After all, which country has the most bird species in the world?

This is an automated translated version of my original post in Portuguese. I haven’t had time to review it and make corrections or adjustments. Feel free to point them.

Recently, a post from the Peru Birdwatchers Club (Club de Observadores de Aves de Perú, COAP) gained some notoriety for stating that “Peru officially became the country with the most birds in the world.” Traditionally, Colombia has always been touted as the country with the richest species. To add to the confusion, the author of the post and other supporters of Peru’s claim pointed to an article to back up the statement. So, how did Peru supposedly surpass the number of Colombian species?

Well, all this is a big confusion from conflicting sources. Before delving into the details of the answer, let us discuss some general ideas.

How do we compile a species list for a place?

The first challenge is, obviously, defining the geographical boundaries of that place. Often, artificial political boundaries (e.g., countries, states/provinces/departments…) are adopted, but it is not always as straightforward as it seems. When boundaries coincide with the coastline, do species seen only in the water count? How far from land? Do distant islands belong to the same territory? And when the boundary is a river, exactly where does the boundary lie? In the center of the riverbed? But how to define the middle, if the banks vary depending on the rainfall regime, etc.?[1] Does a bird that only marginally flies over a small portion of any territory, without landing on it (and without biologically interacting with any organism or even substrate of that territory), belong to that territory?[2]

Note that we are now advancing to a new question, beyond geography: once the geographical boundaries of the place are defined, what are the criteria for including a species on a checklist? By definition, these can refer to both occurrence (spatial-temporal) patterns and types of records. Generally, most authors accept only documented records (by physical material or audiovisual media) that allow independent verification of the species’ identity. However, especially in the 1980s and 1990s (in Brazil), and still today particularly for spatially more restricted locations (e.g., protected areas; third-level (and below) political divisions), visual records were/are accepted as indicative of a species’ occurrence in a certain place. The Brazilian Committee of Ornithological Records (CBRO) adopts an “intermediate” stance: a few species without verifiable documentation (i.e., recorded only visually) are accepted as part of the Brazilian avifauna if their occurrence aligns with the recognized biogeographical pattern for the species as a whole. In other words, if the Brazilian record was (implicitly) made in an environment known to be used by the species and in the same general region as other documented records of the species, it’s deemed reliable; if the record contradicts the prevailing biogeographical knowledge, it is, by caution, discarded due to its improbability.

There’s also the question of bird (and other animal) vagrancy. Does a species whose occurrence in a place is known by a single individual that strayed and accidentally reached a certain territory (even if properly documented) count as part of its avifauna? Or should only populations (and not individuals) be counted? What about introduced species? Escapes from captivity? Individuals that accidentally arrive assisted by ships or other means of transport?

A Redwing (Turdus iliacus), which naturally occurs in Europe, Asia and northern Africa, apparently crossed the Atlantic and landed on a ship in Brazilian territorial waters. Should it be considered a Brazilian species because of this? What is the chance of other individuals appearing in Brazil? On the other hand, in 1986, an Australian Gannet (Morus serrator) was found in Santa Catarina. I always wondered when and especially if someone would ever find another individual in Brazil. To my surprise, it did not even take 40 years to be found again in the country (and again in Santa Catarina). Moreover, knowing that all biological communities change over time, until when should a historical record be counted? The Antarctic Tern (Sterna vittata) has a single record in Brazil, a bird collected almost 150 years ago by a ship at sea on the border of Santa Catarina and Rio Grande do Sul. Should it continue to be included in the Brazilian avifauna?

There are many variables, and except for a general demand for more rigor in species identification and documentation, most studies end up opting for more “inclusive and permissive” decisions to all the questions raised above, although for almost all of them there are equally valid arguments in favor of more restrictive treatments. However, it is clear that different checklists can be constructed for the very same place and based on the very same data, and that any two checklists commonly have different criteria for species inclusion.

Taxonomic freedom – after all, what is a species?

An aspect as important as or even more important than the criteria for species inclusion in a checklist is the definition of which and how bird populations will be recognized as independent species. This isn’t an exclusive checklist problem, but is one of biology’s universal questions: what is a species?

The discussion is old and far from over (the most recent chapter seems to be an interesting and quite recent article discussing again the philosophical and operational bases of what a species is). In ornithology, there are different conceptualization proposals and even operational ones (i.e., once philosophically defined what a species is, how can we recognize them in nature). Different authors have unique and personal views on what a species is and end up adopting different proposals. It is debated, for example, whether two related populations will be recognized as variations of a single species or as two different species, and this directly affects the final number of a place’s species checklist.

The CBRO adopts a species definition that tends to recognize more populations as independent species compared to the traditional “biological species concept” adopted by many authors. There are even more discrepant (in terms of final numbers) proposals for some other countries, like Mexico (see here and here). But, as mentioned at the beginning of this section, one thing is the theoretical concept, another is the application of that concept, which can diverge even within the same philosophy of what a species is since different authors judge the available data on a population’s evolutionary independence differently. Therefore, it is not surprising that the four main global bird checklists (i.e., lists of all existing bird species in the world) had different treatments for several populations and, consequently, different final species numbers. Precisely to avoid such discrepancies, those responsible for generating all these four checklists joined efforts to produce a single, “consensual” world bird checklist (to be released in 2024). It is worth noting here that none of the four checklists (or any other checklist that diverges by opting for different concepts and applications of species definition criteria) is “more correct” than another. Like the inclusion criteria discussed above, the definition of what constitutes a species is also largely a matter of personal choice, with good arguments for and against each choice.

The different sources of information and the validity of comparisons

Let us return to the COAP post. The first point to question is the term “officially.” There simply is not a global authority that defines the number of bird species for each country that could “endorse” the numbers from Peru or any other country. What commonly exists are committees or related groups within each country that assess available data to define a checklist of bird species occurring in that country (whether the checklist is officially recognized by the government or not). Both Peru and Colombia (more recently) as well as Brazil—countries considered the richest in bird species—have such committees.

So, what are the numbers? Peru: 1879 species; Colombia: 1966 species; Brazil: 1971 species! So, is Brazil the country with the most bird species in the world? Actually, no! As mentioned earlier, the CBRO uses a species definition concept that tends to recognize more populations as full species. As I said, inclusion criteria and species definition criteria need to be the same between any two checklists for comparison! The Peruvian and Colombian committees follow the taxonomic classification of the South American Classification Committee (SACC), so their numbers are directly comparable, unlike Brazil. However, repeating what I’ve been saying from the beginning, inclusion criteria need to be the same. While the Peruvian committee only counts documented species, the Colombian committee (CCRO) also includes species with only visual records and even those species considered uncertain (whose identification or documentation origin does not securely confirm their occurrence in Colombia; n = 10 species). Thus, if one counts hypothetical species from both countries, Peru would have 1913 species compared to 1956 in Colombia (and Brazil with 1873, within the same SACC classification [not the CBRO classification]); if only properly documented species are counted, Colombia still surpasses Peru with 1896 vs. 1879 species (and 1859 in Brazil). So why did the COAP post claimed that Peru had surpassed Colombia? Because the information source used for comparison in text published in the Boletín UNOP had not been the most recent Colombian checklist, but rather a 2018 checklist available on the SACC website.

Therefore, we can concluded that, using the most recent available information sources and under the same taxonomic and species inclusion criteria, Colombia would be the country with the richest bird species diversity in the world, followed by Peru and Brazil (and Indonesia). Nevertheless, perhaps the most important thing in all this is to recognize that all these countries (and some others) are equivalently admirable for hosting a mega-diversity of bird species, and this brings enormous responsibility to their citizens. That is exactly what Echeverry-Galvis et al. (2022) discuss about their Colombian checklist: Beyond the number of species, which undoubtedly instills great pride and responsibility, the information in this updated checklist of birds from Colombia reflects the solid work of an ornithological community in collaboration with birdwatchers and local communities who are interested in contributing to science as a foundation for the conservation of national avifauna.

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Final comments:

• This is the reality today (January/2024). With further research, maybe Peru, Brazil or even Indonesia may become the most species-rich country in the world, even though my opinion is that Colombia indeed has the highest potential to be ever the contry with most bird species.

• Birdlife has similar results, although it is hard to trace back the source of their numbers.
  

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Notes:

1. By definition, when a political boundary is a river, the boundary always follows the riverbed (or channel) at its deepest point across any section of the river. However, do bird recordists know and adhere to this definition?

2. Most people tend to agree that colonies of oilbirds in caves whose entrance is in Venezuela, but whose galleries extend into the underground of Brazilian territory, would be legitimate records for Brazil

Bibliography

Bege, L. A. R. & Pauli, B. T. (1990). Two birds new to the Brazilian avifauna. Bull. Brit. Orn. Club. 110(2): 93–94.

Brito, G.R.R., Nacinovic, J.B. & Teixeira, D.M. (2013). First record of Redwing Turdus iliacus in South America. Bull. Brit. Orn. Club. 133(4): 316–317.

Echeverry-Galvis, M.A., O. Acevedo-Charry, J. E. Avendaño, C. Gómez, F.G. Stiles, F.A. Estela & A.M. Cuervo. (2022). Lista oficial de las aves de Colombia 2022: Adiciones, cambios taxonómicos y actualizaciones de estado. Ornitología Colombiana 22: 25-51.

Lees, A. & Gilroy, J. (2021). Vagrancy in birds. Christopher Helm, London, 400 pp.

Maddison, W. P., & Whitton, J. (2023). The Species as a Reproductive Community Emerging From the Past. Bulletin of the Society of Systematic Biologists, 2(1), 1–35.

Navarro-Sigüenza, A.G. & Peterson, A.T. (2004)An alternative species taxonomy of the birds of Mexico. Biota Neotropica 4: 1-32.

Pacheco, J.F., Silveira, L.F., Aleixo, A. et al. (2021). Annotated checklist of the birds of Brazil by the Brazilian Ornithological Records Committee—second edition. Ornithol. Res. 29, 94–105 (see publications)

Peterson, A.T. & Navarro-Sigüenza, A.G. (2009) Constructing check-lists and avifauna-wide reviews: Mexican bird taxonomy revisited. Auk 126(4): 915–921.

Ugarte, M., Angulo, F. & R. Gutiérrez (2023). Actualización de la lista oficial de aves del Perú. Boletín de la Unión de Ornitólogos del Perú (UNOP), 18(1): 8-16.