Putative Eastern Lesser Whitethroat in Berlenga island

The Lesser whitethroat complex is quite confusing. Part of this is due to a wide distribution area from west Europe to eastern Siberia, eastern Mongolia, and parts of China, creating a subtle diversity in the different populations: coloration, biometrics, and vocalizations. Also, the lack of studies and samples lead to poor knowledge of population boundaries and contact zones. Any Lesser whitethroat in Portugal is rare (with only one accepted record by the Portuguese rarities committee), and a couple more still pending.

For the west Europe context, and Portugal in particular, we should be aware of three different taxa (as proposed by Olsson, U., 2013), all rare in Portugal:

European Lesser Whitethroat (Curruca curruca curruca)

• Breeding range: eastern Europe to western Russia (approximately till the Ural Mountains), the Caucasus, Turkey, and western Iran.

Siberian Lesser Whitethroat (Curruca (curruca) blythi)

• Breeding range: eastern Siberia to the north of Altai and the north of Mongolia

Desert Lesser Whitethroat (Curruca (curruca) halimodendri)

• Breeding range: central Asian steppes, from the lower Ural River and Transcapia east to western Mongolia

Taxonomic considerations on the three taxa 

The major issue, which is still not totally set, remains with the status of blythi, as authors go back and forth on considering it a subspecies: Sylvia curruca blythi, or being a synonym with curruca. (Vaurie (1959); Mayr and Cottrell (1986); Cramp (1992); Shirihai et al. (2001); Loskot (2005); del Hoyo et al. (2006); Olsson (2013)). 

The Desert Lesser Whitethroat (Sylvia curruca halimodendri), generates more consensus on being considered a subspecies. The birds occurring SE of the Caspian sea,  roughly Turkmenistan, Afghanistan, and Tajikistan used to be considered (before Olsson 2013) Sylvia curruca minula, which extended the distribution also into China. The Olsson et al. research revealed that minula only occurs in China, and all the other populations, previously classified has minula, were in fact halimodendri.

By the book morphological differences beetween the three taxa

I have to state that this differences are guidelines and reflect typical birds, there is not enough knowledge to classify with confidence the origin of some birds, due to the high individual and clinal variation, as well as the lack of knowledge on/of contact zones. Adapted from Garner (2014):

European Lesser Whitethroat (Curruca curruca curruca)

• A little more darkish grey in the head, particularly in the nape
• Greyish-brown mantle (darker than in other subspecies)
• Slightly longer primary projection, around 60% (50%-70%) of tertial length
• Less blunt wing with 2nd primary=5/6
• Outer tail feathers (R6) in adults with more white than 1cy, but typically with brownish mark along the shaft and brownish-grey smudge on the inner web. 1cy has more brownish wash along the inner web of outer tail-feathers.
• The call is usually a spaced dry "tack"

Siberian Lesser Whitethroat (Curruca (curruca) blythi)​

• Clinal variation, with the far northern birds closely resembling nominate curruca, southern birds closer to halimodendri, with paler brownish-grey mantle and brownish infusions in the nape.
• Primary projection around 50%-60% of tertials.
• 2nd primary = 6/7 or 7/8
• Some birds, in 1cy,  seem to moult some tail feathers into adult-type
• Outer tail feathers more white than nominate curruca in adults and in some 1cy. Some 1cy can have a similar pattern of outer tail feathers to nominate curruca (variable!)
• Tips of R5 and R4 with average more white than nominate curruca
• The call can be similar to the curruca "tack" and has also a more rapid rattle

Call resembling curruca from a DNA confirmed blythi

Rattle call, from a DNA confirmed blythi

Desert Lesser Whitethroat (Curruca (curruca) halimodendri)

• Paler, more sand-brown on upperparts than blythi or curruca
• Short primary projection 40-50% of tertials
• Short wing
• 2nd primary =7/8
• Tail pattern of 1cy with all pure white outer tail feathers (R6). Extensive white tips into R5 and R4, and sometimes well into R1.
• Not known to give the "tack" call, but an almost buzzing like "dze ze ze ze"

The "buzzing" call of halimodendri

Is it possible to separate the three taxa in the field?

In my opinion? No, not with confidence! Confronted with a classic looking 1cy curruca, for example in Portugal, there is little reason to suspect that it's not the nominate, but one can never be sure. It can be blythi from the contact zone, and consequently closer in appearance to curruca. More eastern blythi or halimodendri should raise flags right away, with the overall paleness, structure and tail pattern. Since in Portugal we have so few records of any curruca, it's highly advisable to collect the most information possible on every bird. Fecal or feather samples to obtain DNA are crucial to id at the subspecies level. Also, sound recordings are very important, given some differences, in particular with halimodendri. Quality photos, including the open tail, are very important not just for the pattern of the tail, but also to age the bird properly.

In the ringing table, it should be easy to obtain DNA samples and detailed photos, but also accurate measurements, including wing formula and primary projection.

Now the Berlenga bird

On the 10th of October 2020, a group of brave photographers and myself, went on a pelagic trip off Peniche, with a stop at Berlengas island to look for migrants. The sea was a little rough, but the birding was going well, with a good number of Great, Sooty and Cory's shearwaters, around Berlenga island, including an interesting and brief Leach-storm petrel.

Around noon we stopped at Berlenga. A favourite and famous spot to look first is "The Fig trees", a couple of sheltered and dwarf fig trees that survive in the harbour bay (the few trees in the island). This spot usually attracts migrants to feed on the figs and insects. To avoid a bigger gathering at one spot, I decided to leave this place for last and check other areas, including the plateau around the lighthouse. With my phone dead of cell coverage, I was completely unaware of the buzz in the harbour. When I came back to check the harbour, on my descent, there were waving arms at me telling me to rush:
- "There is a Lesser whitethroat in the fig tree... we tried to call you!", Teresa said.

It took me about 10 seconds to locate the bird, it was very active feeding up and down the branches.
It was quite striking right away how pale this bird was overall. My previous experience with the species was in Poland some years ago, but it's a species I try to look out every year in Portugal, so I was fairly aware of the possibility of an eastern taxon.

The tail pattern was obviously pale, and on the pale limit, in my mind, for nominate curruca. I was concerned to age the bird properly because if it was a 1cy with that amount of white, it had to be something from the east.

Photo: Pedro Marques

The mantle was quite pale, brownish-grey and changed with light. But from different angles, it consistently gave the same pale impression. Subtle infusions of brown into the nape and the suggestion of a dark mask.

​Plain brown iris, wear on flight feathers, in particular tertials are typical of 1cy.

Photo: Pedro Marques

Note the overall paleness of this bird. Sandy infusions on undertail coverts, upper ties and chest.

Photo: Pedro Marques

​A different angle makes the upperparts a little darker, but still fairly pale, in particular the upper mantle.

Photo: Tiago Guerreiro

Note the plain brown iris of 1cy and the suggestion of a dark mask, only noticeable in some angles.

Apparently a short primary projection, which I've tried to estimate using different photographs. It falls somewhere between 45-50% of tertials length, which in theory is slightly shorter than that of nominate curruca.

Photo: Milas Santos

The tail feathers look pretty much juvenile judging by the tip shape (slightly lanceolated), except for the outer right tail feather R6 (note the round broad tip).

On both adult and 1cy type the white in the outer feathers is quite extensive,  in particular it forms a tip down and along the shaft. It's possible to see some darkish infusion along with the inner web of right R6, putting this bird a little off for typical halimodendri.

Photo: Pedro Maia

There are white tips extending to R4-R3.

Photo António Martins

Photo: José Frade

Depending on light and angle, the bird can look different. Prolongated views are essential to complement photograhs.

And what do I think about this bird?

Like I stated before, it's not possible to be sure about the subspecies we are dealing with without DNA sampling, sound recordings, wing formula, or, ideally, all of them. So, no definite identification of the subspecies will ever come out of this analysis, but I consider it a very interesting learning essay.

​I have troubles fitting this bird comfortably on the typical nominate curruca, based on the overall paleness, tail pattern, and primary projection. I would also exclude halimodendri, based on what is known for typical characters of this subspecies (shorter wing, brownish infusions more extensive into the crown, and cleaner - pure white - outer tail feathers).

For me, it fits better on a blythi type bird: the overall paleness, the white on the tail and the "in between curruca and halimodendri" primary projection. Is it a true blythi? Is it a curruca from a  contact zone with blythi? I guess we will never know!

I would love to read what you think about this bird, please use the comments section below.

Acknowledgments

To all photographers that kindly sent me their photos and Yoav Perlman for giving me some insight on the status of S. curruca that pass through Israel, and how variable they are.

References

​Garner, M. (2014) Birding Frontiers Challenge Series: Autumn, Birding Frontiers​
Olsson, U., Leader, P.J., Carey, G.J., Khan, A.A., Svensson, L. & Alström, P. 2013. New insights into the intricate taxonomy and phylogeny of the Sylvia curruca complex. Molecular Phylogenetics and Evolution 6 7: 72–85
Shirihai, H., Gargallo, G., Helbig, A., Harris, A., Cottridge, D., Roselaar, D., 2001. Sylvia
Warblers: Identification, Taxonomy and Phylogeny of the Genus Sylvia.
Christopher Helm, A & C Black, London.
Svensson, L., 1992. Identification Guide to European Passerines, fourth ed.
Stockholm.
Williamson, K. 1976. Identification for Ringers 3: The Genus Sylvia, Second
Reprinted ed. British Trust for Ornithology, Tring.