One of the greatest challenges facing the publishers in producing their Checklist, was the question of how to deal with the tempest of taxonomic changes that has rained on the parade of bird book publishers, ornithologists and general birders since Sibley & Monroe’s 1990 DNA-DNA hybridization study changed the landscape of avian classification forever. With papers on new phylogenetic studies appearing every month, it has become increasingly hard to keep track of all the splits and lumps (taxonomic, not medical terms). But to start at the start, one of the main problems that the authors of The Checklist had to deal with, was the following very simple question:
What is a species?
What is a species? This deceivingly simple sequence of four words forms a question so basic that even my three year old son may ask it. However, providing him with an answer is virtually impossible. After centuries of research, spanning the academic careers of thousands of grey-bearded biological theoreticians, we still simply don’t have an easy answer.
ABOVE: The attractive cover of the first volume of the HBW / BirdLife International Checklist depicts the relatively stable higher-level classification of bird orders and families. (Image: Lynx Edicions).
What characters do the Tobias critera consider?
When comparing two similar taxa, a score of 7 that would indicate different species can be achieved by adding smaller scores based on differences in appearance and distribution. Appearance criteria include colours, patterns and structure of plumage and bare parts; morphometrics (i.e. measurements); vocalisations, which are quantified by measurements from spectrographs/sonograms; and to a lesser extent, ecology and behaviour. To prevent simply adding more and more scores of 1 to arrive at the 7-threshold, only three appearance differences may be scored. Likewise, only two morphometric or vocal characters may be used, and only one difference in ecology or behaviour (which always receives a score of 1 anyways). Each is scored statistically, to fall in one of four levels:
- Minor difference e.g. a subtly different colour wash (1 point)
- Medium difference e.g. a distinct difference in colour tone (2 points)
- Major difference e.g. a different pattern or colour (3 points)
- Exceptional difference e.g. a striking difference applying to a large area of the body, or any trait directly involved in courtship and mate choice.
Distributional criteria are divided into 5 classes. Here it gets a little tricky. When comparing the degree of overlap in the distributions of two taxa, the following five categories are defined.
- Allopatry: if the two species have non-overlapping ranges, a score of 0 is automatically ascribed, as allopatry is not informative from an evolutionary speciation point of view.
- Sympatry: if the ranges of two taxa overlap and they still behave and look like two different species, then they are different species. So they automatically receive a score of 7.
- Broad hybrid zone: there is little resistance to phenotypic merging if the hybrid zone is 200 km or more in width; score 1.
- Narrow hybrid zone: there relatively high resistance to merging; score 2.
- Paraptry: the ranges of the two species adjoin, but there is little or no hybridization along the very narrow contact/overlap zone. The theory here is that the two species exclude each other in the absence of a geographical barrier; score 3.
An important distinction here is that, apparently counter-intuitively, hybridization is treated as an argument against treating two taxa as the same species – quite the opposite of the BSC. The theory is that if two species hybridize, even frequently, but still retain their genomic and phenotypic integrity, without their genomes merging completely, they really are two different species. As I said, a little tricky to wrap your head around, but very logical nonetheless.
What characters do the Tobias criteria NOT consider?
Genetics, for a start. Although The Checklist does incorporate a wealth of information on differences and similarities in genetics, such molecular differences are not scored, and cannot help bring the taxon in question up to a score of 7. In addition the team of taxonomists responsible for the species-level decisions in the book had to find ways of incorporating evidence from a number of obscure characters: number of leg scales in rheas, feather stiffness and louse edemism in kiwis, bone morphology in petrels and egg colour in nightjars being some examples. Some of the cryptic tubenose petrels provided a special challenge, in that they seem to lack diagnosable differences in appearance altogether – simply put, they all look the same! How do these petrels mate with the right species then, especially when they return to their nesting islands at night, or are scampering around in their burrows? Probably by a combination of subtle differences in call and by using scent (think of how well chumming works!).
Vocal evidence needs to be interpreted with great care. Firstly, you want to make sure you’re comparing the same call between species, given that many birds have very extensive repertoires. Secondly, sample size becomes very important in birds that learn sounds from their environment, such as parrots. On the note of sample size, the Tobias criteria set n = 10 as a minimum sample size when adjudicating any criterion (whether physical or vocal). Obviously this is not always practical, especially in cases where the species is known only from a single specimen. There are twelve non-passerine taxa in this category, including the famous Nechisar Nightjar Caprimulgus solala.
Spill! What were the results?
Birders tend to be splitters, and not lumpers, and will consequently be glad to hear that splits outnumber lumps 15 times. Only 30 species were lumped (into 22), while 462 new splits are recognized (the book includes 4,471 non-passerines in total). A few families in particular were shaken, not stirred, with over 20 new members each, among the pigeons, woodpeckers, hummingbirds, kingfishers, owls and parrots. The biggest proportional increases were in the albatrosses, motmots, kingfishers, toucans and barbets. Africa’s avifauna is now 10% richer in species, while Asia’s bird community has increased by 15%. For some more juicy specifics you’ll have to wait for the next blog post, which will be a combination of book review and lump/split spoilers. Thanks for reading this post – I hope you feel as enthralled by the magical diversity of our planet’s birds as I do!