The Royal Tern is split into two species by the ‘IOC World Bird List’ in January 2020.
The split is based on research published in 2017 and reported in this blog (see below) at the time. The original title of the blog when first published was “African Royal Tern should be split from the American species”.
The original text:
The West African Royal Tern should be split as a separate species because it’s not closely related to the American Royal Tern according to new research.
The Royal Tern, as currently recognised, comprises of two geographically separated ‘subspecies’: the nominate Thalasseus maximus maximus in the Americas and Thalasseus maximus albididorsalis in West Africa.
The birds of the two subspecies are morphologically near-identical except for some differences in bill morphology (redder and deeper in American birds) and biometric variations in mass and wing (the American Royal Tern is on average slightly bigger).
DNA tells a different story
To elucidate the true phylogenetic relationship between the American and the West African Royal Terns, Martin Collinson and his coauthors analysed the DNA from birds of both populations as well as from the related ‘orange-billed terns’ (genus Thalasseus). The new research has just been published in the Biological Journal of the Linnean Society.
In spite of the morphological similarity, the DNA analysis revealed that the West African Royal Tern‘s closest relative is not the American Royal Tern, but the Lesser Crested Tern (Thalasseus bengalensis) which is similar but smaller and with a yellower bill. This relationship is well supported by both the mitochondrial and nuclear DNA.
As currently defined, the Royal Tern is a paraphyletic species, and retaining maximus and albididorsalis in the same species with the exclusion of bengalensis does not reflects the true evolutionary relationships between all these taxa. The study thus suggested that the Royal Tern should be split into two species by elevating the West African Royal Tern to the species rank: Thalasseus albididorsalis.
Collinson, J. M., Dufour, P., Hamza, A. A., Lawrie, Y., Elliott, M., Barlow, C. & Crochet, P.-A. 2017. When morphology is not reflected by molecular phylogeny: the case of three ‘orange-billed terns’ Thalasseus maximus, Thalasseus bergii and Thalasseus bengalensis (Charadriiformes: Laridae). Biological Journal of the Linnean Society 121: 439-445. doi: 10.1093/biolinnean/blw049
Update (Jan. 2020):
Based on this study, the ‘IOC World Bird List’ (Version 10.1) officially split the Royal Tern into two species. The English name chosen for Thalasseus albididorsalis is ‘West African Crested Tern’. The American Thalasseus maximus remains Royal Tern sensu stricto.
It makes sense to drop the word ‘royal’ from the name of the African species. But, I am still wondering why not calling it just ‘African Crested Tern’ because the only orange-billed terns breeding in Africa already have well established and descriptive names: Greater Crested Tern and Lesser Crested Tern. Moreover, the latter two species are widely distributed outside Africa (so the ‘African’ qualifier won’t really apply to them).