OCCURRENCE AND IDENTIFICATION OF THE LEACH'S STORM-PETREL COMPLEX OFF SOUTHERN CALIFORNIA 
Figure 19. Unidentified "Leach's Storm-Petrel" off San Diego, 
California, 20 July 2008. Under typical at-sea conditions, and 
even with reasonable photographs, many birds are best left 
identified as simply "members of the Leach's Storm-Petrel com- 
plex,"given our limited understanding of identification criteria. 
This bird may be a California-breeding Leach's, which appear 
smaller and less rangy than North Atlantic and Aleutian Leach's. 
Photograph by Steve N. G. Howell. 
Townsend’s, with a longer tail, and a longer 
tail projection beyond the white tail coverts 
(Table 1). Specimens of Ainley’s appear bulky 
relative to those of Townsend’s Storm-Petrel 
(Figures 17, 18), suggesting that these species 
may have different wing-loadings and thus 
different flight manners, but to our knowl- 
edge the flight manner of Ainley’s is unde- 
scribed. Note that Ainley’s fledge in fresh Ju- 
venal plumage at the same time as fresh- 
plumaged adult Townsend’s are starting to 
breed, and vice versa. 
Ainley’s Storm-Petrel (length 7-7.5” 
[178-190 mm], wingspan 16.7-18.3” 
[424-465 mmj). Size similar to Chapman’s 
but averaging smaller, with a shallower tail 
fork (Table 1). The uppertail coverts are white 
with a variable and irregular dusky median 
stripe or dusky markings (rump score = 
1.5-4, mainly 2-3; Figures 3, 16), and the 
white rump patch typically comprises a little 
more than a third of the rump/tail projection 
beyond the trailing edge of the wings. No 
dark-rumped Ainley’s have been certainly re- 
ported, although a mummihed, dark-rumped 
“chick” found in June 1968 was presumed to 
have been from the winter-breeding popula- 
tion (Crossin 1974). Such birds might be dif- 
ficult if not impossible to distinguish at sea 
from Chapman’s. 
Wing molt timing needs study, but breed- 
ing birds are not usually molting, and it is rea- 
sonable to assume that the adult wing molt 
extends from April to October. First-year 
birds probably molt earlier than adults, as in 
other storm-petrels, and the first-year wing 
molt of Ainley’s thus may be mainly during 
January-July. Given that the summer-breed- 
ing Leach’s, Chapman's, and Townsend’s 
would be in fresh plumage or completing 
wing molt in spring, any “Leach’s” in the ear- 
ly stages of wing molt during spring, or in ad- 
vanced wing molt during sum- 
mer, would be a good candidate 
for Ainley’s Storm-Petrel. 
Relative to Leach’s, Ainley’s is 
smaller, with shorter, slightly less- 
pointed wings and a proportion- 
ately longer but less deeply forked 
tail (Table 1). Ainley’s and Chap- 
man’s are similar in size, but most 
Chapman’s are dark-rumped (a 
condition unconfirmed in Ain- 
ley’s) or have white patches on the 
rump sides in a pattern atypical of 
Ainley's (Figures 6-9, 16). Wing- 
rnolt timing of adult Ainley’s prob- 
ably spans April-October, versus 
August-April in adult Leach’s and 
Chapman’s. Further work based 
upon birds of known taxon may 
bring to light other characteristics, such as 
structure or flight manner, that could aid in at- 
sea identification. 
Occurrence off Southern California 
off California as a whole, Leach’s Storm-Pe- 
trels occur mainly between March and No- 
vember (Ainley 1976). Off southern Califor- 
nia, the Leach’s Storm-Petrel complex is con- 
sidered commonest in June through October 
and least common in December through May 
(Briggs et al. 1987). Leach’s Storm-Petrels 
tend to forage well offshore over warmer wa- 
ters and thus are rarely found on pelagic day 
trips into the relatively cooler waters off cen- 
tral and northern California. In southern Cal- 
ifornia, most records of the Leach’s complex 
are from late summer and fall, which coin- 
cides both with warmer waters (warmer than 
in spring) and with the July-September peri- 
od when most offshore pelagic trips occur. 
Figure 20. Unidentified "Leach's Storm-Petrel" off Santa Barbara, California, 21 
July 2007. This all-dark bird (rump score 5) may simply be a Chapman's Storm-Pe- 
trel, but its relatively compact shape and shallow tail fork — both perhaps artifacts 
of the angle of the photograph — might suggest Townsend's. Photograph by Steve 
N. 6. Howell. 
Figure 21. Presumed Chapman's Storm-Petrel near Sixtymile 
Bank off San Diego, California, 7 September 2006. The brown 
plumage tones (rump score of 4.5 apparent in other photo- 
graphs), rangy structure, and distinct, deep tail notch indicate 
that this bird is not a Townsend's Storm-Petrel. This bird ap- 
pears typical of what many birders simply call "Leach's" (or 
"dark-rumped Leach's") off southern California. Photograph by 
Steve N.G. Howell. 
Figure 22. Probable Townsend's Storm-Petrel near Sixtymile 
Bank off San Diego, California, 7 September 2006. The appar- 
ently blackish plumage tones, compact shape, and bold white 
on the outer uppertail coverts suggest that this bird is a 
Townsend's Storm-Petrel (compare with Figure 21), but how 
much of this could be caused by the angle of the photograph? 
We suspect that this bird is indeed a Townsend's Storm-Petrel, 
but attempting an identification from a single photograph is 
probably unwise in this case and in many others. Birders fortu- 
nate enough to observe storm-petrels in the Leach's complex in 
the Pacific would do well to take extensive series of photo- 
graphs of the birds they observe and to circulate these photo- 
graphs for discussion, as we continue to refine our understand- 
ing of this group. Photograph by Steve N. 6. Howell. 
VOLUME 63 (2009) • NUMBER 4 
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