OCCURRENCE AND IDENTIFICATION OF THE LEACH'S STORM-PETREL COMPLEX OFF SOUTHERN CALIFORNIA 
Figure 16. Typical variation in rump pattern of Ainley's Storm-Petrels collected in winter at Guadalupe Island, Baja California, from 
mostly white (score 1 .5) to heavily mottled with dusky (score 3.5); 
SDNHM specimens. Photograph by Steve N. 6. Howell. 
Figure 17. Comparison of fresh-plumaged adult Townsend's 
Storm-Petrel (left; SDNHM specimen 17672, 13 July; rump score 
1 ) and fresh-plumaged juvenile Ainley's Storm-Petrel (right; 
SDNHM specimen 37473, 14 April; rump score 2). Despite overlap 
in most conventional measurements between these two species, 
the larger bulk of Ainley's is usually readily apparent when the 
two are compared directly. Photograph by Steve N. G. Howell. 
Chapman's is still generally a stronger flier 
with fairly clipped and jerky wingbeats rela- 
tive to the more fluttery Ashy. 
I Ainley's with all-white or all-dark rumps have been confirmed. 
Townsends Storm-Petrel (length 6. 5-7. 2” 
[165-183 mm], wingspan 16.2-17.5” 
[411-444 mm]) is the smallest and shortest- 
tailed taxon in the Leach’s complex (Ober- 
holser 1919, Ainley 1980, Table 1). The up- 
pertail coverts are variable, being bright white 
on some birds (rump score = 1-3, mainly 
1-2), mostly dark on. others (rump score = 
mainly 4; Figures 2, 9). Some 80-90% of birds 
on Islote Negro are dark-rumped, whereas on 
the nearby Islote Afuera 70-90% are white- 
rumped (Crossin 1974, Ainley 1983). Analy- 
ses of vocalizations and genetics of these pop- 
ulations might shed light on their taxonomic 
status. The tail is relatively short, and the 
white rump patch typically comprises half or 
more of the rump/tail projection beyond the 
trailing edge of the wings (Figures 9-15). 
Assuming molt relates to breeding as in 
Leach’s Storm-Petrel (and given that Town- 
send’s is smaller and may require less time for 
its complete molt), wing molt in breeding 
adults likely starts in August-October and 
completes in February-March; the hrst-year 
wing molt may start in June-August and com- 
plete in October-December. 'We have not seen 
any presumed Townsend’s in wing molt off 
southern California in July-September. 
The flight manner of Townsend’s requires 
critical study, but in calm to light winds it is a 
fairly fast and strong Hier, with fairly deep, 
clipped wing beats and a more direct and 
steadier (less three-dimensional) flight than 
the jerkier, more-confident bounding flight of 
Leach’s. Relative to Leach’s and Chapman’s, 
Townsend’s is smaller (which can be readily 
apparent, even when direct comparison is 
Figure 18. Comparison of fresh-plumaged adult Townsend's 
Storm-Petrel (left; SDNHM specimen 17672, 13 July) and fresh- 
plumaged adult Ainley's Storm-Petrel (right; SDNHM specimen 
30177, 2 November). Collected at corresponding points in their 
breeding cycles, these specimens with comparable plumage 
wear illustrate the appreciably darker plumage aspect of 
Townsend's. Also, as in Figure 17, note the obviously larger bulk 
of Ainley's. Photograph by Steve N. G. Howell. 
lacking) and shorter-tailed, with a shallower 
tail fork, accounting for its more compact, 
less rangy shape (Figure 9). The plumage of 
Townsend's is darker overall, more blackish 
than brownish, and the white rump patch is 
often solidly white and more extensive, in rel- 
ative terms, than on Leach’s, with a shorter 
tail projection beyond the white (Table 1); 
thus it may even suggest a Wedge-rumped 
Storm-Petrel (Oceanodroma [Halocyptena] 
tethys). In dark-rumped birds, the pale upper- 
wing band of Townsend’s averages duller than 
the relatively bolder, brighter band of Chap- 
man’s (Figures 7-9, 11-14), and it is possible 
to mistake dark-rumped Townsend’s for Least 
Storm-Petrel (Oceanodroma [Halocyptena] mi- 
crosofna) (Figure 15). 
Whether Townsend’s and Ainley’s Storm- 
Petrels can be distinguished at sea is not cur- 
rently known, but from specimens, Ainley’s 
appears more like a typical Leach’s and is thus 
generally slightly paler overall than 
Townsend’s, with a duller white rump patch 
that usually has a dusky median stripe or 
messy dusky markings that, in series, appear 
subtly but qualitatively different from Leach’s 
(Figure 16). Ainley’s averages larger than 
546 
NORTH AMERICAN BIRDS 
