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NORTHWESTERN NATURALIST
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90:238–243
WINTER 2009
TIMING AND SUCCESS OF BREEDING IN PELAGIC CORMORANTS
AT TRIANGLE ISLAND, BRITISH COLUMBIA, 2003–2008
J MARK HIPFNER
AND JENNIFER
Key words: breeding biology, British Columbia, interannual variability, northeast Pacific, Pelagic Cormorant, Phalocrocorax pelagicus,
Triangle Island
Of the 6 cormorant species that inhabit the
North Pacific, the Pelagic Cormorant (Phalocrocorax pelagicus) is the smallest and most widely
distributed (Hobson 1997). Pelagic Cormorants
breed from Baja California in the eastern Pacific
north to the Bering and Chukchi Seas, and south
across the western Pacific to southern China.
North America supports an estimated one-third
of the global population of 400,000 birds (Johnsgard 1993), with the bulk breeding in Alaska.
Campbell and others (1990) estimated that only
about 4200 pairs bred in British Columbia,
mostly at small and widely dispersed sites
centered within the Strait of Georgia.
Information on the population status of
Pelagic Cormorants in British Columbia is
limited. Populations appear to have declined
over recent decades in the Haida Gwaii archipelago (Fraser and others 1999), although there
is some question about the reliability of the
baseline data (Harfenist and others 2002). More
conclusive evidence suggests that dramatic
declines have occurred in the southern Strait
of Georgia (by about 50% since 1987; Chatwin
and others 2002), and along the west coast of
Vancouver Island (by about 85% since 1969;
Carter and others 2007). The declines have been
attributed to disturbance at breeding colonies
by humans and Bald Eagles (Haliaeetus leucocephalus), and reductions in prey availability due
to oceanographic change (Chatwin and others
2002; Carter and others 2007). Persistent contamination and mortality due to oiling and
fisheries practices might also be contributing
factors (Harris and others 2005; Smith and
Morgan 2005).
L GREENWOOD
There is at present very little information on the
timing and success of Pelagic Cormorant breeding
at colony sites in British Columbia, especially for
regions outside of the Strait of Georgia (Drent and
others 1964; Harris and others 2005). This
information is pertinent to understanding causes
of regional population change, and given declining population trajectories, there is a need for
basic demographic measures.
Triangle Island is situated about 40 km west
of Cape Scott off the northwestern tip of
Vancouver Island. This site supports one of
British Columbia’s largest Pelagic Cormorant
breeding aggregations, comprising over 400
breeding pairs in 1989 (Rodway and others
1990). Here we report on a 6-y study (2003–
2008) of the timing and success of breeding in
Pelagic Cormorants at Triangle Island, through
a period of extreme oceanographic variation
along the British Columbia coast (Mackas and
others 2007). This report constitutes the most
comprehensive account of the biology of the
species at this comparatively pristine site, and
should provide a baseline against which future
changes can be measured.
Field crews collected data on Triangle Island
(UTM: Zone 09, 0494480E 5634395N, WGS84)
from late March until late August in each year
between 2003 and 2007, and from mid-May
until mid-August in 2008. Beginning on arrival
in the spring, we logged systematic, detailed
observations of all cormorant nests situated
within the borders of a small cliff-face plot
(approximately 25 m2) established in 2002 on
Puffin Rock, a small islet adjoined to the main
island at low tide. Our viewing blind was
situated about 50 m from and 2 m above the
highest nest on the plot, and provided a good
view into all nests when adults stood up.
For all active nests, we recorded initial laying
dates, hatching dates, and dates when a chick
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TABLE 1. Reproductive parameters for Pelagic Cormorants at Triangle Island, British Columbia, 2003–2008.
First clutches
Year
n
Hatch
(%)
2003
2004
2005
2006
2007
2008
24
20
15
13
15
0
16 (67)
16 (80)
2 (13)
8 (62)
13 (87)
—
Fledge
(%)
9 (56)
13 (81)
0 (0)
6 (75)
2 (15)
—
Replacement clutches
Successful
(%)
n.(%)1
9 (38)
13 (65)
0 (0)
6 (46)
2 (13)
—
2
2
3
1
0
(25)
(50)
(23)
(20)
(0)
—
Hatch
(%)
Fledge
(%)
1
2
0
0
0 (0)
2 (100)
—
—
—
—
(50)
(100)
(0)
(0)
—
—
Totals
Successful
(%)
0
2
0
0
(0)
(100)
(0)
(0)
—
—
No.
successful
nests (%)
9
15
0
6
2
(38)
(75)
(0)
(46)
(13)
0
Chicks
fledged,
all nests
12
26
0
9
2
0
1
Number and percentage of active nests where the 1st clutch failed to hatch and was subsequently replaced. For example, in 2003, 24 first eggs
were laid, of which 16 hatched and 8 did not. Of the 8 eggs that failed, 2 (25%) were replaced.
was no longer present. We used these observations to determine hatching success (the proportion of active nests in which at least 1 egg
hatched), fledging success (the proportion of
nests in which at least 1 nestling survived at
least 30 d), and breeding success (the proportion of nests in which at least 1 nestling
survived to fledge). To facilitate comparisons
of overall production among colonies, however,
we also report frequency distributions for clutch
size (total eggs), and the number of fledglings
produced per nest. Because Pelagic Cormorants
often replace lost clutches, we followed failed
nests and recorded the same set of parameters if
a replacement clutch was laid.
Timing of Breeding
The number of active Pelagic Cormorant nests
on our monitoring plot varied from 0 (2008) to
24 (2003) across the 6 y, with lower numbers in
more recent years (Table 1). The season’s 1st
egg was laid as early as 15 May in 2003 (day
135) and as late as 4 June in 2007 (day 155),
while median laying dates varied from 30 May
in 2003 (day 150) to 19 June in 2006 (day
170)(Fig. 1). No nests were built and no eggs
were laid on the breeding plot in 2008. Across
all years, individual pairs initiated 1st clutches
between mid-May and mid-July. Laying was
especially synchronous in 2007 and especially
asynchronous in 2004 and 2005 (Fig. 1). Annually, replacement clutches were laid at 0 to 25%
of nests at which the 1st clutch was lost prior to
hatching (Table 1), and these replacement
clutches extended clutch initiation dates to as
late as 3 August in 2006 (day 215)(Fig. 1).
Historical information on timing of breeding at
Triangle Island is sparse. Rodway and others
(1990) reported that nests contained eggs in mid-
June in 1989, and in mid-July in 1982, 1984, and
1985. Chicks were seen in nests on 19 and 23 July
in 1982 (days 200 and 204), with 1 chick described
as ‘‘40 cm tall, starting to feather’’, suggesting that
it was several weeks old. In 1989, Rodway and
others (1990) reported simply that ‘‘most nests
held large young’’ in early August. In more recent
years (Triangle Island Research Station, unpubl.
data), no chicks were seen in 22 nests on 21 June
1998 (day 172), but 9 of these nests held chicks on
8 July (day 189). Similarly, only 1 of 65 nests held a
chick on 26 June in 2001 (day 177), but 5 of these
nests held chicks on 28 June (day 179). In 2002, the
1st chick of the season in 32 nests hatched
between 15 and 19 June (days 166 and 170). Based
on these observations, it appears that there has
been little systematic change in the timing of
breeding over recent decades, while timing varied
markedly from year to year (Fig. 1). The same is
true at other Pelagic Cormorant breeding sites
(Boekelheide and others 1990).
The timing of breeding at Triangle Island is
similar to that at other sites in British Columbia.
Peak laying occurs between late May and midJune at Mandarte Island (Drent and others
1964), and between early June and early July
in Pacific Rim National Park (Hatler and others
1978). Timing at Triangle Island also is similar
to that at the Farallon Islands, California, where
laying peaks in late May (Boekelheide and
others 1990), and on Middleton Island, Alaska,
where laying peaks in late May to mid-June
(Hatch and Hatch 1990).
Clutch size
Clutch sizes varied from 1 to 4 eggs in 2003,
2004, and 2005, and from 2 to 4 eggs in 2006 and
2007 (Fig. 2). Again, no eggs were laid on the
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FIGURE 1. Timing and success of breeding for Pelagic Cormorants at Triangle Island, British Columbia, in
2003–2007. Laying dates are grouped by 5-d intervals and represent clutch initiation dates. No nests were built
and no eggs were laid on the study plot in 2008.
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FIGURE 2. Distributions of clutch size (1–4 eggs, at top) and number of fledglings per nest (0–3, at bottom) of
Pelagic Cormorants at Triangle Island, British Columbia in 2003–2007. No pairs bred on the study plot in 2008.
study plot in 2008. There was little seasonal
trend in clutch size, although weak seasonal
declines have been reported elsewhere (Boekelheide and others 1990). The modal clutch size
was 3 eggs in all 5 y from 2003 to 2007, with
means between 2.3 (2005) and 3.2 (2007) eggs.
Replacement clutches were somewhat smaller,
consisting of 2 (in 6 of 8 cases) or occasionally 3
(2 cases) eggs. While Pelagic Cormorants occasionally lay up to 8 eggs (Campbell and others
1990), clutch sizes at Triangle Island were
within the normal range for the species; means
across the northeast Pacific range from 2.5 to 4
eggs, with no geographic cline evident (Hobson
1997; Sydeman and others 2001).
Breeding success
Breeding success at Triangle Island varied
dramatically over the 5 y of our study: 13 to
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87% of egg-laying pairs hatched 1 or more eggs,
and 0 to 65% of pairs that hatched at least 1 egg
had 1 or more chick survive to fledge (Table 1).
Overall, at least 1 chick fledged from 0 to 65% of
1st clutches, depending on year. Only 2 of 8
pairs that re-laid after losing their 1st clutch
fledged a chick, both in 2004. Including replacement clutches, 0 to 75% of pairs succeeded
annually (Table 1). Of note, no pairs bred
successfully in 2005, a year of widespread
failure for seabird breeding across the California Current system (Sydeman and others 2006).
In addition, the lack of breeding on the study
plot in 2008 apparently was due to constant
disturbance from Bald Eagles, possibly associated with inactivity at a Peregrine Falcon (Falco
peregrinus) eyrie near the cormorant plot. In
other years, the falcon parents kept Bald Eagles
out of the area.
We rarely knew the cause of breeding failure.
Most failures occurred after hatching (Table 1),
possibly because chicks simply starved to death
or were evicted by older and stronger nestmates. Except in 2004, when all 12 clutches laid
on or before the median laying date were
successful compared to only 3 of 8 laid after
the median date, there was little systematic
within-season variation in breeding success
(Fig. 1). The modal number of fledglings produced per nest was 0 in 4 of 5 y; however, in
2004 the modal number was 2 and 1 nest
fledged 3 chicks (Fig. 2). In total, 0 to 26
fledgling cormorants were produced annually
from within the borders of our monitoring plot.
Such extreme interannual variation in breeding success appears to be typical of Pelagic
Cormorants throughout their northeast Pacific
range. At the Farallon Islands, California,
Pelagic Cormorants produced 0 to 2.4 fledglings
per active nest in 11 y (Boekelheide and others
1990). More recent analyses suggest little temporal change since then (Sydeman and others
2001). Further north, Pelagic Cormorants produced 0.1 to 1.4 fledglings per active nest on the
Semidi Islands, Alaska, over 5 y (Hatch and
Hatch 1990). Success was more consistent in just
2 y at Mandarte Island, British Columbia in the
late 1950s where 74 and 79% of active nests
were successful (Drent and others 1964). However, Harris and others (2005) reported considerably greater variance in reproductive success
(0 to 2.0 fledglings per active nest) at Mandarte
90(3)
Island and other colonies in the Strait of Georgia
between 1989 and 1994. On the whole, it
appears that Pelagic Cormorant breeding success at Triangle Island during the 5 y of this
study was fairly typical of the species.
Acknowledgments.—We thank many field workers
for their efforts collecting these data. Funding was
provided by the Nestucca Oil Spill Trust Fund, the
Centre for Wildlife Ecology (Simon Fraser University
and the Canadian Wildlife Service), World Wildlife
Fund Canada, and the Migratory Birds and Science
Horizons program of Environment Canada. We
received invaluable ship and helicopter support from
the Canadian Coast Guard. Thanks to M Court, J
Higham, and C Smith for logistical support from
Vancouver.
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