I don't think the underwing pattern really fits either species perfectly. It is very pale for Short-tailed and the primaries don't seem to be dark enough for Sooty. However, for such a pale underwing, the underparts seem to be dark for Short-tailed. The bill seems rather long which favours Sooty. The forehead seems rather steep which favours Short-tailed. What is very compelling for me though is the tail, this is clearly long and wedge shaped, which seems to eliminate Short-tailed.
