Skip to content

Reprise 11: The Four Different Checklists of Birds of the World

May 1, 2020

Editor’s Note: Entry number eleven in our tenth anniversary blog tour ran March 19, 2019 and is eighth in popularity. I found it on BirdChat (National Birding Hotline Cooperative) to which I subscribe and recommend, and loved it. Author Eran Tomer gave me permission to reprint it, I added some formatting and photos, and it was an instant hit. [Chuck Almdale]


You may be surprised to learn that there are not one, but four different checklists of all the birds in the world.

As you will read below, out of a current total 11,524 extant and recently extinct species, the four lists agree on 9,968 (86.5%) of them, leaving 1556 species considered by some but not by others to be “good” species. You may think, “So what?” but if you’ve seen (or think you’ve seen) Hoary Redpoll, Franklin’s Grouse or Thayer’s Gull, read on.

Birders interested in classification, nomenclature, systematics, status, distribution, world birding, or the ticking off of lifers will find this food for thought. We all use checklists, even if only in our own backyard, and these checklists are based on something. This article delves into that something.

This also gives us a glimpse into the inner processes of science, which very often begin with the observation and naming of “things” and then attempts to organize those “things” into a coherent system. This can get messy at times as scientists do research, present their discoveries, argue their views and disagreements inevitably arise. It is misleading to think that “Science says…” as if “science” were a monolithic entity with a single and unified voice. Science is many people, many voices, perpetually changing and growing. Most importantly, it is inherently self-correcting because every scientist has a horde of other scientists peeking over their shoulder, looking for errors upon which they can pounce. This is human nature, inquisitive and acquisitive, organized for our common good.

This article first appeared on the excellent, informative and entertaining listserve BIRDCHAT as a three-part email essay.  I liked it very much and – having received permission from the author – hope to widen it’s audience. Near its end there are fourteen species mentioned which are treated uniquely by one or another of the four checklists. I have added photos of these particular species, alongside their more commonly accepted classification.    [Chuck Almdale]

For the original PDF file, without photos, click Avian Taxonomies Compared.

Avian Taxonomies Compared

By Eran Tomer
February 2019

Recently one of my projects prompted me to compare the four major ornithological taxonomies:

  • Clements/eBird
  • BirdLife International/Handbook of the Birds of the World (BirdLife/HBW)
  • International Ornithological Committee / Union (IOC)
  • Howard & Moore

I thought I’d share the highlights, especially since I couldn’t find similar information on the Web. First the facts & figures, then the overall conclusions.

Only Howard & Moore still split Caribbean Coot from American Coot.
Left: Caribbean Coot – Fulica caribaea (Mario Espinosa – eBird)
Right: American Coot – F. americana (Mike BairdWikipedia)

Facts and Figures

As of this writing, the four taxonomies list 11,524 bird forms among them. 162 of these have become extinct over the past five centuries. Thus if all forms were recognized as full species, we’d have 11,362 today. No taxonomic authority recognizes all of these:

  • BirdLife – 11,126 species
  • IOC – 10,896 (230 fewer than BirdLife)
  • Clements – 10,585 (541 fewer than Birdlife, 311 fewer than IOC)
  • Howard & Moore – 10,175 (951 fewer than BirdLife, 721 fewer than IOC, 410 fewer than Clements)

Currently 9,968 species, 86.5% of the 11,524 total, are recognized by all four authorities though some decisions are still contestable and changing. Scientific and English names mostly match across lists but some vary. So the prevailing taxonomic disarray is actually limited. The status of only 1556 possible species, or 13.5% of the total, is debated:

  • 496 species, or 4.3% of the total, are recognized by three taxonomic authorities.
  • 362 species, or 3.14%, are recognized by two authorities.
  • 698 species, or 6.1%, are recognized by a single authority only.

These breakdowns vary among taxonomies:

  • BirdLife: 89.6% of species shared with all others; 5.6% shared with one or two other lists; 4.8% unique to itself.
  • Clements: 94.2% of species shared with all others, 5.6% with 1-2 other lists, 0.21% unique to itself.
  • IOC: 91.5% of species shared with all others, 7.4% with 1-2 other lists, 1.13% unique to itself.
  • Howard & Moore: 98% of species shared with all others, 1.8% with 1-2 other lists, 0.2% unique to itself.

Only Howard & Moore still split Thayer’s Gull from Iceland Gull.
Left: 1st year Iceland Gull – Larus glaucoides (Luke Seitz -Macaulay Library)
Right: 1st Year Thayer’s Gull – L. thayeri (Brian Sullivan Macaulay Library)
Both from

So most authorities accept Howard & Moore species, but not vice versa. BirdLife recognizes numerous species that all others don’t. IOC recognizes many species rejected by two authorities but accepted by one other authority. Clements accepts many species recognized by two or three other authorities; fewer recognized by only one other authority; and very few that no one else recognizes (same for Howard & Moore).

Only the IOC recognizes the split of the Cattle Egret into Eastern and Western species.
Left: Eastern Cattle Egret – Bubulcus coromandus (Su Neko)
Right: Western Cattle Egret – B. ibis (HoodedWarbler12)    Both: Wikipedia

Of the 698 species recognized by a single authority:

  • 532 occur only on BirdLife’s list (this organization assesses species differently from the rest; see below).
  • 123 occur only on IOC.
  • 22 occur only on Clements.
  • 21 occur only on Howard & Moore.

Of the 496 species recognized by three of the four authorities:

  • 490 are recognized by IOC
  • 427 by BirdLife
  • 420 by Clements
  • 151 by Howard & Moore

Only the IOC recognizes the split of the Osprey into Eastern and Western species.
Left: Eastern Osprey – Pandion cristatus (Psylexic)
Right: Western Osprey – P. haliaetus (Mike Baird)      Both: Wikipedia

Thus the top three share most of these species, while Howard & Moore doesn’t recognize them. Another perspective:

  • 6 species are recognized by everyone except IOC.
  • 69 species are recognized by everyone except BirdLife.
  • 76 species are recognized by everyone except Clements.
  • 345 species are recognized by everyone except Howard & Moore.

IOC readily accepts splits while Howard & Moore is far more reserved than others about this. BirdLife and Clements are intermediate.

This trend continues with the 362 species recognized by two authorities and rejected by the other two:

  • 315 of these are recognized by IOC.
  • 199 by BirdLife.
  • 175 by Clements
  • 35 by Howard & Moore.
  • 160 species are recognized by BirdLife and IOC, not Clements or Howard & Moore.
  • 149 species are recognized Clements and IOC, not BirdLife or Howard & Moore.
  • 21 species are recognized by BirdLife and Howard & Moore, not Clements or IOC.
  • 18 species are recognized by BirdLife and Clements, not IOC or Howard & Moore.
  • 8 species are recognized by Clements and Howard & Moore, not BirdLife or IOC.
  • 6 species are recognized by IOC and Howard & Moore, not Clements or BirdLife.

Again, most of these 2-lists-only species are ‘shared’ among IOC (especially), Clements and BirdLife.

Another pairwise comparison: which taxonomies are most similar? Clements and IOC are closest with 96.2% of their species overlapping, yet this is asymmetrical. IOC recognizes 365 species that Clements doesn’t, while Clements recognizes only 54 species that IOC doesn’t. That’s because the IOC list is 311 species longer overall than Clements, and more likely to accept splits. So the overlap zone covers most of Clements but less of IOC.

BirdLife and Howard & Moore are least similar with 89.7% of species overlapping. The imbalance here is even more striking because the BirdLife list is 951 species longer than Howard & Moore. The two authorities also differ much in approach and methodology. BirdLife recognizes 1055 species that Howard & Moore doesn’t, vs. 104 species recognized by Howard & Moore but not BirdLife.

Only Clements/eBird still recognizes the split of New Zealand’s Red-billed Gull from the Silver Gull.
Red-billed Gull & chick – Chroicocephalus scopulinus (Jorg Hempel – Wikipedia)
Right: Silver Gull – C. novaehollandae (Joseph C Boone – Wikipedia)

The other four pair combinations are intermediate between these. BirdLife and IOC are most ‘balanced’ since they are of similar length and don’t recognize many of each other’s species. BirdLife recognizes 577 species that IOC doesn’t while IOC recognizes 347 species that BirdLife doesn’t.

IOC recognizes most species that Clements (especially) and BirdLife do: IOC and BirdLife recognize 10,549 species between them; IOC and Clements recognize 10,531. BirdLife and Clements together, without IOC, recognize 10,337 species between them – about 200 fewer. All combinations with Howard & Moore are lower but follow the same pattern: 10,119 species with HM+IOC, 10,071 with HM+BirdLife, 10,051 with HM+Clements.

Only Clements/eBird does not recognize the split between the Red-tailed and Kurdish Wheatears.
Left: Red-tailed Wheatear – Oenanthe chrysopygia (Pekka Fagel via Kuwait Birds)
Right: Kurdish Wheatear O. xanthoprymna (Christoph Moning – Macaulay Library via eBird)


These are the highlights so I’ll stop here with the figures (which go on). Finally, in plain English, how do the ornithological taxonomic authorities compare overall? A reminder first: they all agree on the decisive majority of species.

Howard & Moore is decidedly the most conservative and traditional taxonomy. It doesn’t recognize many lumps and splits that others do. The species it does recognize are widely accepted. Hence it is less contestable and more stable than the rest in an age of taxonomic turmoil. It serves as an unofficial benchmark. If Howard & Moore recognizes a species, in all likelihood it is well-defined and taxonomically “solid”, not a debatable split.

Afghan Sparrow is currently recognized only by Howard & Moore.
Left: Afghan Sparrow – Passer yatii John Gerrard Keulemans
Right: Dead Sea Sparrow – P. moabiticus (Danielle Occhiato)

This authority still accepts a few species that others don’t recognize, or that others have split or lumped. For example, currently it is the only taxonomy that accepts Thayer’s Gull, Caribbean Coot and Afghan Sparrow as full species. Website:

Only IOC recognized the split of Green-winged from Common (or Eurasian) Teal.
Left: Green-winged Teal – Anas carolinensis (Jeff Stacey – Macaulay Library)
Right: Common (or Eurasian) Teal – A. crecca (Guido Bennen – Maucaulay Library)

Conversely, IOC is the most liberal taxonomy. It accepts a very large proportion of proposed splits, including many that some others reject and quite a few that no one else recognizes. For example, currently it is the only taxonomy that splits the Palearctic’s Common Teal (Anas crecca) from the New World’s Green-winged Teal (Anas carolinensis). It is also the only one that splits both Osprey and Cattle Egret each into two species. Only six species are recognized by all authorities save IOC, e.g. Taiwan Thrush.

The Taiwan Thrush is one of only six species recognized by all authorities except the IOC.
Left: Taiwan Thrush – Turdus niveiceps (AlnusWikipedia)
Right: Island Thrush – T. poliocephalus carbonarius (Mark A HarperWikipedia)

Yet some of the species that only IOC recognizes are distinctive and this may be important for certain purposes. E.g. it splits Lava Heron from Green / Striated Heron.

Only the IOC splits the Galapagos’ Lava Heron from the widespread Striated Heron.
Left: Lava Heron – Butorides sundevalli (Victor W. Fazio III – Flickriver)
Right: Striated Heron – B. striata striata (David DishereBird)

Clements / eBird is intermediate. It recognizes many of the splits and species that IOC does, and far more than Howard & Moore. It is said to adhere to North and South American ornithological societies’ checklists (AOS/AOU and SACC) more closely than the other taxonomies. However, it is circumspect about recognizing numerous (probably controversial) splits that IOC accepts, or BirdLife’s independent methodology. It is evidently more inclined to accept species that at least two other taxonomies recognize.

Only Clements/eBird does not recognize the split of Sira Barbet from the Scarlet-banded Barbet.
Left: Sira Barbet – Capito fitzpatricki (Michael G. Harvey – Wikipedia)
Right: Scarlet-banded Barbet – C. wallacei (Andrew Spencer – Macaulay Library eBird)

Nevertheless, currently it doesn’t accept some species that everyone else does, e.g. Red-tailed Wheatear and… Sira Barbet (Capito fitzpatricki, named for the Cornell Lab’s director – manager of the Clements taxonomy). It also rejects various extinct species that other taxonomies accept. Only a few species are recognized by Clements alone, e.g. Red-billed Gull and Margelanic Whitethroat.

Only Clements/eBird accepts splitting the Margelanic from the widespread Lesser Whitethroat.
Left: Margelanic Whitethroat – Sylvia margelanica (Vincent Wang – Macaulay Library via eBird)
Right: Lesser Whitethroat – S. curruca (Dhaval Vargiya – Wikimedia via Wikipedia )

It is a robust taxonomy, more mainstream / standard and less contentious than the others. A natural choice for eBird. Website:

Birdlife / HBW is unique. It used to be very conservative taxonomically until a few years ago, when BirdLife adopted and adapted a new species assessment method. It systematically scores visual, vocal, geographic and other differences among various forms, and assigns a full species rank to those whose score exceeds a certain threshold. It also accounts for genetic differences. This method is disputed scientifically but eminently pragmatic. BirdLife also manages its taxonomy differently from the other authorities. Hence it is idiosyncratic and not as comparable to the rest.

Only Birdlife/HBW lumps Hoary and Common Redpolls.
Left: Common Redpoll – Acanthis flammea (Sharon Watson via Birdshare)
Right: Hoary Redpoll – A. hornemanni (Chris Wood) Both: All About Birds

BirdLife’s list is the longest and contains the most species recognized by no one else. This is due less to acceptance of proposed splits and mostly to BirdLife’s own methodology. It is slightly more split-liberal than Clements as concerns most species. It differs from Howard & Moore about as much as Clements does. However, it rejects many splits that Clements and IOC accept.

So overall, BirdLife doesn’t recognize many species / splits that others do but its own splits are likewise unrecognized by others. For example, it is currently the only authority that lumps Common and Hoary Redpolls but splits Franklin’s Grouse from Spruce Grouse.

BirdLife taxonomy relies more on visual, vocal and behavioral differences to define species than other authorities, and less on genetic differences (which it still considers). This is useful to various ends, e.g. conservation and birding. Website:

Only Birdlife/HBW splits Franklin’s Grouse from the widespread Spruce Grouse.
Left: Franklin’s Grouse – Falcipennis franklinii (Alex Lamoreaux – Macaulay Library)
Right: Spruce Grouse – F. canadensis (Luke Seitz – Macaulay Library)
Both: All About Birds

With all taxonomies, subjective conventions ultimately determine what types and magnitudes of differences suffice to define a given form as a species. Therefore each authority has strengths and weaknesses.

However, the existence of multiple, independent taxonomies (and nomenclatures) engenders decentralization, non-standardization and disorganization. These undermine the very goals of biological classification and constitute a major challenge to ornithology, conservation and birding. Fortunately the problem is recognized and efforts have been undertaken to mitigate it.

I’d like to thank the International Ornithological Committee (IOC) for providing the file with the raw data used in the analysis.

About the author:

Eran Tomer is a birder and naturalist with an academic background in biogeography, ecology and zoology. He resides in Atlanta, Georgia.

Comments are closed.

%d bloggers like this: