Mediterranean Shearwater lump: no more Yelkouan and Balearic?

As a kid studying my battered copy of Collins Bird Guide, shearwaters and petrels always confused me. The prospect of a pelagic or even a scope to use for seawatching seemed impossible to me, so my only choice was to read about them and compare plates instead of actually going out and seeking them. A ‘smudge on the armpit’ or ‘flight distinctly more stiff’ never did much to convince me. As I delved deeper into identification of some species, differences seemed trivial and subjective. One such challenge which I couldn’t wrap my head around was Balearic Shearwater (Puffinus mauretanicus) and Yelkouan Shearwater (P. yelkouan). However, recent literature has showed that my personal incredulity isn’t unfounded, a 2021 study by Obiol and colleagues shows that the two species have less genetic divergence than there is typically between two members of the same subspecies. The data therefore suggests that even subspecies status is dubious, let alone species status!

The IOC has placed a proposed lump between Balearic and Yelkouan Shearwaters in the 13.2 draft. The paper also suggested a lump between Ardenna creatopus and A. carneipes (Pink-footed and Flesh-footed Shearwater), although this has not been acknowledged by the IOC yet.

Balearic and Yelkouan Shearwater lump

Balearic Shearwaters are a critically endangered taxa with only 2000 breeding pairs thought to remain. The species was only recently split from the Yelkouan Shearwater, primarily based off of morphological features. Balearic Shearwaters are conspicuously darker: some are more akin to a small Sooty Shearwater than to a Manx or Yelkouan. Without genetic analysis, the differences seem reasonable. However, a paper by Genovart and colleagues in 2012 questioned this distinction.

Balearic Shearwaters (P. mauretanicus) from four major colonies were studied: genetic material was taken while the birds were also graded on a scale from 1-5 based on how dark their plumage was. Yelkouan Shearwaters (P. yelkouan) from a colony on Sardinia were given similar treatment.

Genetic analysis showed that models could separate P. mauretanicus from P. yelkouan. However, some supposed Balearic Shearwaters on the island of Menorca were found to be more closely related to Yelkouan Shearwaters than to Balearics on other islands. This suggested hybridisation or introgression. Furthermore, it was reported that 95% of variance was explained by differences within populations. In short, while the average bird from Balearic colonies had more in common with another from the same colony, variance within one colony was comparable to variance between colonies.

Plumage data from the study was particularly fascinating:

Table 1:

ISLAND	Dark colouration pattern (% of colony population)	Intermediate colouration pattern (% of colony population)	Light colouration pattern (% of colony population)
Pityusic Islands (Ibiza)	74%	22%	4%
Mallorca	72%	27%	1%
Cabrera	21%	65%	14%
Menorca	3%	33%	64%

This data suggested that the dark colouration typical of Balearic Shearwater was more of a cline: a gradient from light in eastern colonies to dark in western colonies as opposed to two distinct taxa. Both genetic and phenotypic data showed that Shearwaters from Menorca were intermediate between the two ‘species’.

In 2022, two papers by Obiol and colleagues further provided evidence for a Mediterranean shearwater lump. The initial paper (link) was broader in scale and compared DNA from many species of shearwater within three genera. The paper showed practically no divergence between the two Mediterranean taxa, as previously mentioned, less divergence than what is typically expected between two subspecies.

The second paper (link) had more of an emphasis on North Atlantic Puffinus Shearwaters, also treating the Boyd’s and Barolo Shearwater (P. boydi and P. baroli) complex. The mixing of Yelkouan and Balearic was evident, supporting the evidence from Genovart and colleagues’ earlier paper. A high amount of gene flow between Balearic Shearwater colonies and Yelkouan colonies was found. Many Balearic Shearwaters (particularly those from eastern Balearic colonies) were found to be closer to Yelkouan shearwaters than to western Balearic Shearwaters. I invite you to study some of the graphs on the paper, Fig. 2. in particular shows just how little the two populations have diverged from one another.

It was suggested that rather than treating the Menorca colony as ‘admixture’, instead, the two populations are at an early stage of speciation, hypothesised to be due to different migratory strategies (Balearic Shearwaters tend to disperse into the Atlantic, whereas Yelkouan Shearwater tend to stay in the Mediterranean sea).

Conclusions & a quick word on identification

This proposed lump has not yet been accepted, but it seems all but certain given the evidence. With my previous article, I dived into identification of the two treated taxa (Tibetan and Siberian Sand Plovers). However, given the variability seen in the Balearic islands and the lack of any genetic divergence, obvious identification features seem dubious.

Of course, an obviously dark Mediterranean Shearwater is from a Balearic colony, and any Mediterranean Shearwater found in the Atlantic is also probably a Balearic Shearwater given dispersal data. However, the variability in plumage (as seen in table 1) shows just how inconsistent plumage colour can be. In Menorca, just 4% of birds in what is supposed to be a ‘Balearic-type’ colony had dark plumage. Western Mediterranean Shearwaters are essentially a colour-morph probability cline as opposed to populations with any considerable genetic divergence, as shown by the considerably high gene flow.

Identification of the resulting subspecies / races is therefore iffy. Would you consider Menorcan birds to be Balearic, Yelkouan, both or neither? Menorcan birds are more similar in appearance to Yelkouan Shearwater but shared ancestry data showed more in common genetically with Balearic Shearwaters. Assuming that they are considered part of the Balearic race, how would you then go about identification? If you were to lump them with the Yelkouan race, then how would you treat the Cabrera population which is intermediate between the Menorca and Western Balearic Island populations?

What I’m trying to say is that lumping these two taxa into one species solves a lot of ID problems. Nevertheless, I will give a brief overview of separating the two races. Of course, given what has been said, take all ID with a pinch of salt.

Balearic Shearwater (Puffinus yelkouan mauretanicus)

Before the level of gene flow was known, differentiation was considered straight forward. Balearic Shearwater have extensive brown, often with a breast band. Axillaries are typically dark and while the belly is ‘white’ it still has brown mottling. There is never a clean demarcation of brown to white, the white plumage is mottled and ‘messy’.

This image (link to Macaulay library) shows an obvious Balearic Shearwater. Note the overall brown appearance, without a clear demarcation between a white underside and brown back as in Yelkouan.

Yelkouan Shearwater (Puffinus yelkouan yelkouan)

An example Yelkouan: (link).

The pale white underside is striking and distinctive when compared to a typical dark morph Balearic Shearwater. A far greater ID challenge would be differentiation with Manx Shearwater (well, apart from differentiation with a Menorcan Balearic Shearwater which seems all but impossible for now). Features present in Yelkouan Shearwater to discount Manx Shearwater are:

• conspicuous toe projection <=> rare in Manx.
• dark undertail coverts <=> white in Manx.
• pale underside to primary feathers, showing little contrast to underwing <=> Darker primaries in Manx

Cover photo by Julien Renoult (CC BY-SA 4.0).