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The Lesser Whitethroat complex Sylvia curruca sp's
 

From The UAE Checklist:

Lesser Whitethroat Sylvia curruca
S. c. curruca
is probably the commonly occurring subspecies.
S. c. halimodendri winters in se Iran and Pakistan and might occur on migration and in winter. This subspecies tentatively includes blythi, affinis and telengetica.
It has been proposed that halimodendri is a subspecies of S. minula and often is indistinguishable from minula, even in the hand.
In the UAE fairly common to common on migration from mid February to mid May and from September to November. Local in winter in mountain wadis.

Desert Whitethroat Sylvia minula
Common winter visitor from late September to early April.

Hume's Whitethroat Sylvia althaea
Rare to very uncommon migrant and winter visitor from October to May.
 

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From Oscar Campbell (please feel free to make suggestions and corrections in the light of this):

The two references used, BWPi and Sylvia Warblers by Shirihai, Gargallo and Helbig are actually rather consistent in what they say on this magpie’s nest of a ‘species’.

The good news:
S. althaea and S. minula are definitely the most distinctive and classic individuals, if well-photographed and showing the upperside, they probably are identifiable.

The bad news:
Variation is totally clinal, with infinite links and stepping-stones between populations. BWPi notes a panoply of interlinking forms; Shirihai et al subsumes most of these into 5 taxa, nominally of four species.
Only classic, strongly marked individuals are worth considering, let alone claiming. Also, all 5 taxa recognised by Shirihai et al, save for margelanica, breed to the north of us and so all are possible (not least the intermediate curruca which winters in East Africa as does – probably – halimodendri).
On paper, there is no reason why curruca does not come through in good numbers in both spring and autumn (maybe it does?). Indeed, curruca is likely to be much commoner in the UAE than althaea, which is a much shorter-distance migrant (most likely as a winter overshoot, as, for example Hume’s Leaf-Warbler).

 

To simplify a highly complex situation (but note that the following should hold for either autumn or spring birds):

althaea

curruca & halimodendri

minula

Size &
structure

Large and especially strong-billed; long primary projection (as much as ¾ or more of tertials).

Quite large; primary projection comparable to althaea

Small, especially weak-billed and short primary projection: 1/3 to ½ (max) of tertials

Head

Little mask: dark ear coverts and lores obscured by relatively dark crown, forehead, so limited / no contrast. Overall colour dark-slate, smearing onto mantle, and head only slightly greyer than mantle

Mask quite strong and well-marked; certainly darker than crown which is rather ashy grey (mask especially strong on halimodendri?)

Very weak, non-contrasting head pattern; scarcely any trace of mask (darkening on lores only) as ear coverts pale, as is crown.

Upperside

Clearly dark grey and rather slaty. Any brown-tinge very limited, or nil. More likely to have pale panel on folded wing.

Drab grey-brown; neither slate nor sandy; olive tinge perhaps. Slightly purer grey in spring.

Distinct sandy-tinge, so should be some contrast between this and greyer face and crown.

Tail

Distinctly dark, with t6 virtually pure white

Limited white in tail: r6 not pure white (but white more extensive in halimodendri)

Distinctly pale, close to mantle but, as althaea, distinctly pure white t6

Underparts

Distinctly whiter; almost silk-white, especially belly and throat, but often strongly sullied on flanks.

Off white-below; not especially silky with darker flanks, or sandy (paler on halimodendri)

Extensively pale, sandy-buff; so not pure white but flanks rather weakly washed, so lacking contrast.

Margelanica is basically like minula in plumage but as althaea on structure.

 

Call:
Not much help here. Hard ‘tek’ or ‘check’ characteristic of curruca, but probably also used by other taxa. ‘Tzrrrrt’ also given by curruca and noted as churring with a clear rasping tone; similar to call (presumably the ‘tit’ like call commonly heard in the UAE) of minula, but minula may give it more emphatically. Althaea and margelanica probably also give this call. Hence my (our?) general conception that churring equals minula and tekking equals something that may not be minula appears to be a fallacy.

 

Moult details for all:
Post-juvenile is partial in July – early September, before migration. Replace all body feathers, and most wing coverts, but much less commonly, tertials. 1cy in autumn aged by moult limit in greater coverts and by very dark iris, and very sullied and outermost rectrices mostly lacking white, especially in curruca.

1st pre-breeding moult in following January – April also partial, replacing all tertials, some body feathers.

Moult again post-breeding (complete), in July – September, before migration. Adults on migration hence very fresh, and no moult limits. Iris slightly paler, and pure white on r6, especially in althaea and minula.

2nd pre-breeding moult as 1st, and so on.