OCCURRENCE AND IDENTIFICATION OF THE LEACH'S STORM-PETREL COMPLEX OFF SOUTHERN CALIFORNIA 
Table 1. Comparative measurements (ranges, with means in parentheses; in mm) of Leach's (Pacific birds only), 
Chapman's, Townsend's, and Ainley's Storm-Petrels. "Rump"indicates the extent of white uppertail coverts (measured 
at their center, from anterior to posterior edge, mt laterally). This aspect was not measured on Chapman's specimens, 
which are mostly dark-rumped. The measurement"Tail > White" indicates the length of tail projecting beyond white 
uppertail coverts. The Leach's (n=40) were a mix of birds from Alaska and Oregon; all Chapman's, Townsend's, and 
Ainley's were collected from the breeding islands with the exception of two Chapman's, one Townsend's, and the four 
specimens of Ainley's that are discussed in the text. 
Wing 
Chord 
Tail 
Tail Fork 
Rump 
Tail> 
White 
Leach's 
145-162 
70-86 
13-25 
15-25 
36-45 
n = 40 
(151) 
OQ 
i — 
(19) 
(22) 
(42) 
Chapman's 
144-157 
69-82 
14-24 
n = 40 
(151) 
(77) 
(20) 
Townsend's 
135-147 
66-77 
11-17 
20-30 
30-41 
n = 70 
(142) 
(71) 
(15) 
(24) 
(34) 
Ainley's 
141-153 
70-80 
12-20 
20-30 
35-45 
n = 45 
(146) 
(77) 
(16) 
(23) 
(40) 
Figure 10. Variation in rump pattern of Townsend's Storm-Petrels collected in summer at Guadalupe Island, Baja California, 
showing a typical range from all-white (score 1) to mostly dark (score 4.5); some individuals, not shown here, have solidly 
dark rumps (Ainley 1 980). Also note that the short tails have a relatively shallow tail fork. SDNHM specimens. Photograph 
by Steve N.G.Howeli. 
which vary from from all-white to all-dark. A 
scale of “rump” patterns, grading from all- 
white (score 1) to all-dark (score 11), was de- 
vised by Ainley (1980), who scored 391 
Leach’s (47 Atlantic and 344 Pacihc), 103 
Chapman’s, 137 Townsend’s, and 127 Ain- 
ley’s. We use a simplified scale of five steps 
that can be employed more easily in the field 
(Figure 3; Howell, in prep.; cf. Crossin 1974) 
and below have converted “Ainley scale” val- 
ues to our five-step scale; half-step values can 
be estimated in our scale if desired, such as 
when evaluating photographs. Basically, our 
scores of 1-2 equate to white-rumped and 
4-5 to dark-rumped, when viewed under at- 
sea conditions. 
Flight manner obviously varies, depending 
on wind speed and on what a bird is doing. 
The simplified flight descriptions below con- 
vey typical traveling and foraging 
flight manners in light to moderate 
winds, which is how most observers 
see these birds at sea. 
Leach’s Storm-Petrel (length 7.5- 
8.7” [190-221 mm], wingspan 
17.3-19.3” [439-490 mm]). This is 
the largest and longest-winged taxon 
(Table 1), and the most familiar point 
of reference for most observers (Fig- 
ure 4), although California birds aver- 
age smaller and shorter-winged than 
Aleutian and North Atlantic birds. 
The rump patch is mostly white, often 
with variable dusky markings concen- 
trated down the center, but a few 
birds are darker (overall rump score = 
1-5, mainly 1-3; Figure 3). Typically, 
Figure 7. Presumed Chapman's Storm-Petrel off southern California (33° N 
1 20° W), 6 September 2006 (rump score 4.S). Note the fairly rangy and 
long-tailed shape and generally brownish plumage tones. Photograph by 
Steve N.G. Howell. 
Figure 8. Presumed Chapman's Storm-Petrel off Santa Barbara, California, 
21 July 2007 (rump score 4.S). This photograph conveys well the rangy build, 
long and narrow tail, bold pale upperwing bands, and overall sooty-brown- 
ish plumage of this taxon. Photograph by Steve N. G. Howell. 
Figure 9. Presumed Chapman's Storm-Petrel (front; rump score 4) and 
Townsend's Storm-Petrel (behind; rump score 1) off Santa Barbara, Califor- 
nia, 21July 2007. Relative to Chapman's, the Townsend's appears shorter- 
tailed and darker overall, with a broad white rump band and duller pale up- 
perwing bands. Photograph by Steve N. G. Howell. 
544 
NORTH AMERICAN BIRDS 
