumage and DNA (Raty barcode tree).

The Intermediate Egret, Mesophoyx intermedia, has been split into Intermediate Egret, Mesophoyx intermedia, Yellow-billed Egret, Mesophoyx brachyrhyncha (sub-Saharan Africa), and Plumed Egret, Mesophoyx plumifera (Australasia) based on differences in breeding plumage (HBW/BirdLife).

Kushlan and Hancock (2005) and Christidis and Boles (2008) suggested treating the Great Egret as two species: Casmerodius albus and Casmerodius modestus. Certainly, the genetic distance between some of the Great Egret subspecies is quite large, comparable to that between Great and Intermediate Egret (Sheldon, 1987), but the subspecies analyzed are egretta and modestus. This suggest no significant gene flow between egretta and modestus, that they are distinct biological species. But how do the other subspecies (albus and melanorhynchos) fit in? Both Kushlan and Hancock, and Cristidis and Boles, suggest that egretta should be grouped with albus and melanorhynchos. However, Pratt (2011) argues that the split should be between egretta and the rest, mainly on the basis of breeding plumage. However, Raty's 2014 barcoding tree suggests that albus and egretta are more closely related to each other than to modestus (with low support). It also suggests a species level difference between albus and egretta. Further, all 4 subspecies (including melanorhynchos) have distinctive breeding plumages.

Accordingly, the Great Egret, Casmerodius modestus, has been split into Eastern Great Egret, Casmerodius modestus, Great White Egret, Casmerodius albus African Great Egret, Casmerodius melanorhynchos, and American Egret, Casmerodius egretta, based on differences in breeding plumage and except for melanorhynchos, DNA. The order is based on the assumption that the Eastern Great Egret is basal in the group.

The status of the Great White Heron, Ardea herodias occidentalis, remains controversial (e.g., Stevenson and Anderson, 1994). It is very near the borderline for species status. Genetically, it is nested within the larger Great Blue Heron clade. However, in their overlap zone in extreme south Florida, there seems to be little interbreeding between the dimorphic Great White Herons (the dark morph is sometimes called Würdemann's Heron) and the monomorphic Great Blue Herons (McGuire, 2002). Moreover, the Great Blue Herons of the Florida peninsula (wardii) are more closely related to those of the northern US (herodias) than to occidentalis. However wardii and herodias are closer to occidentalis than any of them are to fannini. For the present, I'm following AOU by treating them as one species although I'm not convinced this is correct.

Tigriornithinae: Tiger-Herons Bock, 1956

Cochleariinae: Boat-billed Heron Chenu and des Murs, 1854 (1838)

Botaurinae: Bitterns Reichenbach, 1849-50

Nycticoracinae: Night-Herons Bonaparte, 1854

Ardeinae: Egrets and Herons Leach, 1820

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