Components of the AviList Checklist

Imperial Snipe Gallinago imperialis by Chris Venetz | Ornis Birding Expeditions; Cornell Lab of Ornithology | Macaulay Library

Taxonomy

The core taxonomic fields (taxon rank, order, family name, scientific name) are all adjudicated by the Taxonomic Committee (TaxCom). For the first version (v2025), TaxCom has assessed all cases in which at least one of these fields differed among IOC version 12.1, eBird/Clements v2019, and BirdLife v2019.

In addition, any taxon or taxon group for which TaxCom formally deliberated possible arrangements is accompanied by a decision summary with an issue number that connects to the TaxCom discussion. The summary statement is intended to lay out potential alternative treatments and provide context and a rationale for why AviList has adopted a given treatment. Decision summaries will be of particular interest to many end users. They point to taxonomically controversial complexes in which there is often a need for further research, and which may well undergo future taxonomic change. In this way, it is hoped that AviList will help ornithologists in identifying groups that require more work and thereby assist taxonomic progress in the future.

Future AviList versions will continue this process, implementing changes from v2025 and providing summary statements to explain those changes.

Subspecies

For the first version of AviList (v2025), review and alignment of subspecies has not been a priority. Although all three checklists (IOC World Bird List, BirdLife International, and eBird/Clements) include subspecies, the very large number of discrepancies would have made the reconciliation task unmanageable. Therefore, AviList v2025 started with a baseline list of subspecies from IOC v11.2 (July 2021), and this initial subspecies taxonomy has been largely carried through to AviList v2025. Some recent modifications to subspecies taxonomy as published by IOC World Bird List in v12.1 to v15.1 have been incorporated in AviList; other changes have yet to be implemented, especially where IOC, eBird/Clements, and BirdLife do not align. For example, AviList still treats Fregata magnificens (Magnificent Frigatebird) as monotypic, despite 2–3 subspecies being recognized by modern versions of eBird/Clements, BirdLife, and IOC, with the latter added in v12.1.

An initial task for AviList v2026 will be to bring the subspecies into full alignment with the final version of IOC (15.2, expected in July/August 2025). The AviList team will then set some global standards for consistent definition of subspecies at a global level. For future versions, we expect Regional Advisory Groups to assist in standardizing and modernizing subspecies delineation across AviList.

Nomenclature

While taxonomic incongruences arise from ornithologists’ disagreements about the correct way to delimit species or other taxonomic units, nomenclatural incongruences refer to cases in which there is uncertainty about the correct application of scientific names. Nomenclatural incongruences are much rarer than taxonomic ones, but are no less vexing and need to be addressed.

AviList enlisted a team of experts, forming the Nomenclatural Advisory Group, to discuss, deliberate and vote on all cases in which the three global partner lists exhibited incongruences in the application of scientific names and their spellings.

Akin to the cases adjudicated by TaxCom, AviList version 2025 provides detailed summary statements for all cases of nomenclatural incongruence, setting out the context and offering a rationale for why a certain arrangement or spelling has been adopted.

This process of nomenclatural adjudication is not over. Additional cases of nomenclatural disagreement in the ornithological community will doubtless come to light, and new evidence may arise in the case of issues that were thought to have been laid to rest. AviList will continue to address these issues in future versions.

English Names

For the first version of AviList (v2025), standardization of English names was not formally considered. The English names for v2025 match those names used for the IOC World Bird List 15.1. In cases where species limits differed between AviList and IOC 15.1, the English names were decided upon based on consultation with the IOC team and adoption of IOC standards (e.g., for hyphenation). AviList v2025 also shows the English names used by eBird/Clements and BirdLife International whenever species limits align; the IOC name is not shown separately since it exactly matches the AviList name in almost all cases.

Future versions of AviList will include an official AviList English name decided upon by a newly established English Names Committee commencing its work after the release of version 2025. Even so, the AviList team does not feel that there needs to be only a single prescribed common name for each species, and future AviList versions may display additional widely used alternative English names. Note also that eBird (www.ebird.org) and Birds of the World (birdsoftheworld.org) allows users to set their preferences to any of almost 100 non-English alternate common names as well as regional alternative English names; additionally, Birds of the World provides a link to names that display all the official alternate names in multiple languages (e.g., for Osprey click the “names (81)” link, between IUCN Red List status and “Subspecies”, so see the 81 common name alternatives for Pandion haliaetus.)

Linear Sequence

How does one determine with which species to start and close a list—as well as the order of all the species and other taxonomic units in between? AviList builds on principles commonly employed to best reflect our knowledge of the evolution of birds. This can be visualized using a bifurcating tree, where the basal trunk represents the common ancestor of all living birds and each bifurcation a divergence event. This kind of tree is the common output from phylogenetic studies and is often arranged such that at each bifurcation, the branch leading to the fewest terminal branches (or tips, or leaves) of the tree is drawn first (i.e., arranged towards the left), while the other branch is drawn second (i.e., placed on the right).

All extant birds have evolved for precisely the same amount of time since the common ancestor to living birds first diversified into different groups. Thus, no group of birds is more “primitive” than another; instead, some groups have diversified more and others less. Some have lost more of their branches to extinction through time, and others fewer.

The Primary Principle of Increasing Species Diversity

The primary principle of increasing species diversity guides the process by which the linear sequence reflected in AviList is derived from a tree comprising all extant bird species. No study has produced a single tree comprising all bird species of the world (although some have come close). Therefore, a multitude of scientific studies have been used to infer the linear sequence where possible. This often means integrating results from different studies. As a general rule of thumb, when results differ, integrative and phylogenomic studies are given priority. The principle of increasing species diversity has been implemented on all taxonomic levels, including orders, families, and genera. However, not all genera have been updated to species level in version 2025.

Additional Hierarchically Ordered Principles

At certain bifurcations in a tree, both branches lead to the same number of descendant species. For these cases, we have chosen to implement an additional set of hierarchically ordered principles, which can be consistently applied throughout the list:

  1. The principle of increasing subspecies diversity. Apply the principle of increasing diversity, but at the subspecies level, so that the clade comprising the fewest terminal subspecies is listed first.
  2. The principle of priority. If the number of subspecies is equal between the two clades, list first (a) the clade that was scientifically described first (earliest) or, if not applicable, (b) the clade that comprises the species that was described first.

The principle of priority often has to be applied to sister genera or sister species. In those cases, the genus or species whose name was described first will also be listed first.

In cases that comprise species assemblages below the genus level, AviList first reflects those clades which contain the species first described. For example, take a clade of four monotypic species described in 1882 (species A), 1898 (species B), 1923 (species C), and 1877 (species D), in which species A+B and species C+D are most closely related, respectively. In this clade, C+D will be listed first, so that the linear sequence reflected in the list is D, C, A, B.

Finally, if the year of description is the same, the name published first during that year has been given priority.

Sequence of Subspecies

The sequence of subspecies in the list follows the convention to list these geographically from northwest to southeast based on the approximate range centroid of the ranges. The sequence of this list largely follows that used by the IOC World Bird List which has long adopted these standards for subspecies sequence.

Range Statements

Range statements in AviList are based primarily on those in the eBird/Clements Checklist v2024. These range statements were recently revised in 2023–2024 and contain the most complete, current, and authoritative information on the ranges of each taxon. Under the guidance of Pam Rasmussen, original range statements were drafted for any taxa that did not have an existing range statement in eBird/Clements. Finally, under the guidance from Pam, with help from Juan Freile and Don Roberson, a number of ongoing range statement revisions are underway for eBird/Clements and Birds of the World and these will be shared with AviList for future revisions.

Extinct Species

AviList v2025 incorporates species and subspecies that have become extinct roughly since the year 1500.

Extinct taxa are marked in the taxonomy files in the column headed “Extinct”. Our criteria for v2025 exactly match the criteria used by the IOC World Bird List (v15.2). Our position is to maintain a conservative stance and to retain a fairly high threshold for inclusion. We have limited the list to include extinct species and subspecies that meet the following criteria:

1. The taxon became extinct in its geographical range after 1500 and, by implication, largely as a consequence of the subsequent anthropogenic effects of hunting pressures, environmental alteration, and the introduction of exotic species. It is recognized that humankind has been responsible for the extinction of species from the beginning of our interaction with other life forms, and that this is not a trivial issue. However, extinctions caused by man prior to 1500 are beyond the scope of this list. As a consequence, all extinct taxa included in the list disappeared after 1500 (which is roughly the beginning of the era of European overseas exploration; that exploration and colonization has often—but not always—been a direct or indirect cause of extinction).

2. The taxon is known from physical evidence of its existence (i.e., specimens, skins, skeletal remains, or a credible contemporary illustration, painting, or drawing that leaves little doubt of the taxonomy and uniqueness of the figured bird, such as Tahiti Rail Gallirallus pacificus and Tanna Ground Dove Pampusana ferruginea, but not “Turdus” ulietensis). Thus, taxa known solely from literary sources without corroborating physical or illustrative evidence, compelling as they may be, are excluded from this list (e.g., Reunion Fody Foudia delloni, Guadeloupe Macaw Ara guadeloupensis, and Reunion Gallinule Porphyrio caerulescens).

AviList also includes a column indicating whether a taxon is extinct for subspecies, species, genera, and families. Treatment of a species as extinct defers to the IUCN Red List whenever possible; by the same token, genera and families are marked as extinct if all their composite species are considered extinct. Subspecies are not treated by the Red List, so the AviList team has attempted to apply criteria similar to those in the Red List to indicate taxa that are truly known or believed to be extinct.

The IUCN Red List uses quantitative criteria (analysing records, survey effort, and threats) to accurately and consistently infer extinction probability, classifying species as Extinct (where the probability of extinction is >0.9), Critically Endangered (Possibly Extinct) (0.5-0.89), or Critically Endangered (<0.5). There are however some species that experts on AviList Committees have separately judged as extinct or probably so. We therefore also include, in a separate column to avoid confusion, for some species an unofficial tag (lower case and in parentheses) of ‘(Extinct)’ or ‘(Possibly Extinct)’ for species that might be extinct, but have not yet been classified as such by IUCN. The two approaches, represented in the extinction column and the Red List column of AviList, should help users to better assess for themselves the magnitude of global species loss and the uncertainty surrounding those assessments.

We also hope to inspire more effort to resolve these questions surrounding particular species’ extinction status, either through rediscovery, or by research that can more solidly affirm the sad truth of extinction. We especially applaud the Search for Lost Birds Project, which has focused attention on these most imperiled birds and already has had many successful discoveries, along with disappointments.

Bibliographic Information

The Bibliographic and Nomenclatural Committee regularly met to discuss and implement best practices to reflect the nomenclatural and taxonomic information associated with each taxon listed by AviList. This committee had to resolve how to best address the tens of thousands of little differences arising from the adoption of different formats and standards by the three global partner lists. The committee also had to draw up new rules and standards for how to transcribe names of authors from countries using a non-Latin script.

Importantly, having Avibase as the database backbone for AviList has meant that lots of additional data can be layered in. The Avibase team (i.e., Denis Lepage) has been maintaining these data for many years and while it has not been possible to check every field across tens of thousands of taxa, we have decided to share this data in the interest of making it better. These fields will benefit from feedback on the public release and we expect these to be more complete and more perfect for future versions.

These are the Avibase columns in the “extended” version of the list:

  • Gender of genus
  • Type species of genus
  • Type locality
  • Title of original description
  • Original description URL
  • Protonym

Future Information

AviList v2025 does not contain information on subspecies groups. It also does not contain information on any taxonomic ranks intermediary between the classic Linnaean ranks of order, family, genus, and species (e.g., superfamilies, subfamilies, etc.). This additional information may be added in future versions.

AviList Column Headers

Download the metadata as a PDF (or view in in the “How to use” tab of the XLSX files on the Checklist page) to get the full listing of the AviList column headers along with a short explanation.