Hadoram Shirihai's response to the note published in Birdwatch magazine, concerning a petrel photographed off the Kermadec Islands by Steve Wood:Fig. 1 & 2. Worn Black-winged Petrel, showing clear white collar due to wear and breaching; W Tropical Pacific; photo copyright of Tony Quinn.
It is correct that initially I speculated (as according to the note in Birdwatch, Michael Imber did as well) that the petrel photographed by Steve Wood (click here for link to photo) could be the poorly-known Vanuatu Petrel Pterodroma occulta. However, this was done before I had an opportunity to experience the two forms at sea and around the breeding grounds (i.e. White-necked Petrel Pterodroma cervicalis in the Kermadec Islands, and Vanuatu Petrel around Vanuatu), and also prior to examining the 6 specimens of occulta at the AMNH. In the future, I will be happy to share the results of these studies with others but not in this short response.
As posted on the 26 May (in Seabird News, run by Angus Wilson), and among other speculations, I indicated that Steve Wood's petrel appeared to have "an underwing pattern and overall shape reminiscent of Black-Winged Petrel, but that the black on the head/cap looks too extensive for the latter". But I also speculated "the apparent white collar is not conclusive, as this could possibly be an effect of wear."
Nevertheless, I checked images of the many Black-Winged Petrels that I photographed off the Kermadec Islands and found that some birds do appear to have slightly darker crown or even darker overall head (from certain angels and lights), and some even have a faint pale area on the hindneck. Furthermore, two of my friends, Robert Flood (UK) and Brent Stephenson (NZ), have contact me to say that they too have seen and documented 'white-collared' Black-Winged Petrel. Brent also kindly sent his images taken off northern NZ (during seabird pelagic voyages that he organizes on a regular basis), while Robert drew my attention to images of two such ‘white-collared’ Black-Winged Petrel taken on the Western Pacific Odyssey 2008 by Mike Danzenbaker and Tony Quinn.
In my note I also said "There is probably not much point in discussing the identity of this bird further until more examples are found, and that for me at least, Steve Wood's bird should remain as an unidentified Pterodroma". I will like to revised this statement too by saying that based on clear cut photographic evidences, the petrel photographed by Steve Wood's off Kermadec Islands is MOST LIKELY a 'white-collared' Black-Winged Petrel, at least until someone comes forward with new and well documented evidence to suggest that these could be a different species. I am adding "MOST LIKELY" since there are always caveats in trying to identify a petrel from a single image, and indeed Steve Wood's petrel still has what appears, at least superficially, to be distinctive black cap (see below).
Here is a brief summary of why the petrel photographed by Steve Wood off Kermadec Islands is MOST LIKELY a 'white-collared' Black-Winged Petrel:
1. Overall shape and proportions are most reminiscent of Black-Winged Petrel: it appears to be a rather compact small-sized Pterodroma, with rounded head and wings (the latter being relatively broad, and with short, rounded hand), and rather moderately/longish tail (i.e. resembling a medium-sized and quite heavy but compact cookilaria).
2. Diagnostic underwing pattern of Black-Winged Petrel: with conspicuous diagonal, bold and very broad black covert-band from bend of wing, extending well inward (almost to the armpit but not reaching body), and where narrows slightly, and with a broad black mark on the fore primary-coverts, the wing-tips and the trailing edge.
3. Rather bold dark breast-side patches: also characteristic of Black-Winged Petrel, but not conclusive.
Still one 'concession': the cap (combination of crown and eye-patch) seem to be too dark and solid compared to Black-Winged Petrels that I have seen and photographed. Conceivably, the capped appearance may be exaggerated in this one image by the harsh lighting, and this imaging artifact perhaps also contributes to the white-collared effect. Indeed, it seems that the eye-patch is slightly darker than the crown on Steve Wood's petrel.
Lastly, there is a strong match between Steve Wood’s petrel and the attached great images of Black-Winged Petrels by Mike Danzenbaker and Tony Quinn. These images shows that in a state of heavy wear, Black-Winged Petrels can develop a clear, but narrow, white collar on the hindneck.
The lesson I learned over the years in thinking about such cases is that the simplest explanations are most possibly the correct ones. That said, it is always a benefit to share ideas or to learn from others, and there is value in being flexible until more is known. Thus, Robert Flood and Brent Stephenson should be credited for their important input in considering the identity of the petrel photographed by Steve Wood's off Kermadec Islands, which as I have argue here, is MOST LIKELY an 'white-collared' Black-Winged Petrel. Thanks also to Mike Danzenbaker, Tony Quinn and Brent Stephenson for use of their excellent images.
Hadoram Shirihai, May 2008
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