But how convincing is the supporting evidence?
A quick question: How many subspecies of Manx shearwater (Puffinus puffinus) are there? If you check the Birds of the World website, you will see the label “monotypic” which means no subspecies have been described. However, this classification might change in the near future. A recent paper in the Journal of Avian Biology presents evidence for a subspecies on the Canary Islands.
Our results show that the Canary Islands population is phenotypically distinguishable from other populations in breeding phenology, biometrics, plumage colouration and acoustics traits. In addition, we found an incipient genetic differentiation, representing a new case of cryptic differentiation of peripheral populations in Procellariiformes.
The core breeding populations of Manx shearwater are located in the United Kingdom and Ireland, with several peripheral populations on islands, such as Iceland, Newfoundland and the Azores. Detailed studies on these island populations might lead to the description of more subspecies. But let’s focus on the new subspecies (Puffinus puffinus canariensis) from the Canary Islands and check the supporting evidence.
The researchers compared birds from three locations: Mallaig (Scotland), Corvo (Azores) and La Palma/Tenerife (Canary Islands). First, they looked at fledging date: the Canarian birds fledged on average 31 days earlier than Azorean birds and 52 days earlier than the Mallaig birds. This difference in fledgling date is a common phenomenon in seabirds and is often regarded as a mechanism for (sympatric) speciation. Reproductive isolation can build up when different populations breed at different times (i.e. allochrony). Similar patterns have been reported in other seabird species, such as the storm-petrels (genus Hydrobates, see this blog post).
Another feature that hints at some level of reproductive isolation between different populations of Manx shearwater concerns the sounds that they produce. Acoustic analyses revealed that nine out of twelve acoustic variables were significantly different between the northern population and the Canary Islands. Whether these calls are different enough for the shearwaters to consider birds from other populations as members of a different species remains to be investigated.
Morphology and Genetics
From a morphological point of view, the researchers also documented significant differences. In general, the Canarian birds were lighter and has shorter wings than birds from other populations. In addition, shearwaters from the Canary Islands has darker underwing plumage. Interestingly, these patterns are in line with the Bergmann’s Rule and Gloger’s rule. Bergmann’s Rule states that populations of larger size are found in colder environments, while populations of smaller size occur in warmer regions (see for example this blog post on the line-cheeked spinetail). And Gloger’s Rule describes that more heavily pigmented populations tend to be found in more humid environments, such as near the equator (check out this blog post for more on Gloger’s Rule in gulls).
Finally, genetic analyses of the mitochondrial control region indicated some genetic differentiation between the Canary Islands and the other populations. Eight out of twelve Canarian birds grouped together in the phylogenetic tree, while the remaining four Canarian individuals were scattered across different branches. There is thus some level of genetic differentiation, but the four outliers need to be explained. They might be the result of introgression when birds from different islands interbred, or the outcome of incomplete lineage sorting at the early stages of speciation. Genetic studies with more markers are needed to sort this out.
The Subspecies Concept
Based on these thorough analyses, the researchers thus recommended to consider the Canary Island population as a distinct subspecies. The use of subspecies is quite common in ornithology, although it has led to some heated debate in the scientific literature. The application of subspecies names was introduced by Carl Bruch in 1828 to indicate geographically different forms within a species. In the early twentieth century, the system became commonplace due to the promotion of Elliot Coues in America, and Ernst Hartert and Henry Seebohm in Britain. However, in 1953, two entomologists – Edward Osborn Wilson and William Louis Brown Jr. – heavily criticized the subspecies concept, writing that “the subspecies concept is the most critical and disorderly area of modern systematic theory”. The debate continued and is still raging today (see for example, this recent edition of Ornithological Monographs, dedicated to the subspecies concept).
I won’t go into this tricky discussion, but I will highlight one important argument in favor of describing subspecies. Giving certain populations a formal name draws the attention of government officials and can lead to faster action in conservation efforts. Indeed, the authors indicate that “the taxonomic identification of the Canarian Manx shearwater as a new subspecies should lead to prioritisation of its conservation through an action plan.” And by implementing certain measures, such as excluding introduced predators or managing powerlines, we will not only protect the Manx shearwater, but also countless other species on the Canary Islands.
Rodríguez, et al. (2020) Cryptic differentiation in the Manx shearwater hinders the identification of a new endemic subspecies. Journal of Avian Biology, 51(11).
Featured image: Manx shearwater (Puffinus puffinus) © Martin Reith | Wikimedia Commons