Identification

In general, Middendorff’s has a warm olive-brown crown and mantle, with a contrasting, un-streaked tawny-brown rump, upper-tail coverts and upper-tail, with all but the central tail feathers being darker (especially on the inner web) and tipped whitish. Subdued darker mottling on the mantle and crown, formed by darker feather centres visible in the hand may not be visible in the field. Below, the chin and throat are creamy white, becoming sandy brown on the breast centre, but darker olive brown on the breast sides and flanks and whitish on the belly. The under-tail coverts are pale cream and unmarked.

Detailed description of ochotensis
The head shows a clearly defined and conspicuous supercilium in an otherwise plain face: in late winter the supercilium can appear rather diffuse and poorly defined behind the eye; in summer it is usually fairly obvious. The crown and nape are warm brown and uniform, lacking obvious dark centres.

The mantle is similar in colour, with maybe a warm olive tone and there is usually a hint of darker feather centres, which produces a very faintly mottled or streaked appearance to the upper-parts. Observing this feature in the field can be very difficult and much depends upon the angle at which the mantle is viewed; on some individuals the mottling can be entirely absent, especially on freshly moulted birds on the breeding grounds but is usually more obvious on worn individuals in late winter. The rump and upper-tail coverts are a slightly brighter tawny- or rufous-brown, and are completely unmarked. The tertials, greater and median coverts are dark brown, very slightly darker than the mantle and show narrow warm-brown fringes the same colour as the mantle. The tips to the inner webs of the tertials show a characteristic whitish notch, which is also shared with Pallas’s and Styan’s Grasshopper Warblers. This notch is smaller and less obvious on Middendorff’s than shown by Pallas’s and furthermore it is frequently absent due to abrasion of the tips of the tertials. The closed primaries are dull brown with narrow whitish tips, lost due to abrasion in late summer or late winter. The outer web of the outermost visible primary (p2) is whitish and often quite conspicuous.

The tail is long and distinctly graduated with the outer pair of retrices falling 15-23mm short of the central pair. All the retrices, apart from the central pair, show a large white tip, a feature shared with Pallas’s and Styan’s, but again these tips can be lost to abrasion in late winter. The upper surface of the tail is the same warm tawny- or rufous-brown as the upper-tail coverts, the central pair being slightly brighter than the remainder. In flight this gives the bird a rich brown appearance with an obviously brighter rump and tail.

The under-parts of adult Middendorff’s Grasshopper are pale and unmarked. They never show any trace of spotting on the lower throat or upper breast – although on some specimens there may be very fine, dark shaft streaks on the breast; the flanks are plain and unmarked and the under-tail coverts are uniform and lack pale tips. In adults the chin and throat are whitish and unmarked and there is a faint brown wash across the breast and flanks. The belly is white and the under-tail coverts are creamy-white, lacking any streaking.

In fresh first-winter birds the under-parts usually show a creamy yellowish wash of variable intensity, this being strongest on the chin and throat but becoming paler towards the belly. The under-tail coverts are buff. Some birds of this age may show faint mottling on the lower throat and breast, forming light streaking. As first-winter birds wear and bleach on the winter quarters the yellow tones to the under-parts fade to cream and by late winter a worn, abraded and bleached Middendorff’s Grasshopper Warbler appears very different to a freshly moulted bird in late May.

Subcerthiola
Swinhoe (1874) recorded that a specimen he initially thought to be a pale ochotensis, was similar to specimens from Kamchatka (labelled as L certhiola) sent to him from St Petersburg by von Schrenck. He describes it as olive-grey above, more richly coloured on the rump and tail edges, with the crown and mantle feathers (’obscurely’) darker centred; below, as white on the throat and belly and ochreous olive-grey on the breast sides and flanks – he states that the under-parts lack the ‘golden’ colour of ochotensis.

The Talan Island ‘subcerthiola’ share the plumage features of ochotensis and is obviously very similar. It tends to be more contrastingly marked than nominate, with more Pallas’s-like features: a slightly more streaked crown; a whitish supercilium that broadens behind the eye; more distinct mottling on the mantle; blackish centres to the wing coverts; the under-parts are whiter, lacking the slight buff wash of ochotensis; brownish buff under-tail coverts tipped white – broadly so on the longest. It is also at the large end of the range of measurements for Middendorff’s.

Voice
Middendorff’s Grasshopper Warbler starts to sing immediately upon arrival on the breeding grounds in late May and continues singing into July; it also sings on the wintering grounds from at least February onwards, possibly earlier. The song structure falls within that of the certhiola/ochotensis/pleskei super-species, yet it remains distinctive and is immediately recognisable as differing from the songs of other species. It is given both from a perch and in a display flight: the bird rises up to approximately ten metres, gives the song and descends back into cover.

Unlike the song of some species in the genus Locustella, the song of Middendorff’s is not a continuous reeling or buzzing, instead consisting of a series of three or four individual notes, which rise and fall in pitch, and terminating in a quite fluid and musical flourish; occasionally, the song may be followed by a rather nasal charring. Each song phrase lasts for approximately four seconds with the terminal flourish lasting approximately 1.5 seconds. When the full song phrase is heard, it can be transcribed as drrrt-chrit-chrit-chit-cherwee-cherwee-cherwee-wer-wer-wer.

The call of Middendorff’s is similar to the notes that precede the flourish of the song and can be heard at any time of the year.

Separation from Styan’s Grasshopper Warbler Locustella pleskei
Styan’s Grasshopper Warbler, from Korea, islands in the Sea of Japan and southern Japan, is on average, a larger, plainer bird, lacking the warmth of Middendorff’s – notably on the rump and tail. Its larger bill and apparent lack of streaking makes it appear similar to Oriental Reed Warbler Acrocephalus orientalis or Gray’s Grasshopper Warbler Locustella fasciolata.

The colour of the mantle, rump and upper-tail coverts are the most reliable features for separating Middendorff’s and Styan’s Grasshopper Warblers. The upper-parts of Middendorff’s are warmer and browner than those of Styan’s, which are colder and greyer and usually lack any trace of diffuse mottling; however the mantle mottling is variable with apparent overlap so separation using this feature alone is unreliable. The rump and upper-tail coverts of Styan’s Grasshopper are grey-brown and very similar in tone to the mantle, in flight these appear uniform grey-brown rather than the bright tawny-brown of Middendorff’s. Styan’s also lacks the prominent pale edge to the outer primary.

Middendorff’s shows a slightly more obvious supercilium compared with Styan’s, while Styan’s usually shows a well-marked eye-ring. Again there is individual variation and overlap and these features should not be used in isolation but they appear to be consistent.

In the hand, measurements can be used to reliably separate this species pair: bill length alone is diagnostic with Styan’s always the longer billed; the tail:wing ratio is usually reliable with few exceptions. The only caveat to this being the large birds found in the north of the range of Middendorff’s range and migrants trapped in Hong Kong, which are both within the overlap range with Styan’s.