From July 11-23, 2015, Sheridan and I spent four days in the central Chiapas area as part of the GBNA biennial meeting, and then a week in south/central Oaxaca as part of a private group, ably organized and lead by Michael Retter.
These Wedge-rumped Storm-Petrels Oceanodroma (Halocyptena) tethys were some of the c.30 birds seen near the boat on our Pelagic out of Huatulco, Oaxaca.
There are two populations of this species: nominate tethys breeds on the Galapagos Islands; kelsalli breeds on islands offshore from northern Peru. Both have been documented from waters as far north as southernmost California, but it is thought that most birds seen from pelagics out from the northern and central coasts of Mexico are kelsalli.
Here is a summary (drawn from Steve Howell's excellent N.A. tubenose book) of the differences between the subspecies:-
tethys: longer white rump/utc patch with less-extensive white wrap-around; less-forked tail; larger (clearly larger than Least SP); longer (and narrower) wings, slightly stouter bill.
kelsalli: shorter white rump/utc patch with more-extensive ventral white wrap-around; more-forked tail; smaller (barely larger than Least SP); shorter (and broader) wings; less-stout bill.
Initially I had thought that almost all the birds I could see well seemed to match the description for kelsalli rather than nominate tethys, but after looking at all of my photos I feel that a subspecific assignation may not be possible, in part due to individual variation and also the effects of molt.
No obvious molt, but perhaps the white uppertail coverts are not quite fully-grown?:
B) What is going on at the rear of this bird? The distal end of the visible leg is where the tip of the toes should be, yet there is the suggestion (based on shape) that this is not the end of the leg - and there is a toe-like projection beyond the end of the fresh tail tips. The fresh central tail feathers do not seem short when compared to other individuals.
If the central extension beyond the even tips of the fresh tail feathers is indeed toes, what does this say about the structure of this bird?
It this is not toes but feathers, which feathers are they -presumably the older unmolted outermost tail feathers that are being held centrally? I lean towards the latter explanation, but it is sobering to note just how much this image looks as though there are toes projecting beyond the tail... :
C) This individual has a "young jaeger-like" extension of the central tail feathers, with all the retrices seeming to be fresh except the left-side outermost retrix, which appears to be pale and longer. Compare this individual to the one above:
D) This is the individual with the longest white rump/utc patch that I managed to photograph. I saw at least one other bird that seemed to have the white reach almost to the tip of the tail, possibly because the tail was not fully-grown?:
E) This individual is at the opposite end of the spectrum, compared to the bird above. Some of the reduced white is due to ongoing molt.
F) Another individual where the shape and extent of the white patch is affected by molt:
The uneven nature of the trailing edge of the white patch indicates that molt in these tracts is ongoing:
This individual has an extensive white patch, plus extensive white wrapping around onto the underside (the buffy tones there are perhaps due to staining from the chum?):