Thanks for the links, but (as ever with photos on the net) be aware that many of these are misidentified, although is not surprising given the confused taxonomy of the Sand and Pale Martin complexes and the lack of references which deal with them in sufficent detail.
The easiest way to separate them is structure with Pale Martin having wings longer than the tail at rest and a very shallow fork to the tail (recalling Asian House Martin), whereas on Sand Martin the wings are about the same length (or shorter) than the tail at rest and the tail has a deep fork, rather like a juvenile Barn Swallow.
In general Pale Martin is the most widespread in China with three or four different and quite distinct subspecies occuring. Sand Martin would appear to be largely restricted to eastern China.
