SHORT-TAILED/SOOTY SHEARWATER
Congratulations to those who spotted this bird and appreciated its potential to be a Sooty Shearwater. Was hoping that some additional photos might be posted to help with id of this interesting bird. On the two we have, I think its going to have to remain one of those mysteries.
There are pluses and minuses. The one photo of the underwing is certainly interesting. Short-tailed is very variable and the pattern could fit either species on first impression. However, to my eye, the white on the inner underwing is a bit too wide for Sooty, where the white is usually limited to the median coverts in the middle of the wing and does not typically extend so far back towards the trailing edge. This results in the most prominent and widest area of paleness on the underwing of Sooties being on the primary coverts on the outer wing rather than sharing prominence with the inner wing like the photo suggests.
Other comparisons are structural. Sooty has a longer and less slender bill than Short-tailed, and this is accentuated by a more sloping forehead. This is hard to judge in the photos as the bill is not shown side-on but again it appears to be closer to Short-tailed and not as long as expected for Sooty. Also in the lower photo, the tail end appears "lumpy" suggesting that feet may be projecting beyond the tail. This point is in favour of Short-tailed but not Sooty. Lastly to my eye, the wings are not quite as long in proportion to the body as I would expect for a Sooty, but OK for Short-tailed.
However, many of these points are subtle and a wider series of photos backed by field descriptions of flight style and other observations would help with a decision one way or the other.
Lastly, it is worth noting that there are estimated to be over 20 million Short-tailed Shearwaters and they are known to pass up the East Pacific in our spring in big numbers after leaving their Australian breeding grounds, whereas the numbers of Sooty Shearwaters are an order of magnitude less at several million and their numbers are split between New Zealand and Chile. Furthermore, NZ birds appear to take a more northerly route not close to East Asia and as a consequence complete their clockwise circuit of the Pacific more to the east. This results in them being much more numerous on the west coast of USA on their return journey than Short-tailed. For any betting man, the odds on Sooty in the South China Sea are thus much longer.
Mike Chalmers
