Identification of in-hand Storm-Petrels in Eastern Tropical Pacific
A quick review
text by Don Roberson
The Problem:

Tom Schulenberg writes: "Manuel Plenge forwarded to me the attached images of a handheld storm petrel. The bird was captured at or off of Lobos de Tierra in northern Peru. [The original observer has a lot to learn about how to photograph birds for later identification!]

Relevant taxa include:
Oceanites oceanicus Wilson's Storm-Petrel
Oceanites gracilis White-vented Storm-Petrel
Oceanodrama tethys  Wedge-rumped Storm-Petrel
Oceanodroma leucorrhoa  Leach's Storm-Petrel
Oceanodrama castro  Band-rumped Storm-Petrel
Unfortunately there seem to be no measurements or body mass data to go along with the photos. Can it be identified?"

I believe it was Sherlock Holmes who said "eliminate the impossible and whatever remains, no matter how unlikely, must be true." I'll start with this "eliminative" approach although one suspects that "whatever is left" will not be "unlikely" but, rather, a common bird in ocean west of northern Peru.

Fortunately, I have my own in-hand photos of a couple of Leach's Storm-petrels that came aboard our research vessel in fall 1989 in the eastern tropical Pacific. This first one is from 26 Oct 1989 at about 2°N, 88°W, which is a bit northeast of the Galapagos. It measured it in-hand but screwed up the wing measurement, but exposed culmen 14mm and tarsus 24mm are fine for either O. l. leucorrhoa or O.l. beali. The second bird was on 30 Oct at about 5°S,97°W, southwest of the Galapagos and about 500nmi or so west of northern Peru. I post two shots of that bird, also measured in had: wing 146mm, culmen 13, tarsus 22. It is actually fairly hard to take measurements at night on a rolling ship, but these are okay for O. leucorrhoa, esp. beali, although the bill is a tad short, per Crosin, R.S. 1974. "The Storm Petrels" in Pelagic Studies of Seabirds in the central and eastern Pacific Ocean (W.G. King, ed.), Smithsonian Contrib. Zool. 158.

This illustrates that other photographers (i.e., me) have failed to provide very useful shots, but at least it shows the general size of a Leach's Storm-Petrel in hand and the general bill size.
These two photos of another Leach's are consistent with the first shot above. The woman holding this one is a smaller person than the guy above, so the bird looks even bigger in her hand. Bill size is the same.

Things to note: (a) longest tail feathers extend basically as long as the primaries; (b) the usual "divided" white rump patch of Leach's, (c) the rather little white extending down on flanks, and (d) the largish bill.

I think we can rule out Leach's for your mystery bird on these points: (a) mystery bird has wings substantially longer than tail, (b) mystery bird has an extensive white rump without divided line, (c) mystery bird has lots of white on flanks, and (d) mystery bird looks a bit smaller-billed than Leach's. Further, it appears smaller in the hand than either Leach's, even without knowing the size of the person.

When I looked at the mystery photos, I immediately thought Leach's was eliminated by the extensively white flanks of the mystery bird. When sitting on the water, Leach's does not show white on the waterline, while Wilson's, Wedge-rumped, and Band-rumped all do (pers. obs.). The mystery bird clearly looks like it would show lots of white when sitting on the water. So I am confident Leach's is ruled out.

As it happens, I did a bunch of museum research on storm-petrel i.d. for a paper that never got completed, but has the following draft directed at in-hand i.d.:

In-hand characters :  Should a bird be found blown or washed ashore, or aboard a ship at sea, there are additional in-hand characteristics to check.  Some authorities emphasize that Band-rumped is the only storm-petrel to have clear-cut broad black tips to the white rump feathers (Palmer 1962, Cramp and Simmons 1977), but this point is variable, and some Band-rumped specimens show only dark bars, edges,  or black spots (D. Lee, in litt.). Many Leach's have a darkish shafts or a dark subterminal patch on the longest rump feathers (see classes 5 and 6 of figure 1 in Ainley 1980; also Figure 4), and some have dark blotches to the tips to the white feathers, overlapping some specimens of Band-rumped (D. Lee, in litt., and specimens at LACM and CAS). No Leach's, however, has the broad and distinct black tips to the white feathers, along with white shafts, shown by the Band-rumped Storm-Petrel.  The Band-rumped also has a shorter, stubbier bill than Leach's (good sketches in Ainley et al. 1980 and Lee 1984), but this subjective character is not likely to be useful except in direct comparison.

An additional in-hand character I noted in specimens of Pacific Band-rumpeds was the presence of extensive white at the base of the outer three rectrices. This part of the tail is covered by the uppertail coverts in the field, but in the hand the white, often triangular, basal portions of these feathers can be located.  The white always extended up from 2 to 3.5mm from the base of the feather on all 30 Band-rumpeds in the CAS collection. I found white at the base of the rectrices on only 3 of 30 randomly-selected Leach's at the CAS collection, and always restricted to less than 1.5mm of the base of the rectrix.  No white was found on these feathers in any Wilson's or Wedge-rumped storm-petrel Oceanodroma tethys I examined, nor on the extinct Guadalupe Storm-Petrel O. macrodactyla.  Of the white-rumped, blackish-colored storm-petrels in the eastern Pacific, only the White-vented Storm-Petrel Oceanites gracilis possessed white at the base of any rectrix, and that species is easily distinguished by its small size and white belly.

Here is a Band-rumped O. castro specimens from AMNH from 4 Apr 1891 at Wenman I., Galapagos. Note the broad black tip to the most visible white upper tail covert feather.

We can see the tips of the longest uppertail coverts well on the mystery storm-petrel. These feathers are clean white to the tip, no hint of any black.

We can eliminate Band-rumped Storm-Petrel.

This leaves Wilson's, Wedge-rumped, and White-vented. On the mystery bird, there is no hint of white or whitish to the base of the outer rectrices. Further, should this bird have had a white patch on the belly, surely they would have photographed the underside. And while the mystery bird is smaller than Leach's, it is not absolutely tiny in the hand. For all these reasons, I think White-vented is eliminated.

This leaves Wilson's and Wedge-rumped. Wilson's has very long legs with yellow webs. We can see this but, on the other hand, one might have thought they'd photograph interestingly colored feet. Probably more important, the overall tone of this bird is a grayish-black, not black-black, and I generally find Wilson's to be "black-black" birds. Wilson's has a longer tail than Band-rumped, and the tail of Band-rumped still is almost as long as the wings in the specimen (above). The mystery bird has a very short tail; too short, I think, for Wilson's. So although a bit more tentative, I think Wilson's is eliminated.

This leaves Wedge-rumped. That is, in fact, what I thought the bird would turn out to be before I started this analysis. It looks to have a very short and almost wedge-shaped tail. It looks to be decidedly smaller than Leach's and even Band-rumped/Wilson's, so the size looks good for Wedge-rumped. [White-vented is also small but doesn't have this shape of tail.]  The culmen lengths on my two Leach's were 13 and 14mm. Just 'eye-balling' the mystery storm-petrel, the bill looks a tad shorter, maybe 11 or 12mm. That is, in fact, the range of O. t. kelsalli race of Wedge-rumped that presumably nests in Peru. Finally, I can see very faint but distinct shaft streaks on the white coverts of the mystery bird. I have noticed that before on Wedge-rumpeds but no other storm-petrel. I have never tried to quantify this character and don't know if it is useful, but it is curious . . .

My own notes from late October 1989 show that Wedge-rumped, and Leach's, and Band-rumped, were all seen regularly well offshore northern Peru. I did not see Wilson's far offshore (and in California they are usually fairly nearshore at pockets of productivity). White-vented was seen only around the Galapagos on my trip. But Wedge-rumped is an excellent candidate on distribution as well.

All photos & text © 2005 Don Roberson, except those of the "mystery storm-petrel" provided by Schulenberg and Plenge; all rights reserved.




Page created 25-26 Jan 2005