Peter Donaldson of Pearl City, Hawaii writes: "This bird was found on a cruise ship coming into port on the island of Maui in Hawaii. The bird was captured and photographed, and when released it flew off strongly. The pictures were provided by Dr. Fern Duvall, Department of the Hawaii Land and Natural Resources, Division of Forestry & Wildlife.
The bird seems to be one of the smaller gadfly [Pterodroma] petrels. The bird looks too dark, especially on the head, to be a Cook's/DeFilippi's or maybe even Pycroft's. The underwing pattern seems to rule out species with more distinctive patterns likeBonin or Black-winged. So I think the ID question is centered on Pycroft's, Stejneger's and Gould's/Collared. Is the head and nape too dark to be a Pycroft's? a Stejneger's? Are the grayish outer tail feathers significant? Is there too little black on the underwings for this to be a Gould's/Collard Petrel?"
We welcome comments on the
Fig. 1. Lateral view showing rich blue-gray mantle with paler edging on the tips of the feathers. Greater coverts and primaries blackish. Crown and nape dark extending to enclose the feather around the eye. Whitish, slightly mottled, forehead. Bill chunky.
Fig. 2. Spread wing showing limited extent of the dark carpal bar. As Peter says, this pretty clearly rules out Bonin Petrel which are relatively common breeders in the archipelago.
Fig. 3. Lateral view showing pale forehead, dark cap and shoulder to good effect. The toes appear bluish with a hint of pink in the webbing. The bill is comparatively chunky.
Fig. 4. View of spread tail.
Fig. 5. Another dorsal view with rule for scale. The contrast between the nape and mantle is particularly obvious here.
Here are some supportive comments:
Angus -- It looks like a Stejneger's Petrel to me. I attach a couple photos of specimens from CAS that I took years ago; the film is starting to turn color but it still shows some points well. These are the 3 P. longirostris that were collected ~300 mi off California on 19 Nov 1906. The tail is of one of them, also.
The underwing pattern rules out all but Stejneger's/Cooks/De Filippi's/Pycroft's. White-winged/Collared is easily ruled out on that point, plus they have much more extensive blackish "shawls" that cover more of the nape/upperback/shoulders.
Obviously, Cook's/De Filippi's is out on head pattern (and tail pattern), so we are left with Stejneger's v. Pycroft's. I have the least information about the latter taxon, but in our review of specimens for Roberson & Bailey (1991), we found none with extensive black over the crown [Steve Bailey did much of the pycrofti review because he visited museums in New Zealand; there are very few in American collections].
The bird in question fits all aspects of Stejneger's that I can see. It has the appropriate amount of black on the head (most like the top bird in the specimens below) that nicely contrasts with gray back; it has the right tail pattern (rather wedge-shaped, blackest on centrals, and pale grayish outers); the right underwing; and the right bill. Stejneger's has a rather thick bill compared to somewhat thinner bills on Cook's/Pycroft's (plus the latter is short billed).
A fall bird near Hawaii would be appropriate range for this transequatorial migrant.
I haven't done any research -- this is just off the top of my head, with the photos in my collection. But Stejneger's looks right to me.
Cheers, Don Roberson, Pacific Grove, California
[NB Keith Hansen's plate from the Roberson & Bailey 1991 paper can be view on Don's petrel page together with a host interesting seabird photos.]
Thanks for that. A friend, Ross Silcock, emailed me about this the other day also (he is on Hawaii birding). I have had a quick look through the photos etc and would confer with Stejneger's also. However, as with yourself this is without proper consultation of the literature. However, the very dark head and prominent white forehead seem to fit ok for me.
Regards, Brent Stephenson, New Zealand
Had a look at your 'Maui' petrel photos. Beyond reasonable doubt a juvenile Stejneger's Petrel. Too much darkness on rear crown/nape for Pycroft's which is the most likely confusion here. Not nearly enough black on underwing stripe for Gould's, nape and rear neck not dark enough and size looks too small. Freshness of plumage, dull blackness of primaries and clean, black bill indicate a recent fledgling.
Cheers, Mike Imber, New Zealand
I agree with most of Don's comments on this bird. The absence of a white supercilium and the width of the white forehead are diagnostic and the absence of a dark "shawl" is 99.9% effective. The overall darkness of the crown is a bit of a red herring as Pycroft's (especially) is unbelievably variable -I have seen Pycrofts this dark but they did have a supercilium. Measurements would clinch it (and I would be interested in them too).
Regards, Paul Scofield, Curator of Vertebrate Zoology, Canterbury Museum, New Zealand