What causes taxonomic disagreement between bird checklists?

Study identifies a combination of research bias and differences in divergence rates.

Why can’t we all just get along? This question often comes to my mind when I follow the heated debates between taxonomists. Personally, I find the quest to pigeonhole the immense biodiversity into species and subspecies not that interesting. I prefer to focus on the ecological and evolutionary processes that gave rise to all this diversity. For conservationists and policy makers, however, a reliable overview of the species in an area can be crucial. It is thus certainly important to continue with the correct classification of life.

But what is the correct way to classify organisms? The species problem has been haunting biologists for centuries. Charles Darwin already indicated that “No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species.” And the struggle continues to this day (see this blog post). Taxonomists often disagree about the species status of a certain population. A first step in solving these taxonomic disputes is to understand what factors cause the disagreements. And that is exactly what Montague Neate-Clegg and his colleagues did. They scoured four bird checklists for taxonomic disagreements and tried to identify the underlying causes. Their findings recently appeared in the journal Global Ecology and Biogeography.

Research Bias

The researchers examined four commonly used checklists for the world’s avifauna:

  • Howard and Moore Checklist of the Birds of the World
  • eBird/Clements Checklist of Birds of the World
  • Birdlife International Digital Checklist of Birds of the World
  • International Ornithological Community (IOC) World Bird List

This extensive comparison culminated in 11,389 extant species names of which 9,894 were recognized by the four checklists. Some simple math reveals 1,495 problematic cases. A minority of these (18 cases) were newly discovered species, whereas the rest were genuine taxonomic disagreements, such as the redpolls (genus Acanthis) and the crossbills (genus Loxia).

A detailed look at the disputed taxa showed a clear geographical research bias. The researchers noted that “taxonomic agreement was lowest for species in Southeast Asia/Australasia and the Southern Ocean, understudied regions where islands have driven high levels of cryptic diversification. In contrast, agreement was highest in the temperate Northern Hemisphere where diversity is lower and research is more extensive.” Luckily, ornithologists are working hard to stabilize the taxonomy in Southeast Asia and Australasia (see for example here and here).

Taxonomic agreement in birds across regions of the world for (a) all checklists and (b) excluding Howard & Moore (which has not been updated since 2014). From: Neate‐Clegg et al. (2021).

Ecological Reasons

Apart from the research bias, the analyses also pointed to several ecological traits. First, bigger species – especially with a body mass over 500 grams – were less likely to be in dispute. Obviously, big species are easier to observe and study. But there are also ecological reasons: larger body size is often associated with longer lifespans, smaller clutch sizes and larger home ranges. Together, these factors might lead to lower diversification rates, and consequently less diversity which is easier to classify.

Second, taxonomic agreement was higher in open-habitat species and migratory species. These traits tend to result in high levels of dispersal, resulting in gene flow between populations. The homogenizing exchange of genetic material prevents the formation of new species, resulting in clearly delineated species. This situation contrasts with altitudinal migrants that are partly depended on forest habitats. Here, the researchers found more taxonomic disagreements which they explained as follows:

Our results may, therefore, support the theory that intermediate dispersal ability leads to higher rates of diversification whereby sufficient dispersal capability is required to colonise new areas but not so much dispersal ability that gene flow prevents speciation. The greatest potential for cryptic lineage differentiation may, therefore, occur in lineages with intermediate forest dependence and intermediate mobility.

Ecological predictors of taxonomic agreement for the world’s birds for all checklists (in black) and excluding Howard & Moore (in grey, which has not been updated since 2014). From: Neate‐Clegg et al. (2021).

Species Concepts

The ecological traits discussed above could push some taxa into the “grey zone” of the speciation process where different species concepts support different taxonomic decisions. This conflict between species concepts is apparent when comparing certain check lists. Birdlife, Clements and Howard and Moore follow the Biological Species Concept (focusing in reproductive isolation) whereas the IOC adheres to the Evolutionary Species Concept (focusing on lineage differentiation, an approach that I also prefer). The reliance on and application of these different species concepts partly explains some taxonomic disagreements.

Hence, aligning the bird checklists will thus involve finding a consensus on the “best” species concept. It seems that we are right back from where we started. Those dreaded species concepts…


Neate‐Clegg, M. H., Blount, J. D., & Şekercioğlu, Ç. H. (2021). Ecological and biogeographical predictors of taxonomic discord across the world’s birds. Global Ecology and Biogeography30(6), 1258-1270.

Featured image: Common Redpoll (Acanthis flammea) © Jyrki Salmi | Wikimedia Commons

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