The 2020 AOS Supplement is out!

by Michael L. P. Retter

Every summer, birders anxiously await publication of the “Check-list Supplement” by the American Ornithological Society’s Committee on Classification and Nomenclature of North and Middle American Birds (a.k.a. the NACC). The supplement (available here) details revisions to the NACC’s Check-list. Below is rundown of the more significant revisions. You can read all the proposals on which the NACC voted this year at Later in the year, be sure to check out ABA’s annual “Check-list Redux” in North American Birds magazine. There, you’ll find photos, maps, and more detailed analysis of these changes.

Nowadays, it can be assumed that any change in taxonomy is due (at least partly) to analysis of new genetic data, so that is not always mentioned below. As a general policy, the NACC accepts as additions to its North American Check-list any species the ABA’s Checklist Committee adds to its list. Those changes are not listed here. In instances where new species appear on the Check-list because of a split, the sequence in which they are listed here is the sequence in which they appear on the Check-list. Species marked with single asterisks (*) below are those which do not appear on the ABA Checklist. Those which do not appear on AOS’s North American Check-list are marked with double asterisks (**). Extinct species are marked with daggers (†).

This year, the topics most likely to generate discussion within the ABA Area are the split of Mallard and the lump of Northwestern and American crows, an extralimital split that changed the scientific name of Dusky Thrush, and genus-level changes affecting some hummingbirds.

Further south, in Middle America, Black-faced Antthrush and Paltry Tyrannulet were split, TSFKAP (the species formerly known as Puerto Rican Screech-Owl) got new English and scientific names, Checker-throated Antwren got a new English name, and some large antbirds, Buff-throated Foliage-gleaner, White-crowned Manakin, and many more hummingbirds were moved to “new” genera.


Say hello to Mexican Duck!

(Two-way split of Mallard)

Mallard (Anas platyrhynchos)

Mexican Duck (Anas diazi)

Some of you may remember Mexican Duck as a species from pre-1983 bird books, such as the original Golden Guide. Well, it’s back! After rejecting this split just two years ago, the committee reevaluated the situation in light of new genetic evidence and accepted the treatment of Mexican Duck as a full species. This follows the decision of Clements/eBird in 2018 to do the same, and it eliminated one of the four instances in which the AOS and eBird lists disagreed within the ABA Area. (See the white-eye account below for another. The two others still in existence involve swamphens and hwameis. The Florida swamphen population is represented by the AOS-unsplit Purple Swamphen and the Clements/eBird-split Gray-headed Swamphen. The Hawaiian hwamei population is represented by the AOS-unsplit Hwamei and the Clement/eBird-split Chinese Hwamei.)

Like American Black and Mottled ducks, Mexican Ducks are “brown mallards” that lack a distinctive adult male plumage. Adult males resemble slightly darker versions of females with brighter yellow bills. Mexican Ducks most closely resemble “Texas” Mottled Ducks (Anas fulvigula maculosa) but differ in having white borders to the speculum, slightly paler body plumage, a less prominent or absent black gape spot, and grayer (vs. peachy) cheeks with fine streaking.

Mexican Duck was lumped with Mallard for the last 37 years because the two interbreed in the southwestern U.S. Indeed, hybrids with Mallard are common at many locations, such as in Albuquerque, New Mexico. Compared to female Mallards, Mexican Ducks have darker (not white) tails, darker undertail coverts, darker bodies, duller bills, and have narrower white borders to the speculum; they also tend to have greener speculums. Mallard ancestry is readily detectable in males by the presence of green on the head, rust on the breast, and/or curled central tail feathers. (Confusingly, an illustration of a “good” male Mexican Duck long appeared in the National Geographic guide with the label “intergrade”. Thankfully, this is no longer the case. And confusingly, the Sibley Guide states that “virtually no pure Mexican [Ducks] occur in North America”–yet the species is a North American endemic!)

Mexican Ducks are mostly resident in interior Mexico north to the southwestern United States. They are commonly found as far north as Albuquerque, New Mexico, as far west as Yuma, Arizona, and as far east as McAllen, Texas. Post-breeding dispersal is made apparent by sporadic records from as far north as Utah, Wyoming, and Nebraska, as far west as coastal southern California, and as far east as South Padre Island, Texas. The species’s status further north and east in Texas, the Great Plains, and Tamaulipas is greatly complicated by the presence of the extremely similar-looking “Texas” Mottled Duck, with which it hybridizes. (But with Mexican Duck’s newfound status as a full species, birders in these areas may be more likely to scrutinize “brown mallards” and elucidate the situation there.) As many birders who have searched for wild Muscovy Ducks on the Rio Grande in south Texas know, Mexican x Mottled hybrids are routinely seen there. And as if things weren’t already confusing, the occasional presence of feral Mallards in the same area makes the possibility of three-species hybrids (!) not unlikely, and I have personally seen birds whose identities are best explained by this hypothesis.

For more information on Mexican Ducks, check out this article from the February 2020 issue of Birding.

What are these birds flying over the Rio Grande? The male at top left might be pure Mottled Duck or a (Mottled x Mexican) hybrid. The reduced upper speculum border makes the female at top right a pretty clear (Mottled X Mexican) or (Mottled x Mallard x Mexican), Mottled X Mexican. Among many other things, the green on the head of the male at bottom right makes Mallard one of its parents; the bold speculum edges make Mottled Duck seem an unlikely parent, at least not in the recent past, so it’s probably (Mallard x Mexican) or (Mallard x Mexican x Mottled). Finally, the female at bottom left could be (Mexican x Mallard) or (Mexican x Mallard x Mottled). Whatever the individual identifications, Collectively, there is clear evidence of all three species among these four birds. Salineño, Starr Co., Texas. 14 November 2017. Photo (C) Alex Lamoreaux.

Say goodbye to Northwestern Crow!

(Lump of Northwestern and American crows)

What many of us have long known has come to pass. In the words of the Supplement authors, Northwestern Crow is neither a species nor even a subspecies, but rather, a “geographic trend” within American Crow. Now we can all stop fighting with one another over how far north you have to be along the Pacific coast of North American to be able count a “Northwestern Crow”. So long–I will not miss thee!

Split of Japanese White-eye

Warbling White-eye (Zosterops japonicus)

Swinhoe’s White-eye** (Zosterops simplex)

If you’ve birded Hawaii, you know Japanese White-eye as the little green non-native warbler-like bird that’s common all over the lower elevations. And if you have birded coastal southern California in the last decade, you have probably also run into this introduced species. Or so you may have thought…

Recent genetic data suggest that Japanese White-eye is more than one species. One Chinese population (Swinhoe’s White-eye) is more closely related to, among others, Abyssinian White-eye of northeastern Africa and the Arabian Peninsula than it is to the Japanese population (Warbling White-eye). So the paraphyletic Japanese White-eye had to be split.

White-eyes in southern California are multiplying so quickly that they are now among the most common urban birds in coastal Orange County, where they were first noted around 2006 (Kimball Garret, pers. comm.). In addition, they have been found as far away from this stronghold as Malibu, Santa Catalina Island, Pomona, and the Tijuana River mouth. To which species do these birds belong? So far, all signs point to Swinhoe’s White-eye, but identification among white-eyes is very tricky. Once genetic confirmation of this hypothesis is attained, and if the population continues to thrive and spread, Swinhoe’s White-eye seems destined to appear on the ABA and AOU checklists within the next decade or so. Until then, birders in southern California (and northern Baja California!) should keep eBirding them to help document their spread.

Warbling White-eye has a well established population in Hawaii and is common on all of the main islands. This species tends to have a warm brownish wash to the flanks and little yellow in the lores. Swinhoe’s White-eye averages having a larger yellow loral patch and cleaner, whitish flanks which contrast more readily with the yellow vent and bib. There may also be vocal differences. Given current knowledge, range seems the best identification tool for white-eyes found in the United States.

Split of Dusky Thrush

Naumann’s Thrush** (Turdus naumanni)

Dusky Thrush (Turdus eunomus)

In North America, this split effectively results in a change of scientific name for Dusky Thrush. Naumann’s Thrush breeds in interior Siberia west of the Lena–Aldan River. Dusky Thrush’s breeding range overlaps that of Naumann’s but is much more extensive, especially to the north, west, and east–almost to the easternmost tip of the Russian Far East. Hybridization between the two, which led to their previously lumped status, complicates what would otherwise be a fairly straightforward identification of a bird photographed at Gambell, St. Lawrence Island, Alaska on 5 June 2015.

The bird looks very much like Naumann’s Thrush (Turdus naumanni, sensu stricto), but the record has not yet been ruled on by the Alaska records committee, and the ABA Checklist Committee usually waits for state and provincial committees before its considers a record, so for now the species appears on neither the ABA nor AOS checklists.


Most of the other changes which affect the ABA Area are changes to scientific names and the sequence of species on the checklist.

Split of Royal Tern

Royal Tern (Thalasseus maximus)

West African Crested Tern** (Thalasseus albididorsalis)

Large orange-billed terns breeding off the coast of western Africa have long been considered a subspecies of Royal Tern, but no longer. Genetics show them to be more closely related to Lesser Crested Tern** (T. bengalensis). So the paraphyletic Royal Tern had to be split. Compared to Royal, West African averages slighter and smaller, with a longer, thinner, paler, and yellower (less red/orange) bill. There are no records of West African Crested Tern in North America, and it appears so similar to Royal Tern that field identification would likely be difficult given current knowledge.

Goodbye, Atthis!

Bumblebee Hummingbird (Atthis heloisa Selasphorus heloisa)

Wine-throated Hummingbird* (Atthis ellioti Selasphorus ellioti)

The genus Atthis (formerly home to Bumblebee and Wine-throated hummingbirds) has been absorbed into Selasphorus, which was paraphyletic without this change. This follows the treatment in Howell and Webb’s Birds of Mexico and Northern Central America, which was published in 1995. Better late than never…

Selasphorus reshuffle

And as a consequence of the preceding change, the linear sequence of the genus Selasphorus has changed to the following:

Calliope Hummingbird

Rufous Hummingbird

Allen’s Hummingbird

Broad-tailed Hummingbird

Bumblebee Hummingbird

Wine-throated Hummingbird*

Volcano Hummingbird*

Scintillant Hummingbird*

Glow-throated Hummingbird*

Major hummingbird reshuffle

A wholesale reclassification of hummingbirds is underway, and this is part of that process. Many “new” genera have been created or resurrected. Of particular interest is the confirmation that Mexican Woodnymph is not a woodnymph, but rather, a member of “white-tailed hummingbird” group. I look forward to English name changes to be considered for it and others below. The sequence for species from Golden-crowned Emerald to Xantus’s Hummingbird is now as follows:

Dusky Hummingbird* (Cynanthus sordida Phaeoptila sordida)
Cuban Emerald* (Chlorostilbon ricordii Riccordia ricordii)
Brace’s Emerald†* (Chlorostilbon braceiRiccordia bracei)
Hispaniolan Emerald* (Chlorostilbon swainsoniiRiccordia swainsonii)
Puerto Rican Emerald* (Chlorostilbon maugaeusRiccordia maugaeus)
Blue-headed Hummingbird* (Cyanophaia bicolorRiccordia bicolor)
Broad-billed Hummingbird
Golden-crowned Emerald* (Chlorostilbon auricepsCynanthus auriceps)
Cozumel Emerald* (Chlorostilbon forficatusCynanthus forficatus)
Canivet’s Emerald* (Chlorostilbon canivetiiCynanthus canivetii)
Garden Emerald*
White-eared Hummingbird (Hylocharis leucotisBasilinna leucotis)
Xantus’s Hummingbird (Hylocharis xantusiiBasilinna xantusii)
Wedge-tailed Sabrewing* (Campylopterus curvipennisPampa curvipennis)
Long-tailed Sabrewing* (Campylopterus excellens Pampa excellens)
Rufous Sabrewing* (Campylopterus rufus Pampa rufa)
Emerald-chinned Hummingbird*
Violet-headed Hummingbird*
Antillean Crested Hummingbird*
Violet Sabrewing*
Bronze-tailed Plumeleteer*
White-vented Plumeleteer*
Crowned Woodnymph*
Coppery-headed Emerald* (Elvira cupreiceps Microchera cupreiceps)
White-tailed Emerald* (Elvira chionuraMicrochera chionura)
Violet-capped Hummingbird*
Pirre Hummingbird* (Geothalsia bella Goldmania bella)
Mexican Woodnymph* (Thalurania ridgwayiEupherusa ridgwayi)
White-tailed Hummingbird*
Blue-capped Hummingbird*
Stripe-tailed Hummingbird*
Black-bellied Hummingbird*
Scaly-breasted Hummingbird*
Violet-crowned Hummingbird (Amazilia violicepsLeucolia violiceps)
Green-fronted Hummingbird* (Amazilia viridifronsLeucolia viridifrons)
Azure-crowned Hummingbird* (Amazilia cyanocephalaSaucerottia cyanocephala)
Blue-vented Hummingbird* (Amazilia hoffmanniSaucerottia hoffmanni)
Berylline Hummingbird (Amazilia beryllinaSaucerottia beryllina)
Blue-tailed Hummingbird* (Amazilia cyanuraSaucerottia cyanura)
Snowy-bellied Hummingbird* (Amazilia edwardSaucerottia edward)
Cinnamon Hummingbird
Buff-bellied Hummingbird
Rufous-tailed Hummingbird*
Honduran Emerald* (placement uncertain, possibly in Polyerata)
Mangrove Hummingbird* (placement uncertain, possibly in Polyerata)
Sapphire-throated Hummingbird* (Lepidopyga coeruleogularis Chrysuronia coeruleogularis)
Humboldt’s Sapphire* (Hylocharis humboldtii Chrysuronia humboldtii)
Blue-chested Hummingbird* (Amazilia amabilis Polyerata amabilis)
Charming Hummingbird* (Amazilia decora Polyerata decora)
White-bellied Emerald* (Amazilia candidaChlorestes candida)
Blue-throated Goldentail* (Hylocharis eliciae Chlorestes eliciae)
Violet-bellied Hummingbird* (Juliamyia julie Chlorestes julie)

Phasianidae Reshuffle

A lot has changed at higher taxonomic levels within the pheasant family, Phasianidae. Subfamilies are no longer recognized, and the linear sequence has changed to the following:

WIld Turkey

Ocellated Turkey

Ruffed Grouse

Spruce Grouse

Willow Ptarmigan

Rock Ptarmigan

White-tailed Ptarmigan

Greater Sage-Grouse

Gunnison Sage-Grouse

Dusky Grouse

Sooty Grouse

Sharp-tailed Grouse

Greater Prairie-Chicken

Lesser Prairie-Chicken

Gray Partridge

Ring-necked Pheasant

Kalij Pheasant

Indian Peafowl

Gray Francolin

Black Francolin

Red Junglefowl

Himalayan Snowcock


Japanese Quail*

Erckel’s Francolin

New sequence for the rail family

Colombian Crake*

Paint-billed Crake

Zapata Rail*

Spotted Rail

Uniform Crake*

Rufous-necked Wood-Rail*

Russet-naped Wood-Rail*

Gray-cowled Wood-Rail*

Ridgway’s Rail

Clapper Rail

Aztec Rail*

Mangrove Rail*

King Rail

Virginia Rail

Western Water-Rail

Corn Crake


Spotted Crake

Common Gallinule

Common Moorhen

Eurasian Coot

Hawaiian Coot

American Coot

Purple Gallinule

Azure Gallinule*

Purple Swamphen

Ocellated Crake*

Yellow Rail

Yellow-breasted Crake*

Ruddy Crake*

White-throated Crake*

Gray-breasted Crake*

Black Rail

Laysan Rail

Hawaiian Rail

New sequence for families in the order Suliformes

Fregatidae (frigatebirds)

Sulidae (boobies and gannets)

Anhingidae (anhingas)

Phalacrocoracidae (cormorants)

New sequence cormorant species

Brandt’s Cormorant

Red-faced Cormorant

Pelagic Cormorant

Great Cormorant

Double-crested Cormorant

Neotropic Cormorant

New sequence for New World vulture species

California Condor

King Vulture*

Black Vulture

Turkey Vulture

Lesser Yellow-headed Vulture*

New sequence for Chloroceryle kingfishers

Amazon Kingfisher

American Pygmy Kingfisher

Green Kingfisher

Green-and-rufous Kingfisher

New sequence for Progne martins

Brown-chested Martin

Purple Martin

Southern Martin

Gray-breasted Martin

Sinaloa Martin

Cuban Martin

Caribbean Martin

New sequence for grasshopper-warbler family (Locustellidae)

Middendorf’s Grasshopper-Warbler

Lanceolated Warbler

River Warbler

New sequence for owls from Flammulated Owl to Puerto Rican Screech-Owl

Flammulated Owl

Puerto Rican Owl* (see below)

Whiskered Screech-Owl

Bare-shanked Screech-Owl*

Tropical Screech-Owl*

Bearded Screech-Owl*

Pacific Screech-Owl*

Western Screech-Owl

Eastern Screech-Owl

Balsas Screech-Owl*

Middle American Screech-Owl*

Chocó Screech-Owl*

=== Further changes affecting only Middle America ===

(Single asterisks are no longer used.)

¡Hola, Puerto Rican Owl!

Puerto Rican Screech-Owl (Megascops nudipes) ➛ Puerto Rican Owl (Gymnasio nudipes)

It seems that this species isn’t a screech-owl after all. It and Flammulated Owl are instead more distant relatives of the screech-owls (Megascops).

Say hello to the Mayan Antthrush!

Mayan Antthrush (Formicarius moniliger)

Black-faced Antthrush (Formicarius analis)

This is one of those obvious splits that birders on the ground have known about and “used” for a long time. Indeed, Howell and Webb’s Birds of Mexico and Northern Central America split this more northerly population 25 years ago under the name “Mexican Antthrush”. While the two species look similar, they sound almost nothing alike. Mayan Antthrush’s song is a loud, far-carrying, and halting series of strong whistles, beginning with a solitary whistle and a pause, followed by an accelerating and slightly ascending series of 10–15 additional whistles. In Central America, Black-faced Antthrush’s (sensu stricto) song is much simpler and consists of a short strident whistle followed by a pause and one to three downslurred whistles. In eastern Honduras, the only place where both species are known to be present, Mayan Anththrush is found in foothill rainforest, while Black-faced is found in lowland rainforest. Non-vocalizing birds seen there may be identified by the presence (Mayan) or absence (Black-faced) of a narrow rufous collar across the upper breast just below the black throat. Mayan Anthrush is found from southern Mexico to eastern Honduras, making it a North American endemic.


Say Goodbye to Paltry Tyrannulet!

(Four-way split of Paltry Tyrannulet)

Guatemalan Tyrannulet (Zimmerius vilissimus)

Mistletoe Tyrannulet (Zimmerius parvus)

Spectacled Tyrannulet** (Zimmerius improbus)

Venezuelan Tyrannulet** (Zimmerius petersi)

The tiny Zimmerius tyrannulets of Middle America now belong to one of two species. Guatemalan Tyrannulet is resident in foothills and highlands from Chiapas through central and S Guatemala to El Salvador, making it a North American endemic. Mistletoe Tyrannulet is found from southern Belize through the Izabal region of northeastern Guatemala and south to northwestern Colombia; in Belize and Guatemala, it is a lowland species. Spectacled and Venezuelan tyrannulets are endemic to northwestern South America.

Guatemalan Tyrannulet has noticeably streaked underparts, brownish auriculars, a rather long tail, thin yellow wing margins, broad white eyebrows that connect over the top of the bill, and a short, thick, straight bill. The last two features (eyebrow and bill) combine to give it an appearance recalling Philadelphia Vireo. Its typical vocalizations consist of short, somewhat nasal, piping whistles of variable pitch given approximately once a second and seldom if ever paired (e.g., hyip, pyup, pyup, pyip, peep peep, pyip, pyoo); they seem to end with a P sound.

Mistletoe Tyrannulet has subtly streaked underparts, olivaceous auriculars, a medium-length tail, bolder yellow wing margins, narrower eyebrows that are washed with yellow and do not meet over the top of the bill, and a slightly longer bill with a slight but noticeable downward curve throughout. Its typical vocalization is an airy whistle given at a slow pace, often ascending and then descending, and with a breathy ending; it is seldom if ever paired (e.g., hueeeuh hyuuuh hyuuuh). I can personally attest that the vocalizations of the two species are so dissimilar that they routinely confuse experienced birders who are hearing their second of the pair for the first time.

Say hello to the Checker-throated Stipplethroat!

Checker-throated Antwren ➛ Checker-throated Stipplethroat

The English name of Epinecrophylla fulviventris has changed from Checker-throated Antwren to Checker-throated Stipplethroat. The same has happened to additional South American members of this antbird genus, which are governed by the NACC’s sister committee, the SACC (South American Check-list Committee). The two committees try to keep their checklists in broad agreement, and this change was in line with that goal.

Split of Comb Duck

Comb Duck (Sarkidiornis sylvicola)

Knob-billed Duck** (Sarkidiornis melanotos)

The Old World and New World populations of Sarkidiornis have been split. The lumped species was known as Comb Duck (Sarkidiornis melanotos), so this change results in a “new” scientific name for Comb Duck (sensu stricto).

Split of and new sequence for the antbird genus Myrmeciza

White-bellied Antbird

Zeledon’s Antbird (placement uncertain, possibly in Hafferia)

Chestnut-backed Antbird (Myrmeciza exsulPoliocrania exsul)

Dull-mantled Antbird (Myrmeciza laemostictaSipia laemosticta)

Chestnut-backed and Dull-mantled antbirds have been moved out of the genus Myrmeciza and each into a “new” genus. The Check-list authors note that the current placement of Zeledon’s Antbird (formerly lumped as Immaculate Antbird) within the genus is uncertain.

“New” genus for Buff-fronted Foliage-gleaner

Philydor rufum Dendroma rufa

There is no change in sequence.

“New” genus for White-crowned Manakin

Dixiphia pipra Pseudopipra pipra

There is no change in sequence.

“New” genus for White-shouldered Tanager

Tachyphonus luctuosus Loriotus luctuosus

There is no change in sequence.

Scientific name change for Dwarf Jay

Cyanolyca nana Cyanolyca nanus

If you’re into arcane rules about the gender of nouns and adjectives in Latin, then this change is for you!

New sequence for the wood-partridges

Buffy-crowned Wood-Partridge

Long-tailed Wood-Partridge

Bearded Wood-Partridge

New sequence for Ara macaws

Blue-and-yellow Macaw

Chestnut-fronted Macaw

Cuban Macaw

Scarlet Macaw

Red-and-green Macaw

Military Macaw

Great Green Macaw

New sequence for Forpus parrotlets

Mexican Parrotlet

Green-rumped Parrotlet

Spectacled Parrotlet


Proposals not accepted include

Split of Great White Heron from Great Blue Heron

Split of Haida Gwaii Saw-whet Owl from Northern Saw-whet Owl

Split of Garnet-throated Hummingbird

Split of Unicolored Jay

Split of Horned Lark

Change in English name of Blue-headed Quail-Dove

Change in English names of the scrub-jays

Change in English name of Olive Warbler
Recognition of “Guanacaste Hummingbird” as a species

On a personal note, I am sad but not surprised to learn that Ocotero wasn’t chosen over the horrible name Olive Warbler for the neither olive nor warbler Peucedramus taeniatus. And likewise that Canada didn’t get its first endemic species in the Haida Gwaii Saw-whet Owl.

I thank Jeff Bouton, Terry Chesser, Jon Dunn, Kimball Garrett, Mary Gustafson, Marshall Iliff, and Guy McCaskie for their expertise in helping craft this missive.



Michael L. P. Retter is the editor of North American Birds magazine, editor of special issues of Birding magazine, and past editor of Birder’s Guide magazine. He is also author of the ABA Field Guide to Birds of Illinois and a tour leader for BRANT Nature Tours.  During college, Michael took regular road trips with friends into Mexico during holiday vacations and has been tourleading there ever since. He is currently putting his knowledge of the area’s birds to paper by writing an upcoming Princeton identification guide to the birds of Mexico, Guatemala, and Belize. Michael runs QBNA, the continent’s email list and informal club for LGBTQ+ birders. He currently lives and gardens in Fort Worth, Texas.