ID photo guide: subalpine warblers
Western Subalpine, Eastern Subalpine and Moltoni’s Warblers are all rarities in Britain and Ireland. While distinctive as a group, identifying them to species level is tricky. Andy Stoddart provides the information you need to recognise this similar trio.
A comprehensive taxonomic revision of the subalpine warbler complex concluded that it contains three species-level taxa, with what was once considered one species eventually split into three: Western Subalpine, Eastern Subalpine and Moltoni’s Warblers. All three warblers have been recorded in Britain and Ireland, although the third one is very rare. While distinctive as a group, identifying the birds to species level is a challenge. Andy Stoddart provides the information you need to recognise these three similar warblers should you be lucky enough to discover one in the field.
BASIC PRINCIPLES
‘Subalpine Warbler’ is a delightful small warbler of the Mediterranean region, where it typically favours scrub interspersed with taller bushes on dry slopes. It is a summer migrant, returning in March and April from its winter quarters in Africa.
It has traditionally been treated as a single species, although significant variation in plumage and calls across its range has long been recognised and multiple subspecies have been described. In recent years, these subspecies have come under increasing scrutiny and, assisted by evidence from genetic analysis, their true distinctiveness has been revealed.
‘Subalpine Warbler’ was split by the British Ornithologists’ Union’s Records Committee (BOURC) in 2014 into two species: Subalpine Warbler (with four subspecies) and Moltoni’s Warbler (monotypic). Within the former, two western subspecies were generally grouped together as ‘Western Subalpine Warbler’, with two eastern subspecies treated as ‘Eastern Subalpine Warbler’. These two groupings were then split by the International Ornithological Congress (IOC) in 2020 (with one of the two western subspecies regarded as invalid).
These taxonomic changes have, in turn, required some nomenclatural adjustments, and the whole complex, along with many other former Sylvia
warblers, has been transferred into the new genus Curruca.
The ‘Subalpine Warbler’ complex now comprises three ‘cryptic’ species, as follows:
• Western Subalpine Warbler Curruca iberiae:
monotypic, breeding in northwest Africa, Portugal, Spain, southern France and the far north-west corner of Italy.
• Eastern Subalpine Warbler Curruca cantillans: polytypic, comprising two subspecies – nominate
cantillans, breeding in central and southern mainland Italy and Sicily, and albistriata, breeding in the far north-east corner of Italy, the Balkans, Greece and western Turkey.
• Moltoni’s Warbler
Curruca subalpina: monotypic, breeding in the Balearic Islands, Corsica, Sardinia and northern mainland Italy.
Subalpine warblers are annual vagrants to Britain in small numbers and are regular enough to represent a highly viable bird-finding target. Most records are in spring
(but there are also a good number of autumn birds) and the majority occur in coastal counties, with the south and east coasts (as far north as Shetland) most favoured.
All three of the newly split species occur here, but establishing the numbers of each in the historical record is problematic with many not sufficiently documented to be identifiable and so treated simply as ‘Subalpine Warbler species’. The official statistics for 1950-2019 show 20 Westerns, 103 Easterns and 10 Moltoni’s, as well as 735 unidentified ‘Subalpine Warblers’.
These figures are inevitably less than definitive and almost certainly do not give an accurate picture of the relative status of each species. Moltoni’s Warbler seems to be genuinely rare, but the true proportions of Western and Eastern Subalpines are likely to be more evenly balanced than the statistics suggest.
Subalpine warblers as a group are relatively easily identified. They are small, a little smaller than a Lesser Whitethroat, and more compact and short tailed, with paler ear coverts and brownish-pink legs.
Spring males are grey above and reddish below (albeit variable in colouration and extent), with red orbital and eyerings and a white submoustachial. Autumn adult males have fresh whitish feather tips in the underparts which partially obscure their colour and make them appear more pink.
Females and first-year birds are dull and brown above and whitish below, slightly greyer around the head, with a reddish orbital ring and whitish eyering. Some adult females in spring have more peachy underparts and can even show a weak white submoustachial.
However, the recent taxonomic developments have led inevitably to greater identification difficulties. As a result, new criteria have been developed, previously understood criteria revised and the thresholds for certain identification and official acceptance raised.
A firm identification to species level is now very challenging and only the most distinctive spring males are likely to be identifiable in the field. The British Birds Rarities Committee now requires submissions of males in spring to contain two of the following: details of the underparts colouration, a photograph of the pattern of the second outermost tail feather (T5), and a description or (preferably) a recording of the call. For females and all birds in autumn, submissions should include both the tail pattern details and call. Even so, these features are all somewhat variable and the occasional ‘anomalous’ individual can be found.
In all cases, however, DNA analysis can provide a certain identification.
Inevitably, many ‘Subalpine Warblers’ will have to remain unidentified to species. Nevertheless, this should not be a deterrent to looking at each one carefully and noting as many features as possible. The currently understood identification criteria for spring males of each species (ranked approximately in order of importance) are as follows:
Western Subalpine Warbler
■ Underparts extensively warm orange-red, lacking contrast between the throat/ breast and the rest of the underparts.
■ Call a hard tack like Lesser Whitethroat.
■ A square white tip to T5.
■ Primary projection relatively short and broad, with four to six primary tips visible.
■ Usually short and narrow white submoustachial.
Eastern Subalpine Warbler
■ Underparts orange-red (nominate cantillans) or more brownish-red, less orange (albistriata), colour a little weaker on flanks (cantillans) or sometimes confined to throat (albistriata), belly more extensively white.
■ Call an abrupt tek (cantillans) or an often doubled t-tret (albistriata).
■ A narrow white wedge alongside the shaft on the inner web of T5.
■ Primary projection relatively long and narrow, with six to seven primary tips visible.
■ Usually long and broad white submoustachial.
Moltoni’s Warbler
■ Underparts extensively salmon-pink or rose-pink, lacking contrast between the throat/breast and the rest of the underparts.
■ Call a soft, Wren-like rattle.
■ A square white tip to T5.
■ Usually short and narrow white submoustachial.
■ (Primary projection variable and hence not useful). ■