Interesting pics also in the old BF thread you linked. As I said I agree with 1cy (Sand?) Martins - all of them. I think you're right to believe that the outer primaries should be distinctly more worn in adults. So, they must be of an early brood not from the North. Still, I find it astonishing how pale the fringes of the tertials are. Maybe a feature of the local race or already newly grown adult type feathers?
Thanks for the contribution, Roland. It's helpful.
I think that in the first bird in #1 all the tertials have already been replaced and are adult. In the second bird, the shortest tertial looks browner and is probably still juvenile. The first bird in #1, then, will have finished a complete post-juvenile moult by October of its first year, which I find quite astonishing for a northern migrant!
I've read somewhere that Sand Martins in the UK double brood, and that juveniles from the first brood migrate as early as mid/late July. So, there is a wave of migration composed entirely of juveniles in mid/late summer in Europe. If such a pattern were repeated here, that might explain the sudden and unexpected arrival of exclusively juvenile birds in late August/September.
However, according to HBW, Sand Martins in the Far East typically only have a single brood. I suspected this might be the case as Sand Martins here migrate much later than they do in Europe. There is northbound migration through outlying islands (Kinmen/Matzu) in late April/May (with the bulk of the birds in May) of birds which must be northern
ijimae as
fohkienensis Pale Martin is already on its breeding grounds. So, as Sand Martins here migrate later, I guess they breed (and fledge and moult and disperse) later than do European birds, making an early 'wave' of first brood juveniles less likely in this region (though Mac's comments in #4 suggest there are early arrivals).
As the timing of post-juvenile moult is in keeping with the 'southern set' of resident swallows that breed here (which will complete post-juvenile moult by October), I've fancied that these birds too might belong to this set. If so, that would make them
fohkienensis Pale Martins from just across the water in Fujian. Perhaps there is some post-fledging dispersal of this form?
So, you see where I am with these! If I could establish the rule that juvenile
ijimae retained juvenile plumage until October/November (after its migration), then these birds could safely be identified as Pale Martin by timing of moult. However, this 'first wave, second wave' business poses real problems for this assumption! Given that, is anyone able to answer my question in #5?