OCCURRENCE AND IDENTIFICATION OF THE LEACH'S STORM-PETREL COMPLEX OFF SOUTHERN CALIFORNIA 
Figure 11. Presumed Townsend's Storm-Petrel off San Diego, California, 20 
July 2008. This photograph conveys well the relatively compact build, dark 
plumage aspect, relatively duller pale upperwing bands than Chapman's, 
and large white rump patch of this taxon. Note that the extent of the white 
rump patch (score 1) appears greater than the short tail. Note also the fairly 
shallow fork in the tail. Photograph by Steve W. G. Howell. 
Figure 12. Presumed Townsend's Storm-Petrel off Santa Barbara, California, 
21 July 2007. Besides the fairly dark plumage aspect, note the large white 
rump patch (score 1) and short tail with a fairly shallow fork. Photograph by 
Steve N. G. Howell. 
figure 13. Presumed Townsend's Storm-Petrel off Santa Barbara, California, 
21 July 2007. This image suggests a small, fairly compact, and short-tailed 
bird relative to North Pacific (and North Atlantic) Leach's, although the tail is 
foreshortened at this angle. The white rump patch (score 1) appears greater 
in extent than the brown of the tail and could suggest to some observers a 
Wedge-rumped Storm-Petrel, the largest individuals of which are about the 
same size as a small Townsend's. Photograph by Steve N. G. Howell. 
Figure 14. Presumed Townsend's Storm-Petrel off Santa Barbara, California, 
21 July 2007. This image conveys well the compact build, dark plumage as- 
pect, and short tail of this species. As in Figures 1, 9, and 1 1 -1 3, the white 
rump patch (score 1) dominates the caudal projection of the bird, being 
greater in extent than the short, shallowly forked tail. Photograph by Steve 
N.G. Howell. 
Figure IS. Presumed Townsend's Storm-Petrel (rump score 4.S), near Six- 
tymile Bank off San Diego, California, 7 September 2006. Although perhaps 
not identifiable to taxon from this photograph, the notably small size and 
compact shape of this individual were readily apparent in the field to How- 
ell and McGrath, who noted the bird's superficial resemblance to Least 
Storm-Petrel. Note the relatively rounded wing-tips, which are typical of 
southern taxa in the Leach's complex (Townsend's, Ainley's, and some Chap- 
man's); the wings of northern-breeding populations tend to be longer and 
more pointed. Photograph by Steve N. G. Howell. 
the white rump patch is rela- 
tively limited in extent, com- 
prising less than half of the 
rump/tail projection beyond 
the trailing edge of the wings 
(Figure 3). Fully dark-rumped 
birds are very rare in Pacific 
Leach’s (and unknown in the 
Atlantic; Flood 2009) but 
common in Chapman’s. From 
Alaska to Washington, Leach’s 
have a rump score of 1-2, 
whereas by central California, 
this changes to a score of 1-5, 
with most being 1-2 (Figure 
5). Good views are needed to 
assess rump score accurately 
at sea, and birds scoring 
3. 5-4. 5 can appear solidly 
“dark-rumped” at a distance. 
Wing molt of breeding 
adults starts in August-Octo- 
ber and completes in Febru- 
ary-April, with the tail molted 
in July-September. First-year 
birds start wing molt in 
May-July and complete it in 
October-December; subse- 
quent molts presumably syn- 
chronize with the adult sched- 
ule (Ainley et al. 1976, Spear 
and Ainley 2007; Howell, un- 
publ. data). A few Leach’s (and 
Chapman’s) off southern Cali- 
fornia in late summer and fall 
show molt of rectrices and in- 
ner primaries, but such birds 
are quite rare, which suggests 
that most Pacific Leach’s may 
remain to the south in their 
first summer. 
The bounding, nighthawk- 
like flight of Leach’s Storm-Pe- 
trel mentioned in field guides 
is a good thing to learn well as 
a yardstick for learning the other 
species. As a rule, relative to other 
taxa in the complex, nominate Leach’s 
exhibits the most unpredictable use of 
three-dimensional space in its flight, 
such that its erratic course is often dif- 
ficult to track with a camera. That 
said, the slightly smaller California 
Leach’s may have a “mellower” flight 
more like that of Chapman’s. 
Chapman’s Storm-Petrel (length 
6.8-7. 7” [173-196 mm], wingspan 
17.2-18.7” [437-475 mmj) averages 
smaller than Leach’s but, on average, 
has a slightly deeper tail fork (Table 1; 
see also Ainley 1980). The uppertail coverts 
range from having obvious white (on some 
Coronado Island birds) to being all-dark (on 
most San Benito Island birds). Overall rump 
score = 2.5-5, mainly 4-5 (Figures 3, 6-9), 
with white-rumped birds comprising about 
10% of the Coronado population and 0% of 
the San Benito population (Ainley 1980). The 
molt schedule of Chapman’s appears similar 
to that of Leach’s. 
The flight of Chapman’s is much like that 
of Leach’s but averages weaker, less confident- 
looking, with slightly quicker and often shal- 
lower wingbeats; Chapman’s tends to be easi- 
er to track with a camera. At times Chapman’s 
can recall a purposeful Ashy Storm-Petrel, but 
VOLUME 63 (2009) • NUMBER 4 
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