Pallas’s Leaf WarblerPhylloscopus proregulus, Jiangsu, China, May. Pallas’s Leaf Warbler is one of the Big 5 leaf warblers of Shanghai. The species passes through the city-province on migration and can be found in winter there. Northeast China and the Russian Far East are part of the breeding range of this East Asian warbler. (Craig Brelsford)
PALLAS’S LEAF WARBLER: IDENTIFICATION
Pallas’s Leaf WarblerPhylloscopus proregulus breeds across southern Siberia and Mongolia to Sakhalin Island; in China in Heilongjiang and eastern Jilin. Winters eastern China south of Yangtze River to Southeast Asia. Common passage migrant and winter visitor Shanghai. Breeds in coniferous forests, to 1700 m (5,580 ft.); on passage and in winter lower and in more varied woodland habitat, including city parks. Very active, constantly moving from perch to perch; often hovers, recalling Goldcrest, with which it is sometimes seen. Tiny, large-headed, short-tailed. The eponym of the Pallas’s Leaf Warbler Complex, of which it is the most brightly colored and boldly marked member. Has well-defined, pale-yellow central crown-stripe equally distinct throughout length, dark olive-grey lateral crown-stripes (greener or even yellowish in front), bold yellow supercilium (turning whitish posteriorly), and long, brownish-black eye-stripes that may be hooked at rear. Wings blackish-grey with prominent yellow wingbars on median and greater coverts and dark bar at base of secondaries. Tertials blackish with white fringing; also remiges (and, to lesser degree, tail feathers) tipped white when fresh. Pale yellow rump conspicuous when bird hovers, contrasting with olive-green mantle and uppertail coverts. Short, dark brownish-grey tail, lacking white. Underparts greyish-white. Pallas’s Leaf Warbler has brighter lemon-yellow supercilium and coronal crown stripe, bolder head pattern, and greener mantle, yellower wingbars, and whiter underparts than Lemon-rumped Warbler P. chloronotus, Gansu Leaf WarblerP. kansuensis, and Sichuan Leaf WarblerP. forresti. Small, delicate bill brownish-black with orange base to lower mandible. Feet orange-brown, usually darker than Yellow-browed WarblerP. inornatus. Pallas’s is also smaller and more compact and has larger and rounder head, shorter bill and tail, darker and broader eye-stripe, and broader supercilium than Yellow-browed Warbler. Song a loud, melodious series of trills and whistles, sometimes lasting several minutes, and delivered from concealed perch in canopy. Call a soft, nasal, distinctly up-turned “dweet,” repeated incessantly. — Craig Brelsford
On Tuesday I arrived at Cape Nanhui too late to see Crow-billed Drongo. My partners Kai Pflug and Elaine Du and I made the fateful decision to cover Hengsha Island in the morning. The alluvial island at the mouth of the Yangtze was decidedly humdrum, with Far Eastern Curlew out on the mud along with 2 Sanderling and a Ruddy Turnstone. The huge new tree plantation on the island failed to deliver any forest birds beyond a single Asian Brown Flycatcher. There was a good count (17) of Richard’s Pipit.
We arrived at Cape Nanhui and found kaca, who mentioned an unusual drongo he had seen that morning. We kept our eyes peeled for dark drongos, finding none. Our Nanhui harvest was limited to expected October birds such as Grey-backed Thrush (6) and Eyebrowed Thrush (2). Asian Brown Flycatcher (26) seemed to be on every tree.
All of Shanghai’s Big 5 Leaf Warblers were present: Pallas’s Leaf Warbler (1), Yellow-browed Warbler (1), Arctic-type Warbler (2), Pale-legged/Sakhalin Leaf Warbler (7), and Eastern Crowned Warbler (2).
SONG CAN SAFELY SEPARATE PALE-LEGGED FROM SAKHALIN
Pale-legged Leaf Warbler is safely separable from Sakhalin Leaf Warbler only by song and call. Every other trait of each can occur in the other. Numerous authorities confirm this. Swedish ornithologist Per Alström calls the two species “almost identical” and “virtually indistinguishable except by song” (Alström 2012). Mark Brazil says field separation of Pale–Sak is “uncertain,” and he warns readers to “beware light conditions” (2009). Clement writes that Pale-legged and Sakhalin are “very similar” and claims, dubiously, that the latter is distinguishable from the former “mainly by greener upperparts and lack of wingbars” (2006). Clements goes on to describe juvenile Pale-legged as being “more greenish on upperparts,” which begs the question of whether the greenish Pale–Sak one is observing is an adult Sakhalin or a juvenile Pale-legged. Moreover, a quick look at Oriental Bird Images shows many Sakhalin Leaf Warbler with wing bars.
Thankfully for us birders, the songs of the two species are distinctive and provide the basis for a safe ID. The song of Pale-legged, occasionally heard in Shanghai in May, is a cricket-like trill, that of Sakhalin a high-pitched, three-note whistle.
Bottom line: In Shanghai, any Pale–Sak one sees is probablyPale-legged Leaf Warbler, the continental breeder, and not Sakhalin Leaf Warbler, the breeder from the eponymous Russian island plus Hokkaido and Honshu; but to claim certainty about any non-singing individual is the taxonomical version of Russian roulette.
Even on a fast-moving Pale–Sak in poor light, the pink of the bill and especially of the legs is readily seen. The distinctive pale color of these bare parts is a handy tool for distinguishing Pale–Sak from birds in the Arctic Warbler Complex, which like Pale–Sak lack a crown stripe and usually show one or two wing bars. (The Arctic Warbler Complex consists of Arctic WarblerPhylloscopus borealis, Kamchatka Leaf WarblerP. examinandus, and Japanese Leaf WarblerP. xanthodryas. In Shanghai, Arctic Warbler is the most common of the three, migrating through Shanghai every spring and autumn.) The pink coloration also distinguishes Pale–Sak from Dusky WarblerP. fuscatus, an uncommon migrant and winter visitor in Shanghai, and the scarce passage migrant Two-barred WarblerP. plumbeitarsus.
The tail-pumping of Pale-legged/Sakhalin is one of the most distinctive behavioral traits of the species pair. The steady movement usually occurs independently of other muscular actions and is slow enough for the eye to see. The tail-flicking of Arctic Warbler, by contrast, is more spasmodic and is often accompanied by wing-flicking.
— Pale-legged/Sakhalin Leaf Warbler is often found on the lower, thicker branches of trees.
With its ability to forage along thick branches and not just glean from the underside of leaves, Pale-legged/Sakhalin can remind one of a nuthatch. Other species such as Arctic Warbler use the lower branches, but sustained observation shows Pale–Sak more often in those areas. Note: In May and June 2016, I studied Pale-legged Leaf Warbler on its breeding grounds in Heilongjiang. There, amid trees older and taller than one usually sees in Shanghai, I most often noted the species far above my head, in the mid-canopy.
Editor’s note: This post caught the attention of Philip D. Round, a professor at Mahidol University in Bangkok and an expert on leaf warblers. In an e-mail to me, Round writes that as discoveries are made and papers published, separating Pale-legged Leaf Warbler from Sakhalin Leaf Warbler on call may become more widespread. Separation on morphology, by contrast, will be much more difficult, though it may eventually turn out to be possible in the hand.
The following paragraphs are from Round’s e-mail to me:
“I enclose a paper that details the first records of both Kamchatka Leaf Warbler and Sakhalin Leaf Warbler from Thailand. [Editor’s note: the paper, “Addition of Kamchatka Leaf Warbler Phylloscopus examinandus and Sakhalin Leaf Warbler P. borealoides to Thailand’s Avifauna,” is available for download through shanghaibirding.com (708 KB).] This has been rather overtaken by events, as we have now caught into the hundreds of Sakhalin LW, mostly on spring passage, and quite a few more Kamchatka. I have an undergraduate student who has carried out DNA assay on about ten percent of all the Pale-legged and Sakhalin LW caught. For many of these we have also recorded call notes on release. When she comes back from overseas study in January 2017 I hope we’ll get a paper out which publishes details of call-note frequency and DNA results for this large sample, which should show the correlation between species and call-note frequency clearly. (Actually this is moderately and anecdotally well-known already. I think either Frank Lambert or Jonathan Martinez was the first to draw my attention to the difference, and it is mentioned by Yap et al. in BirdingASIA with reference to an overwintering Singapore bird.) [Note: Round is referring to Yap, Francis et al., “First wintering record of the Sakhalin Leaf Warbler Phylloscopus borealoides in South-East Asia, with notes on vocalisations,” BirdingASIA 21 (2014): 76–81.]
“I am a bit less sanguine on finding means (other than by call frequency or song) to separate all birds. Even in the hand, it is by no means clear. We can pick out long-winged male Sakhalin, and short-winged female Pale-legged. But there is more overlap than previously realized, and most are in between. There don’t appear to be any 100% consistent wing-formula differences, and plumage and bare-part features, while somewhat indicative, are again less than 100% reliable—especially under field conditions. But probably we are missing something. The next thing to do is to apply PCA or some other multivariate analysis to figure out reliable means of separation of birds in the hand from our large sample, and also to use the information we have to figure out differences in the timing of passage of the two spp.”
Alström, P. (2012). Identification of Phylloscopus & Seicercus Warblers in China. Notes from presentation given to Beijing Birdwatching Society in November 2012. PDF downloadable here (13 MB). Click here for a 5 MB zip archive containing all 40 pages of the report in JPEG form. Those pages can be synced to your smartphone like photographs and consulted in the field. (Accessed: 14 Jul 2020)
Brazil, M. (2009). Birds of East Asia. Princeton, New Jersey: Princeton University Press.
Clement, P. (2006). Family Sylviidae (Old World Warblers). Pp. 663-4 (Pale-legged Leaf Warbler, Sakhalin Leaf Warbler) in del Hoyo, J., Elliott, A. & Christie, D.A. eds. (2006). Handbook of the Birds of the World. Vol. 11. Old World Flycatchers to Old World Warblers. Lynx Edicions, Barcelona, Spain.
Featured image: Shanghai’s Big 5 leaf warblers. (Craig Brelsford)