Birdwatching is an endlessly rewarding hobby, especially when you manage to tick off a new species on your Life List. But what happens when one species becomes two? That’s exactly what happened with Cory’s Shearwater, a bird many of us have enjoyed spotting over the open ocean. Recently, a significant taxonomic split occurred, dividing Cory’s Shearwater (Calonectris borealis) from a close relative or former subspecies, Scopoli’s Shearwater (Calonectris diomedea). If you’ve been tracking your bird sightings, this change could directly impact your records.
The Cory’s Shearwater, long recognized as Calonectris borealis, has been found to share striking similarities with the other species in the genus, the Scopoli’s Shearwater (Calonectris diomedea). Although these two species are similar in appearance and behavior, genetic studies have shown sufficient differences to warrant their separation into distinct species.
Calonectris borealis (Cory’s Shearwater): Found primarily in the North Atlantic, this species breeds on islands such as the Azores, Canary Islands, and Madeira. It is characterized by its large size, powerful flight, and pale underparts.
Calonectris diomedea (Scopoli’s Shearwater): In contrast, Scopoli’s Shearwater breeds in the Mediterranean, on islands like those of Greece and Italy. It is slightly smaller than Cory’s Shearwater, with subtle differences in wing markings and vocalizations. Scopoli’s tends to be smaller and more slender. But the key field mark is that it has more white in the underwing, with white “webbing” on the outermost primaries extending towards the wing tips (20% or more of the outermost primary tends to be white in Scopoli’s). This in contrast to the Cory’s Shearwater you see on the image of the blog, which was shot on the western coast of Ireland in September 2024.
For bird enthusiasts, taxonomic splits like this one carry real-world consequences. If you’ve already spotted what you identified as a Cory’s Shearwater, you may need to reconsider your records and your lists. Depending on where and when you made your observation, you might have seen either Calonectris borealis or Calonectris diomedea. This means a single sighting could now count as two separate species—if you’re lucky enough to have encountered both!
It’s essential to check your photos, field notes and species distribution ranges to update your Life List accurately. For those birding in the Mediterranean, your Cory’s Shearwater sightings might be now Scopoli’s Shearwater, while in the North Atlantic, you’re probagly dealing with the original Cory’s Shearwater. As – former – Cory’s Shearwater tends to migrate or disperse widely after breeding, neither species should be assumed based on location alone. While Cory’s Shearwater tends to be more common than Scopoli’s along the east coast of North America and the western coasts of Europe and Africa, but this is no guarantee.
This split didn’t happen overnight. Ornithologists have long speculated that Calonectris borealis and Calonectris diomedea were separate species based on differences in morphology, vocalizations, and breeding locations. However, the confirmation came through advanced genetic analyses. Studies revealed significant genetic divergence between the two populations, showing that despite their similar looks, these birds have been on separate evolutionary paths for quite some time.
Molecular data, such as mitochondrial DNA differences, played a crucial role in establishing these two groups as distinct species. This approach is common in ornithology, where physical differences alone aren’t always enough to split species, but genetic information provides a clearer picture of their evolutionary histories.
For the avid birder, taxonomic changes like these are a reminder that our understanding of the natural world is dynamic and constantly evolving. If you’ve been lucky enough to encounter Cory’s or Scopoli’s Shearwaters, now’s the time to update your Life List!
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