Asian House Martin (Delichon dasypus) must be a candidate for vagrancy to Western Europe. At least one of the three subspecies is a strong and long distance migrant. However, looking very similar to Northern House Martin, its common European sister-species, how many people would be able to identify one?
The nominate subspecies of Asian House Martin is perhaps the most likely to wander. It breeds in southeast Russia, the Kuril Islands, Japan and Korea and migrates through eastern China to winter in the Malay Peninsula, Borneo, the Philippines, Java and Sumatra.
Ssp cashmeriensis breeds in the Himalayas from Afghanistan east to Sikkim and northwards into Tibet and western and central China. It is a short-range migrant, mainly wintering at lower altitudes in the foothills of the Himalayas.
The third race, ssp nigrimentalis, breeds in southeastern China and southern Siberia. Its wintering grounds are unknown, but birds in Taiwan apparently just move to lower altitudes in winter.
In eastern China any sighting of a House Martin is notable. I have seen a handful of both Asian (presumably the nominate subspecies) and Northern House Martins at Laotieshan in Liaoning Province but, in a sign of just how scarce they are in this region, I have still not seen one of either species in Beijing (they are passage migrants and seen in small numbers each spring and autumn – clearly I just haven’t been trying hard enough!).
My recent visit to Chang Bai Shan in Jilin Province provided an opportunity to get to grips with Asian House Martin as several pairs were nest-building on our hotel, allowing some fantastic views. Capturing any hirundine in flight with a camera is never easy, and the images below won’t win any prizes, but they do show some of the features to look out for in separating Asian House Martin from Northern House Martin. To the European readers of this blog, it’ll be worth making a mental note of these features when checking out those late autumn migrants….!
There are several differences between Asian and Northern House Martin that should make identification relatively straightforward if seen well.
Structurally, Asian House Martin is smaller, more compact and squarer-tailed than Northern but these features aren’t necessarily easy to ascertain on a single bird.
Perhaps the best feature, and one that I have found very helpful in the field, is the colour of the underwing coverts. In the images above, taken in sunny and dull conditions respectively, one can see the relatively dark underwing coverts, a consistent feature of Asian House Martin. Compare with this image of a Northern House Martin. The paler underwing coverts of Northern are not usually as prominent as shown in this linked image (taken in strong light) and can often appear concolourous with the rest of the underwing but a House Martin with obviously dark underwing coverts should be Asian.
Another subtle feature to distinguish these two species is the amount of black on the face. Compare the image above of Asian with this image of Northern. The black on the face generally extends a little lower on Asian.
Another feature is the rump. On Asian House Martin the white rump is usually relatively small and can appear ‘flecked’ with dark streaks, as in the above image. On Northern the white rump is larger (due to more of the uppertail coverts being white) and is usually clean white.
Finally, check out these images from John Holmes in Hong Kong. Asian House Martin often shows ‘dusky’ flanks and belly, whereas Northern usually shows bright white underparts.
In summary, the combination of a smallish white rump (sometimes flecked), dark underwing coverts, ‘dirty’ flanks, a squarer tail and a darker ‘face’ are all characteristics associated with Asian House Martin. Maybe one will turn up at your migration watchpoint this autumn…?
EDIT: I have added this image of an Asian House Martin taken at Chang Bai Shan that shows the dark feathering on the upper chin (ie just below the lower mandible). In Northern House Martin, the chin is white.