OCCURRENCE AND IDENTIFICATION OF THE LEACH'S STORM-PETREL COMPLEX OFF SOUTHERN CALIFORNIA 
Only since the early 2000s have birders start- 
ed to explore offshore southern California 
waters with any regularity, although some 
birders participated in research cruises well 
offshore dating back through the mid-1980s. 
On most recent fall pelagic trips out of San- 
ta Barbara and San Diego, from 60 to 350 
“Leach’s” have been seen daily, mainly over 
deeper offshore waters and around banks 
such as the Tanner Bank, Cortez Bank, and 
Sixtymile Bank (Figure 2). Based on these ob- 
servations, we have adduced the following 
patterns of occurrence for the birds we have 
documented in southern California waters. 
With a limited number of trips from which to 
work, we acknowledge that this portrait of 
status and distribution is provisional, some- 
times conjectural, and certainly incomplete 
(Figures 19-22). 
Leach’s Storm-Petrel. Presumably there is a 
northbound migration through offshore wa- 
ters in February-April, with a southward 
movement back to the tropical Pacific in Sep- 
tember-November, but to what extent the 
larger northern Leach’s occur relative to 
“bcflli” Leach’s is unknown. Non-breeding im- 
matures (but perhaps not many first-year 
birds, see above) probably occur in summer 
and fall off southern California, perhaps 
mainly in July-September, the period during 
which non-breeding immatures of other 
pelagic species are present in numbers off 
California, e.g., Long-tailed Jaeger (Stercorar- 
ius longicaiidiis), Sabine’s Gull (Xema sabini), 
and Common Tern (Sterna hirundo). Most 
presumed Leach’s occur over deeper offshore 
waters, at and beyond the shelf break. 
The breeding population of Leach’s Storm- 
Petrels on the California Channel Islands was 
estimated at 159 pairs in 1991; only 19 birds 
were mist-netted, with about two-thirds 
white-rumped and one third dark-rumped 
(Carter et al. 1992). This population may be 
increasing (fewer than 10 breeding birds were 
estimated in 1975-1977; Hunt et al. 1980), al- 
though more mist-netting effort occurred in 
1991, and the 1970s estimate was derived 
from mark-recapture analyses that may have 
been flawed (H. R. Carter, pers. comm.). Giv- 
en that breeding storm-petrels are probably 
restricted to foraging within a few hundred 
kilometers of colonies, and that large num- 
bers of non-breeding immatures can be pres- 
ent around colonies in the breeding season, 
the Channel Islands population may con- 
tribute appreciably to the numbers of 
“Leach’s” observed in summer off southern 
California. 
Chapman’s Storm-Petrel. This taxon (based 
on the sheer number of dark-rumped birds 
seen) is fairly common off southern Califor- 
nia, at least during April-October. For exam- 
ple, 65% of 100 birds at Cortez Bank, 10 Sep- 
tember 2005, were dark-rumped (score 4-5; 
JF, pers. obs.), as were 25% of 200 birds be- 
tween Cortez Bank and Sixtymile Bank, 7 
September 2006 (SNGH, pers. obs.); 21% of 
190 birds in the vicinity of San Juan 
Seamount, 21 July 2007 Oh pers. obs.); 33% 
of 78 birds south of Santa Cruz Island, 25 July 
2009 (SNGH, pers. obs.); 45% of 102 birds 
from the 4000-meter depth contour west of 
the Bell Bank to the Bell Bank, 25 August 
2009 Oh SNGH, pers. obs.); and about 90% 
of 138 birds north of Ninemile Bank, 7 Sep- 
tember 2009 (TM, WTH pers. obs.). 
In addition to birds from the nearby colony 
at the Coronado Islands, the fall numbers of 
Chapman’s may include birds from the San 
Benito Islands, paralleling the well-known an- 
nual northward fall incursions into California 
waters of Black Storm-Petrels (Oceanodroma 
[Halocyptena] melania) and Least Storm-Pe- 
trels from Mexican colonies. During April-Oc- 
tober, Chapman’s tends to occur nearer to 
shore than nominate Leach’s and is regularly 
found from the Thirtymile Bank out to beyond 
the shelf edge. Small numbers may occur well 
offshore in winter, but at this season most 
“Leach’s” off southern California are apparent- 
ly white-rumped (P. Pyle, pers. comm.). 
Townsend’s Storm-Petrel. In general, 3-5% of 
all Leach’s-type storm-petrels seen on offshore 
trips in the southern half of the Southern Cal- 
ifornia Bight from late July to mid-September 
show characteristics of Townsend’s Storm-Pe- 
trel. Both white-rumped and dark-rumped 
birds have been seen, with more of the former 
perhaps reflecting the relative ease with which 
they can be identified at sea. We have two 
records outside of this period. About 90% of 
110 birds seen 5 June 2008 in the vicinity of 
the Bell Bank (JF, pers. obs.) were thought to 
be white-rumped Townsend’s, suggesting that 
their occurrence in California waters may be 
prone to marked interannual variation. We 
also have one record of a white-rumped bird, 
perhaps a juvenile, on 2 November 2008 west 
of the Cortez Bank (photograph by TM). We 
suspect that this taxon occurs regularly off 
southern California, mainly July-October 
(but see the June observation, above), which 
mirrors the northward dispersal at this season 
by sundry Mexican-breeding seabirds, such as 
Black and Least Storm-Petrels, Guadalupe 
[Xantus’l Murrelet (Synthlihoramphiis [It.] Iiy- 
poleucus), and Craveri’s Murrelet (S. craveri). 
Townsend’s Storm-Petrels have often been ob- 
served over deep water just beyond the shelf 
break, thus occurring in areas where both 
Leach’s and Chapman’s are frequent, 
Ainley’s Storm-Petrel. Despite searching, we 
have seen no birds that we considered candi- 
dates for Ainley’s Storm-Petrel, although dis- 
tinguishing this taxon from California 
Leach’s, and perhaps from white-rumped 
Chapman’s, may not be possible given our 
present (lack of) knowledge. Although the 
limited specimen record suggests a southward 
post-breeding withdrawal during May-Octo- 
ber (noted above), it is conceivable that for- 
aging adults range north into United States 
waters during their winter breeding season 
(perhaps from September or October on- 
wards?), Any pelagic trips far offshore in win- 
ter should be on the lookout for this taxon. A 
report of 20 “Leach’s Storm-Petrels” at the 
Davidson Seamount on 1 December 1979 
(Garrett and Dunn 1981) is intriguing. These 
birds could have been late southbound or 
wintering leucorhoa, possibly birds of the 
smaller southerly breeders thought to winter 
off California (Briggs et al. 1987) — but could 
some have been Ainley’s Storm-Petrels? 
Conclusions 
Mark Twain once said that he would rather 
have his ignorance than another man’s knowl- 
edge, for his ignorance was far greater. In the 
case of the Leach’s Storm-Petrel complex, 
however, we would happily trade a lot of ig- 
norance for a little knowledge, and Ober- 
holser’s statement of 1919 still rings true to- 
day: “Notwithstanding the considerable atten- 
tion that has been paid to petrels of the Ocean- 
odroma laicorboa group, the last word has ev- 
idently not yet been said on the subject.” 
Our provisional observations indicate that 
Chapman’s and Leach’s are the predominant 
taxa in the Leach’s Storm-Petrel complex in 
waters off southern California, but that small 
numbers of Townsend’s Storm-Petrels also oc- 
cur in late summer and fall, mainly July 
through September. As yet, we have no evi- 
dence that Ainley’s Storm-Petrel enters United 
States waters, but it may well occur, perhaps 
mainly in late winter. 
Acknowledgments and Dedication 
We thank Phil Unitt (San Diego Natural His- 
tory Museum; SDNHM), Maureen Flannery 
and Jack Dumbacher (California Academy of 
Sciences; CAS), Carla Cicero (Museum of 
Comparative Zoology, University of Califor- 
nia, Berkeley), and Kimball L. Garrett (Los 
Angeles County Museum) for access to spec- 
imens in their care; Harry R. Carter, and Peter 
Pyle for sharing unpublished data; Howell 
also thanks Kim Kreitinger and Kirsten 
548 
NORTH AMERICAN BIRDS 
