Seabird splits

In recent issues I have endeavoured to predict or speculate on the likely splits in Western Palearctic breeding and potential vagrant species. I have intentionally avoided the majority of seabirds, partly because of the subtlety and complexity of their taxonomy, and also because the history of seawatching has shown that almost anything can turn up at any time!

However, several resident or regularly seen seabirds have recently been shown in the literature to be composed of more than one species, though most of these have yet to be officially accepted by national or regional ornithological authorities or mainstream field guides.

The dark primaries and undertail of Boyd’s Shearwater are visible with good views. Note also the face pattern. Photo by Dominic Mitchell.

Part of the reason that seabirds are so hard to classify is the extreme similarity of the plumage of many of the forms, their remote and inaccessible breeding colonies – meaning that some forms have been under-sampled – and the difficulty of identifying many of them in field conditions. However, their philopatric nature – a strong loyalty to their birthplace – also means that as long as a seabird is seen at its breeding site, you can be fairly sure of its identity. Observation at these places has provided us with the most recent insights into seabird evolution and identification.

The volcanic islands formed by the opening of the mid-Atlantic Ridge are responsible for providing the isolated environments that produce most of the distinctive seabird forms. Unsurprisingly, it is there that we are finding the majority of ‘splittable’ taxa. Several cryptic radiations of tubenoses are currently coming to light in the North Atlantic islands.

Trouble with tubenoses

Tubenose taxonomy in particular suffers from a profusion of polytypic species, concealing a plethora of cryptic forms, and this is certainly the case with ‘Little’ Shearwater. Recent studies have gone some way to sorting out the taxa in the North Atlantic, however, and it is now apparent that two distinct species breed in the Western Palearctic. Neither are actually Little Shearwaters at all, and both are possible vagrants to British and Irish coasts.

Boyd’s Shearwater – formerly often viewed as a subspecies of the ‘old’ polytypic species, Audubon’s Shearwater – is the Cape Verde form of the current Macaronesian Shearwater, with a counterpart in Barolo’s Shearwater found on The Azores, Canaries and Madeira. What is now termed Audubon’s is itself a possible vagrant from the tropical Atlantic, having been seen along the Gulf Stream as far north-east as New England.

Of the three taxa, Barolo’s Shearwater is the only species confirmed from our shores, though it is still extremely rare. Boyd’s has notably dark flight feathers and undertail coverts, while the same feathers are largely white in Barolo’s, the species currently illustrated in most field guides as Macaronesian or Little. It should be said that identification of these extremely similar forms, and indeed most of the seabirds mentioned in this article, is best undertaken with good-quality photos and prolonged and clear observation.

Photographed off North Carolina, this ‘Madeiran’ Storm-petrel is very difficult to assign to one of the newly split species. Photo by Angus Wilson.

The shearwater formerly known as Cory’s has been split into four species, which have the Mediterranean, northern Macaronesia, Cape Verde Islands and Pacific Ocean as their home territories; all have evolved under long-term isolation. The northern Atlantic Islands form is now the nominal Cory’s Shearwater. The hard-to-distinguish Mediterranean form is colloquially known as Scopoli’s, separable in good field conditions by the white bases to the primaries on the underwing. Another yearned-for vagrant is the slightly more distinctive Cape Verde Shearwater. This is much smaller than the other two forms, with a duller, more slender bill, dark cap, lighter throat and more pointed rear end. 

After Scopoli’s Shearwater, another Mediterranean tubenose perhaps worthy of a split is Mediterranean Storm-petrel. Extremely similar physically to the familiar British ‘Stormie’, melitensis has a distinct DNA lineage due to the ancient closing of the Mediterranean basin, and probably originally colonised via the Strait of Gibraltar. While the form seems to stay mostly in the Mediterranean, there are at least two records of Maltese-ringed birds turning up further north, in France and The Netherlands. Unfortunately, DNA and vocal differences uttered at the breeding colonies are unlikely to aid identification should one appear off our coast!

Problem petrels

Just when birders were starting to feel that they had a chance of identifying Fea’s Petrel in the field from the less likely vagrant Zino’s Petrel (both split from Soft-plumaged Petrel some years ago), this was revealed as inadvertent over-confidence when Desertas Petrel was described. This form is what used to be called Fea’s in Madeira, but is probably in fact a different species. Fea’s Petrel is now the name for the Pterodroma petrel from the Cape Verde Islands, this group being the source of the type specimen and having distinct biometrics from those found in the Desertas island group. Desertas birds have a different breeding season and some DNA sequence divergence, and may qualify as vulnerable with the IUCN if fully described as a separate species.

The Desertas form has an even deeper bill than nominate Fea’s, but field identification of these sister-species triplets is still being refined. Unfortunately, most claimed Fea’s-type Petrels are unlikely to have been seen in the detail needed to establish specific identity, even under good field conditions. Interestingly, subfossil remains indicate that gadfly petrels probably assignable to Desertas Petrel on structure and measurements were present at breeding colonies in Scotland, Denmark, Sweden, The Netherlands and Spain less than 2,000 years ago.

This Black-bellied Storm-petrel was photographed off Australia, but is visually almost identical to the Atlantic form. Photo by Ben Lascelles.

Madeiran Storm-petrel has a troubled past on the British list. Historical records have recently been deleted from the national record, but at least three reports of birds seen from land and at sea are currently under consideration. However, recent research indicates that this is another species group that will not be identifiable in the field to split-species level on current knowledge. With its broad white rump, sooty plumage and pale greater-covert bar, it is also known as Band-rumped Storm-petrel in North America, but this name now covers a whole superspecies.

Demonstrating that bird species can be isolated temporally as well as geographically and behaviourally, a recently designated Azores form of Madeiran, which breeds in the summer, has been named as Monteiro’s Storm-petrel, while Grant’s Storm-petrel, also breeding there but in the autumn and winter, has also been named in Robb and Mullarney (2008). The two ecologically separate forms can be differentiated with much difficulty using vocals, moult and wear, but Monteiro’s does have a noticeably deeper tail fork. Grant’s is also found to the north of several of the other Macaronesian islands, sympatrically with the ‘original’ Madeiran Storm-petrel. As if this weren’t disconcerting enough, a further possible vagrant band-rumped form is present in the Cape Verde Islands. All are so physically similar that it is a brave observer who would call them as anything more than ‘Madeiran’ on even good field views.

Phenomenal Fregatta

The sighting of a Fregatta storm-petrel at Severn Beach, Gloucestershire, last November (see Birdwatch 211: 52-53) certainly highlighted the anything-goes possibilities of habitual seawatching. However, the sighting also underlined the difficulties of identifying any of the forms of Black-bellied and White-bellied Storm-petrels under field conditions, as well as the unfinished state of much of Procellariiform systematics.

The nearest breeding colonies of White-bellied Storm-petrel are on the British protectorate of Tristan da Cunha in the South Atlantic, but there is still confusion over whether both species occur there, or indeed whether a white-bellied population of Black-bellied is the true identity of the birds on that wind-swept outpost. Any storm-petrel with white on its underparts is a mega by default (if not an aberrantly plumaged bird!), but can any be truly nailed down to species level in the field?

These two larids are true Atlantic Gulls in their most distinctive plumages – adult winter (left) and first-winter (right). Photo by Dominic Mitchell.

Another potential good species from the Azores, and much publicised after claims of individuals in Britain and Ireland, is Atlantic Gull, the highly distinctive local form of Yellow-legged Gull. This insular taxon seems close to the ancestral ‘Yellow-leg’ and has diagnosable plumages at all ages, but particularly in its first-year and adult winter stages.

This larid exists in its strictest sense only on the Azores. However, forms intermediate in plumage detail appear to make up a stepped or clumped cline across the Canaries, Madeira, Morocco, Spain and Portugal, implying that a ‘clean’ split may be tricky. Regular or intermittent genetic introgression through hybridisation may also hinder true divergence in this form, and the DNA of the Portuguese lusitanicus appears to be intermediate between michahellis and atlantis.

Obvious features include dense head streaking in typical adult winter plumage, forming a discrete diffuse dark grey hood. Juveniles are closer to Lesser Black-backed Gull or even American Herring Gull in appearance, but all ages and plumages really need good images and notes describing a wide combination of features to clinch identification and exclude aberrant plumages of other large white-headed gull species. There have probably been at least 30 reports in Britain and Ireland so far, but none yet accepted.

White-breasted Cormorant is the sub-Saharan form of Cormorant, and is found in the Western Palearctic on the Cape Verde Islands and in Mauritania. This African form is already split by some authorities, and the Moroccan subspecies of Cormorant, isolated from the European population, may well belong to the African form, which it resembles. The Mediterranean form of Shag is also isolated, and typically has more yellow on the bill in adults and obvious extensively pale underparts on immatures compared to north-west European birds. The Moroccan subspecies is intermediate between the two.

It is apparent that the taxonomy of seabirds is one of the trickier aspects of bird classification, and even the most ruddy-cheeked and watery-eyed seawatcher has to accept the rueful fact that not all of their sightings can be attributed to species. However, field marks are constantly being refined and most watchers of the waves will know that nothing will replace good photographs and copious notes when it comes to the future defining of vagrant visitors to our own shores.

References

For a full list of references, please click here.