OCCURRENCE AND IDENTIFICATION OF THE LEACH'S STORM-PETREL COMPLEX OFF SOUTHERN CALIFORNIA 
10 
Figure 3. Variation in uppertail-covert ("rump") pattern for the Leach's Storm-Petrel complex. Simplified scores and half-scores used here are given across 
the top, with corresponding Ainley scale values across the bottom. Scores used in this paper range from 1 (white overall, including solidly white) to S 
(dark overall, including solidly dark). Score 1 can have a few dark marks (rarely visible at sea), and score S can have a few pale markings, likewise rarely 
visible at sea. Score 2 is mostly white, score 3 about 50:50 white/dark, and score 4 mostly dark. Scores of 1 and 2 are typical of white-rumped Townsend's, 
1 .5 and 2.5 are typical of white-rumped Leach's, 2.5-3.5 are typical of Ainley's, and 4-5 are typical of Chapman's and dark-rumped Townsend's. The ex- 
tent of "rump" patches and depth of tail fork shown here are not intended to convey taxonomic significance. Illustration by Ian Lewington. 
apart, and birds breeding 
on the former island have 
been treated as the sub- 
species willetti, which 
has at times been sub- 
sumed with more north- 
ern populations into the 
subspecies heali (Austin 
1952). Power and Ainley 
(1986) showed, however, 
that these two island 
populations were mor- 
phologically similar and 
thus better grouped 
together. 
The wintering range of 
Chapmans is mainly in 
the eastern tropical Pa- 
cific (Crossin 1974, 
Spear and Ainley 2007), 
but some birds may oc- 
cur north to waters off 
Figure 4. Leach's Storm-Petrel in the western Pacific (26° N 142° E), 22 April 2007. 
This bird is probably well away from the geographic range of other taxa and ap- 
pears to be a typical North Pacific example of nominate Leach's. Note the relatively 
limited white rump band (score 1.5-2), its length (measured along the axis of the 
body) about equal to the depth of the tail fork. Photograph by Steve N. G. Howell. 
other taxa in the Leach’s complex as well as to 
other storm-petrels? Here we offer some pre- 
liminary thoughts, based on the literature, 
our review of museum specimens, and our 
observations of thousands of Leach’s Storm- 
Petrels of various taxa on pelagic trips off Cal- 
ifornia (Figure 1); marine topography off 
southern California, with many of the areas 
described below, is illustrated in Figure 2. 
What is known of the overall distribution 
of these four taxa can be summarized as 
follows. 
Leach's Storm-Petrel (0. I. leucorlwa) breeds 
in the North Pacific from Alaska south to Cal- 
ifornia; North Atlantic populations are also 
considered to belong to nominate Icucorhoa. 
Populations from southeastern Alaska to 
northern Baja California average smaller than 
Aleutian (and North Atlantic) 
Leach’s and have been grouped as 
the subspecies beali, including the 
subspecies beldingi described from 
Oregon (Austin 1952, Crossin 
1974). As noted above, the most 
recent analysis (Power and Ainley 
1986) suggests that variation is cli- 
nal in eastern Pacific breeders, 
from larger and whiter-rumped in 
the north to smaller and darker- 
rumped in the south, and that leu- 
corlioa effectively “intergrades” 
with chapmani between the Faral- 
lon Islands and Coronado Islands 
(between 38° and 32° N latitude). 
Birds return to colonies from 
late February through May; egg 
laying is mainly in May-June, with 
fledging from Pacific colonies in 
mid-August to early November (later north- 
wards; Ainley et al. 1974, Gibson and Byrd 
2007). The larger northern-breeding Leach’s 
(with longer and relatively pointed wings) 
winters mainly in the central tropical and 
equatorial Pacific (Crossin 1974, Spear and 
Ainley 2007); Leach’s wintering off California 
(Briggs et al. 1987, P Pyle, pers. comm.) have 
been attributed to the smaller “beali" (with 
shorter and blunter-tipped wings), which 
winters mainly in the eastern Pacific (Crossin 
1974). 
Chapman’s Storm-Petrel (0. I. chapmani) 
breeds on the Coronado Islands and San Ben- 
ito Islands, Mexico, with a nesting schedule 
similar to early-breeding leucorhoa (mainly 
May-September). The Coronado and San 
Benito island groups lie about 500 kilometers 
southern California (P. Pyle, pers. comm.). 
Townsend’s Storm-Petrel (0. socorroensis) 
breeds on islets off the south end of 
Guadalupe Island (mainly Islote Afuera and 
Islote Negro) and perhaps also locally on the 
main island, but most storm-petrels there (in- 
cluding the now-extinct Guadalupe Storm- 
Petrel, 0. macwdactyla) have been eliminated 
by feral cats (Ainley 1980, Ainley 1983, 
Crossin 1974, Jehl and Everett 1985). The 
population on Islote Negro has been estimat- 
ed at 4000 birds and on Islote Afuera at 3000 
birds (Crossin 1974, Ainley 1983). Egg laying 
is mainly in late May-June, with fledging in 
October-November. The known at-sea range 
lies in the eastern Pacific between 35° N and 
10° N (Crossin 1974), but seasonal move- 
ments are not well known. 
Ainley’s Storm-Petrel (0. cheimomnestes) 
breeds on three islets (Islote Negro, Islote 
Afuera, and Gargoyle Rock) off the southern 
end of Guadalupe Island, Mexico (Ainley 
1980, 1983, Crossin 1974, Jehl and Everett 
1985). Crossin (1974) proposed reviving the 
subspecies name kaedingi for the winter- 
breeding population, whose distinctiveness he 
recognized, but the type specimen of kaedingi 
(Carnegie Museum #22219) is from the sum- 
mer-breeding population, for which the name 
socorroensis has priority (Ainley 1980). 
The breeding population of Ainley’s Storm- 
Petrel may not be in excess of a few thousands 
birds. Egg laying is mainly in November-De- 
cember, with fledging in April-May. The at- 
sea range has not been described previously, 
but we presume that birds range over waters 
off the Baja California peninsula during their 
542 
NORTH AMERICAN BIRDS 
