Dear ID-ers
I asked a very experienced seabirder/pelagicist friend of mine in Australia (Roger McGovern) to comment on the ID conundrum of the mystery shearwater seen on this trip. He said that:
Having looked at your bird, I am inclined to say that it is a Short-tailed [...] The bill does not look long and slender enough for a Sooty and the steep forehead suggests Short-tailed rather than Sooty. The white underwing flash extending into the secondary coverts also points to Short-tailed, as does the absence of dark streaking in the coverts. It would have been nice to have a photo showing the feet extension or lack thereof, as clear feet extension would be another strong point in favour of Short-tailed. In the field, I find the flight pattern very useful as Short-taileds tend to fly in short bursts of rapid wing beats (a bit like Flutterers and Huttons) whereas Sootys glide a lot more in the way of Wedge-taileds.
Roger also pointed out that: "The question of sorting Short-taileds and Sootys when they are in ‘overlapping’ plumages is a vexed question which leads to a lot of discussion at the highest level of expertise!" and referred me to a discussion on the Birding Australia mailing list about a photo of a shearwater off Wollongong in January 2012 which was first identified by Nikolas Haass (one of our most knowledgeable field ornithologists in Australia) as a Sooty, but that after much discussion, the consensus was that it was actually a Short-tailed.
I read through the discussions of the Australian bird and have summarised the information on IDing these two birds.
UNDERWING: Short-tailed: shiny underwing with little contrast; especially on the under primary coverts; more silvery, reflective-looking background; low contrast between the the shiny primary bases and the coverts.
Sooty: shows dark streaks which contrast sharply with the very white background; has clearcut areas of dark pigment.
These features can be really hard to be sure of in the field, because they are so light dependent. In the hand they are always easy to tell Sooty and Short-tailed apart, even ignoring measurements, just by the colour of the underwing. However, seeing that in a reliable way in the field is extremely difficult and hugely lighting dependent, so it's very easy to be mislead, even if you know exactly what to look for.
BILL: Short-tailed: small, "cute"-looking, appears 'glued on'.
HEAD: Short-tailed: head small, neck short, forehead steep, almost vertical forehead; has a hood.
Sooty: sloping forehead (though even a Sooty can show a steep forehead).
TAIL: Short-tailed: small/short tail, and long foot projection.
STRUCTURE: Sooty: combination of a relatively muscular body and very long pointed wings.
FLIGHT STYLE: (See Roger's comments above). But one commentator maintained that "Flight style is important - it's often how we pick up something different. But even ignoring human subjectivity, flight style is obviously not a fixed character. See how much wind strength can affect it! So should it be relied upon to identify a rarity? I'm a skeptic. It doesn't constitute any useful evidence in my view. It may be the bird is genuinely different in flight for some perfectly rational (but unknown) reason, or it may just be that people think it looks different due to some illusion, or worse, because someone else said it did. Often it comes down to the question of whether a bird looks different because it is a different species, or for some other reason. It might be just hungry or fat!"
A final caveat was that none of these features are definitive by themselves, and that pictures of both species showing and not showing these features could be found, and this underlines the fact that we always need to look at a combination of field marks.
James
