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By Chris Lotz
In this article I compare three of the main world bird lists. I then also discuss why these lists give different species counts. These lists are dynamic, but the information given is valid on the date I wrote this blog, 31 March 2020.
*July 2021 update: according to Denis Lepage of Avibase, the goal of an ambitious new project is to eventually have one single integrated list. Clements and the IOC seem committed to a full merge, for example.*
The Clements/eBird world bird list produced by the Cornell Lab of Ornithology, recognizes 10721 BIRD SPECIES on the planet. It is the most popular list in North America. It’s become even more widely adopted as eBird (which uses only the Clements list) has become so spectacularly popular among birders. Indeed, since eBird is becoming increasingly popular worldwide so more birders in other parts of the world, e.g. Asia, are now switching to Clements. The eBird app makes it very easy to keep track of one’s life list (and also one’s year list, county list, state list, country list, etc.), and also now “offers” the Top 100 challenge (just the kind of thing so many birders love). So Clements may well become the most widely used world bird list in the not too distant future; that’s my prediction anyway.
The IOC world bird list recognizes 10770 BIRD SPECIES and has, for years, been widely accepted as the most authoritative bird list by a great many birders, field guide authors, conservationists, and biologists outside of the Americas. Currently the IOC list is still followed by a vast number of people worldwide (a smaller proportion in North America, though). Indeed, we at Birding Ecotours still use the IOC list as our default list, but because of what I mentioned above we’re starting to put the Clements/eBird names in parenthesis on our website, tour booklets and trip reports (especially for our American bird tours).
The Birdlife International (BLI)/Handbook of the Birds of the World (HBW) world bird list is an interesting new development and recognizes 11147 BIRD SPECIES. It’s not as widely followed by birders as the above two lists, but, partly since HBW has now generated quite a number of good field guides as well as an impressive illustrated checklist of all the world’s birds, their taxonomy/list is gaining traction.
There are many other lists out there. Howard and Moore is worth mentioning here as it was the fourth list analyzed on a well-known ‘BIRDCHAT” listserv article; it recognizes 10175 species. Then bird book authors for various parts of the world sometimes create their own taxonomies/bird lists (but most bird book authors choose one of the existing major world lists such as one of the major ones mentioned above).
Which software/app to use to manage your life list
Avibase is a brilliant resource for downloading bird lists for all countries in the world, and often also for different states or provinces within a country. It allows you to choose from ten different taxonomies, not just the main ones mentioned above.
As far as listing software goes, we as a company have enjoyed using the free web-based Scythebill, which also allows one to choose between Clements or IOC. But eBird is now gaining a huge amount of momentum not just as a research/conservation/citizen science tool but also as a listing portal for keeping a birder’s life list (one is forced to use Clements, though, which means that those who prefer IOC resist the change to eBird).
Why don’t the major world bird lists recognize the same number of species?
Why, for example, does the Handbook of the Birds of the World (HBW) list recognize 426 more species than the Clements list? All three major lists mentioned above basically use the “Biological Species Concept”. This means that they all define a species the “classic” way: simply put, two birds are considered to belong to different species (rather than just subspecies) if they are unable to breed together and produce viable offspring. If two birds look different but can hybridize freely, they’re considered subspecies, not species.
It’s completely impractical, of course, to test the Biological Species Concept by doing a giant worldwide experiment and trying to breed all the subspecies/species with each other to see if they’re species or not. That would mean capturing all the forms that one suspects are species, putting them together and seeing if they breed and hatch viable young together. So instead biologists rely on looking at differences in plumage, vocalizations, genetics, etc., that will indicate whether they think these species will be able to breed together and produce viable young.
The different world lists emphasize different things to look at when deciding which must be able to breed together, and thus be species, and which aren’t. For example, the Handbook of the Birds of the World list emphasizes plumage, vocalizations, and other non-genetic variables. The IOC list emphasizes genetics a lot more and is also influenced to some extent by the “Phylogenetic Species Concept”, a completely different way of defining a species than the more classic Biological Species Concept. Indeed, the Biological Species Concept only works for certain groups of living things (it works well for birds, but not very well for plants, and of course it doesn’t work at all for groups of organisms that reproduce asexually).
The HBW list uses its own method (the Tobias criteria) for deciding which are species and which are just subspecies. The IOC, on the other hand, bases its decisions on a plethora of peer-reviewed scientific papers (themselves using a wide array of different methods) that justify when subspecies should be “split” into separate species (or the opposite, when two or more species should actually be “lumped” into one).
A lot of these scientific papers that the IOC bases its decisions on are based on genetic work. Just because genetic work has been done, though, doesn’t mean it’s necessarily superior (I’ve noticed that a lot of birders assume this). One of the reasons for this is that many of these scientific studies only sequence one gene (usually cytochrome b) out of tens of thousands of genes each bird has. And even if the whole genome were in fact sequenced (instead of just one gene) the interpretation of what that actually means is extremely complex, not to mention controversial. Gene sequencing is certainly not the only way of studying “genetics”; there are a large number of different methods.
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