Having the chance to photograph Cook’s Swift (aka Pacific or Fork-tailed Swift of the cooki race) at Fraser’s Hill earlier this month reawakened my interest in the field differences between this and Pacific Swift (aka Pacific or Fork-tailed Swift of the nominate race).
A bit of background first.
Wells (1999) recognized three forms of Pacific Swift in the Peninsula: pacificus (a long-distance migrant from the Northern Palearctic), kanoi (from the South-east Palearctic), and cooki (which breeds in South-east Asia), all of which occur as migrants or non-breeding winter visitors.
Paul Leader, in his 2011 paper, Taxonomy of the Pacific Swift Apus pacificus Latham, 1802, complex (Bull. B.O.C. 2011 131(2)), upped the ante by proposing that the group be divided into four full species: Pacific Swift Apus pacificus, Salim Ali’s Swift Apus salimali, Blyth’s Swift Apus leuconyx, and Cook’s Swift Apus cooki. He proposed that kanoi was synonymous with kurodae, which was subsumed under pacificus as a subspecies.
As far as field identification of the three taxa mentioned by Wells is concerned, Leader described the following differences evident on skins which might also be evident in the field:
I have very few photos of pacificus from Malaysia. My best efforts were taken at Tanjung Tuan of a bird arriving from across the Straits of Melaka on 16 March 2011.
This shows rather brown upperparts, with the head slightly paler and browner than the mantle, a broad white rump patch and a well-defined whitish throat (and slight superciliary line).
The same bird from below, again showing rather brown sides of the head, a clearly demarcated and apparently unstreaked white throat, and brown underwing coverts offering relatively little contrast with the rest of the underwing.
Compare that with this cooki, taken at Fraser’s Hill on 7 Feb 2014. It is sooty blackish above, including the head, and there are signs of a glossy sheen on the wiing coverts and mantle. The rump band is narrow and the throat patch relatively inconspicuous.
Cooki from below, showing a strong contrast between blackish underwing coverts and relatively pale undersides to the flight feathers. The pale throat is less sharply defined than in pacificus and looks less white. The sides of the head are slightly brownish-looking, but still sootier than the pacificus above. Finally, the white fringing to the underpart feathers is noticeably broader than on the pacificus. I’m not necessarily proposing that these would be clear field marks, but at least they are visible on reasonable quality photos!
Pacificus left and cooki right, for handy comparison!
This cooki has the faint pale fringes to the scapulars mentioned by Leader.
As far as structure was concerned, of the many photos of cooki on which I can determine the wing formula, none of them show p1 as the wingpoint. However, there does seem to be a distinct difference between pacificus and cooki:
On cooki, P1 and P3 seem to be very similar in length, whereas, at least on this pacificus, P1 looks substantially longer than P3. I should add a substantial caveat or two here. First of all, it’s very hard to get a good view of the wing structure, even on photos, as angles can foreshorten the apparent length of feathers. Secondly, my limited survey of photos on the net seems to show that pacificus has quite a variable wing formula, with some showing P1 and P3 of almost equal length. So, this may not be a useful feature after all. More study needed!
Another structural feature which was apparent in the field was the very short-necked appearance of cooki.
I haven’t had chance to check this on pacificus, but on my photos at least, it looks to have a longer head projection in front of the wings than cooki.
Where and when to see them?
Cooki overwinters in the Peninsula, particularly over the forested central spine, whereas pacificus is only a migrant, wintering well south of the Peninsula. Thus, the best chance of seeing pacificus should be during autumn or spring migration, perhaps at one of the raptor migration watchpoints, while cooki can be seen during the winter months at any of the hill stations, or even over lowland forest in the north of the Peninsula.
Dig deep 
Excellent article! From the pics, cooki seems to have very broad wings that I may call somewhat banana shaped while pacificus seems to have slender crescent-shaped wings. Is this feature a distinctive differentiator of the two species?
Good observation Hor Kee. We are still learning how to do these in the field, but Paul Leader has suggested to me that Pacific looks spindlier, like a true Apus swift, whereas, Cook’s looks more compact, like a swiftlet. A difference in wing breadth such as you mention would contribute to this overall impression. A lot depends on angles though, especially when viewing still photos. It’s important we start to actually look at these things in the field, to get a better impression of jizz.
Great post as usual, Dave. I’ll compile some photos of ‘cooki’ in northern Thailand as well. I somehow have the feeling that the rump patch of ‘cooki’ that I’ve photographed is a bit narrower than the ones you posted here but that’s probably due to feather wear and such.
I’ve looked at your pics on OBI Ayuwat and those birds certainly seem to have narrower rump bands. The ones I’ve seen in northern Malaysia did too – barely visible on some birds. I think there must be quite a bit of variation, whether related to feather wear or not I’m not sure.
Not sure if the tail is helpful, does felt like Pacific seemed to have longer tails against the longer narrower wings and Cook’s rather short tail against broader wings, that may give a different jizz, from some available images on OBI and over here.
Pingback: Doi Ang Khang: 12 October 2014 | ayuwat
Pingback: Busan, June 9 | Birds Korea Blog