All my life I have found nature fascinating, usually amazing and often surprising. But every now and again something happens that just blows me away.
In July, when Paul Holt and I found a small population of Greenish-type Warblers (Phylloscopus trochiloides) at Lingshan, Beijing’s highest mountain, I had no idea that the discovery could represent a missing link in the distribution of what is thought be one of few examples of a “ring species”.
The Greenish Warblers are widespread leaf warblers whose breeding range extends from temperate northeastern Europe to subtropical continental Asia. They are strongly migratory and most winter from India east to Thailand.
According to a theory first put forward by Ticehurst in 1938, Greenish Warblers were once confined to the southern portion of their range and then expanded northward along two pathways, evolving differences as they pushed north. When the two expanding fronts met in central Siberia (W Europe’s viridanus and Siberia’s plumbeitarus), they were different enough not to interbreed. Hence plumbeitarus is now considered a separate species – TWO-BARRED WARBLER.
This unusual situation has been termed a ‘circular overlap’ or ‘ring species’, of which there are very few known examples.
“Map of Asia showing the six subspecies of the greenish warbler described by Ticehurst in 1938. The crosshatched blue and red area in central Siberia shows the contact zone between viridanus and plumbeitarsus, which do not interbreed. Colours grade together where Ticehurst described gradual morphological change. The gap in northern China is most likely the result of habitat destruction.” (emphasis added)
As one might expect when looking at the map, in Beijing we are used to seeing TWO-BARRED WARBLERS (P.plumbeitarus) on migration as they make their way to and from their breeding grounds in NE China and Siberia. However, until a few weeks ago, there were no records in the capital of any of the races of GREENISH (P. trochiloides). Fast forward to 22 June 2015 and Paul Holt and I were in the middle of a 3-day trip to explore Beijing’s highest mountain – Lingshan. We had already encountered the albocoeruleus form of RED-FLANKED BLUETAIL, until very recently thought to be confined to a handful of sites in Qinghai and Gansu Provinces, more than 1,000 km to the southwest. In a relatively small piece of woodland on a northeastern facing slope Paul suddenly heard the distant song of a Greenish-type warbler. Fortunately the path we were following led us towards the sound and, after walking a little further, we could soon hear, and later see, the songster. It was clear that it wasn’t alone and, during the next couple of hours we encountered at least four singing birds. Paul focused on recording the song (see below) as I tried to snatch a photo or two as it flitted almost non-stop amongst the thick foliage in the canopy of the birch trees.
Paul was confident that these birds were not TWO-BARRED WARBLERS and most likely belonged to the obscuratus form of GREENISH WARBLER.
At this point it’s worth outlining the key plumage differences between GREENISH and TWO-BARRED:
- Two-barred is fractionally stronger billed than the viridanus Greenish we see in W. Europe but these figures probably don’t hold up too well when comparing it with the more poorly known obscuratus of Qinghai and Gansu.
- A typical Two-barred should have a broader greater covert wing bar that extends on to more inner greater coverts than viridanus – a greater covert bar that doesn’t taper towards the inner wing as conspicuously as it does on viridanus.
- Two-barred usually also has a second (median covert) wing-bar & this is often whitish or even white – this second bar is rare on viridanus (& when present is indistinct & typically not white).
- A median covert wing-bar is commonly seen on obscuratus (Paul Holt, pers. obs).
- Two-barred is also slightly darker above & whiter below than viridanus – but again that’s difficult/impossible to discern on a lone bird & in any case obscuratus has the darkest upperparts of any subspecies of Greenish Warbler (& is often contrastingly darker on the crown then inviting confusion with Large-billed Leaf Warbler P. magnirostris).
- Two-barred often (‘but not always’ sic Svensson 1992) has a pale yellowish supercilium while obscuratus is ‘apparently on average’ whiter here.
A recording of the Lingshan Greenish Warbler is below. Note that the interval between strophes has been shortened for convenience.
We thought that this newly discovered population was most likely the obscuratus form of GREENISH WARBLER rather than plumbeitarsus (TWO-BARRED WARBLER). Or is it an intermediate form between obscuratus and plumbeitarus?
We are keen to hear the views of others with experience of these species. If it is obscuratus, the find represents the first record for Beijing of GREENISH WARBLER.
Perhaps supporting the identification as obscuratus, it’s worth noting that there are two other species from Gansu/Qinghai that have recently been discovered breeding on mountains in or nearby Beijing – “Gansu” Red-flanked Bluetail (Tarsiger cyanus albocoeruleus) at Haituoshan (a forested mountain in Hebei immediately to the north of Beijing) and Lingshan and Large-billed Leaf Warbler (Phylloscopus magnirostris) at Wulingshan (a forested mountain in Hebei immediately to the northeast of the Chinese capital). It seems likely that small pockets of forest on some of Beijing’s higher mountains are supporting small disjunct populations of these relatively high elevation species, helping to fill in the gap in the map above (attributed to habitat loss).
The finding begs the question: are there other species from the Gansu/Qinghai area that could yet be discovered in the capital? Grey-headed Bullfinch, Chestnut Thrush and Chinese White-browed Rosefinch are possible candidates… In any case the Lingshan Greenish-type Warblers provide yet more evidence that there is still so much to be discovered in Beijing, the most well-watched part of China, let alone the rest of this vast country.