Territory Quality and Reproductive Success of Black Oystercatchers in British Columbia
Author(s): Stephanie L. Hazlitt
Source: The Wilson Bulletin, Vol. 113, No. 4 (Dec., 2001), pp. 404-409
Published by: Wilson Ornithological Society
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Wilson Bull., 113(4), 2001, pp. 404-409
TERRITORY QUALITY AND REPRODUCTIVE SUCCESS OF
BLACK OYSTERCATCHERS IN BRITISH COLUMBIA
STEPHANIE L. HAZLITT' 2
ABSTRACT.-I investigated the relationship between the physical characteristics of breeding territories and
the reproductive success of Black Oystercatchers (Haematopus bachmani) in British Columbia. I compared
measures of the threat of egg and brood depredation, conspecific competition, and food supply with measures
of reproductive success on 38 oystercatcher breeding territories during 1996 and 1997. Oystercatchers breeding
on territories near Glaucous-winged Gull (Larus glaucescens) colonies had smaller first clutches than pairs free
of neighboring gulls. Oystercatcher pairs hatched and consequently produced more young on shallow sloping
intertidal shoreline sites compared to steep-sloped islets and shorelines. Received 28 Feb. 2001, accepted 25
Nov. 2001.
Variation in reproductive success is believed to result when there is variation in resources available to individuals (Noordwijk
and De Jong 1986). For avian species that defend breeding territories where the majority of
foraging and chick provisioning is done, variation in seasonal fecundity should be dependent largely upon territory quality (Hogstedt
1980). Various types of evidence have been
used to infer avian territory quality (reviewed
by Ens et al. 1992). For example, breeding
pairs on territories clustered in space, with
long tenure of use and occupied early in the
season, had high reproductive success and
were therefore considered to be occupying
high quality sites (Ens et al. 1992, Bollmann
et al. 1997). In many studies, physical characteristics of the territory have been associated with breeding success (e.g., Brodmann et
al. 1997). Finally, some studies have experimentally revealed the characteristics that explain the variation in reproductive success,
such as the amount of nest site cover in Magellanic Penguins (Spheniscus magellanicus;
Stokes and Boersma 1998).
Black Oystercatcher (Haematopus bachmani) territories often are clustered together
on small islands, they have a long duration of
use, and they vary in occupation through the
years (Vermeer et al. 1989, Hazlitt and Butler
2001). Reproductive success of individual
Black Oystercatcher pairs is highly variable
I Dept. of Biological Sciences, Simon Fraser Univ.,
Burnaby, BC V5A 1S6, Canada.
2 Current address: Bird Studies Canada, 5421 Robertson Rd., Delta. BC V4K 3N2, Canada;
E-mail: stephanie.hazlitt@ec.gc.ca
(Hartwick 1974, Groves 1984, Andres 1999,
Hazlitt and Butler 2001). Some factors related
to hatching success, fledging success, and productivity of oystercatchers include protection
of nest sites from predators and from surf
wash out (Hartwick 1974, Groves 1982, Nol
1989, Vermeer et al. 1992), conspecific competition in the form of aggressive interactions
among breeding pairs and toward young (Hartwick 1974, Ens 1992, Ens et al. 1992), and
the food supply available in the territory for
provisioning young (Hartwick 1974, Nysewander 1977, Groves 1982, Nol 1989, Ens et
al. 1992, Andres 1999). Of forest-nesting
predators, Northwestern Crows (Corvus caurinus) and Common Ravens (C. corax) represent the greatest risk to oystercatcher eggs and
chicks in British Columbia (Groves 1982;
Vermeer et al. 1989, 1992), with both species
nesting in trees and shrubs on islands in the
Strait of Georgia and foraging near oystercatcher nest sites (Verbeek and Butler 1999).
The purpose of this study was to investigate
the relationship between the physical characteristics of breeding territories and the hatching success, fledging success, and productivity
of Black Oystercatchers in the Strait of Georgia, British Columbia.
METHODS
I conducted the study in the southern Gulf Islands,
Strait of Georgia, British Columbia (480 35' N, 1230
15' W). The Gulf Islands are an archipelago of 284
islands and islets between Vancouver Island and the
mainland of British Columbia. The study area included
21 Gulf Islands and islets near the town of Sidney.
Oystercatchers in this region nest mostly on small
sparsely vegetated islands (Vermeer et al. 1989). All
islands in the study area. regardless of size or forest
404
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Hazlitt
*
405
TERRITORY QUALITY OF BLACK OYSTERCATCHIERS
TABLE 1. Characteristics of 38 Black Oystercatcher breeding territories in the southern Gulf Islands, Strait
of Georgia, British Columbia, 1996 and 1997.
Territory characteristic
Length of shoreline (m)
Intertidal slope (degrees)
Presence of breeding gullsa
Number of neighboring territories
Distance to mainland/forested
island (m)
a
Mean
75
20
0.5
1.3
943
Median
53
18
1.0
2.0
960
SD
Range
53
10
0.5
0.9
25-245
9-45
0-1
0-2
574
0-1760
CV (%)
71.2
51.1
96.1
68.1
60.8
Indicates proximity to Glaucous-winged Gull breeding colony (O = no colony. I = colony on same island).
cover, were searched between 21 April and 30 May
1996 and between 9 May and 30 May 1997 for breeding pairs of Black Oystercatchers. All islands were circled by motorboat and islands where oystercatchers
were observed were searched on foot for evidence of
breeding.
I considered an oystercatcher breeding territory to
be the area of shoreline beach and the adjacent intertidal area that was defended against all other oystercatchers during the breeding season (Askenmo et al.
1994). Territory size was assessed by observations of
territory boundary disputes between neighboring oystercatcher pairs along the linear shoreline, which occur
approximately twice per hour (Purdy and Miller 1988).
The larger or longer territories tended to be entire islets
or rocky outcroppings occupied by a single pair. It was
difficult to assess boundaries in these cases, and hence
the entire islet was considered a territory. I visited established breeding territories by boat approximately
every 5 days until the breeding attempt failed or chicks
had fledged. Reproductive failure was assumed when
no replacement clutch was laid ?14 days after egg
loss, when chicks were not on the territory for two
consecutive visits, or when adults were no longer resident on the territory for two consecutive visits.
I measured five territory characteristics after breeding attempts were completed. These characteristics
were chosen as measures of the threat of egg and brood
depredation, conspecific competition, and food supply.
(I) As an index of predation threat from corvids, I
measured the distance of the nest scrape from the forested mainland or nearest forested island from a 1:
80,000 scale marine chart; (2) I recorded the presence
or absence of breeding Glaucous-winged Gulls (Larus
glaucescens) on the island or islet; and (3) I recorded
the number of immediate neighboring oystercatcher
territories (none, one, or two neighboring pairs) for
each territory. As indices of the intertidal food supply
within each breeding territory, I measured (4) the
length of the shoreline at high tide with a measuring
tape; and (5) I measured the slope of the littoral zone
with a clinometer at high, mid, and low points along
transects perpendicular to the shoreline every 5 m. I
reasoned that a long, shallow-sloped territory had a
greater area and hence a greater food supply than a
shorter, steep-sloped territory for a given tidal cycle.
The measures of reproductive success were size of
first clutch, size of replacement clutch, hatching success (percent eggs hatched), brood size at hatching,
fledging success (percent hatchlings fledged), and pair
productivity (number of fledglings/pair). First clutch
size was the clutch size observed before May 30 or
known to be the first clutch. Some pairs that lost their
eggs early in the season laid a replacement clutch. I
used the total number of eggs laid for each pair to
calculate hatching success; however, to test whether
the low rate of hatching success on steep territories
was related to the high rate of first clutch loss, I calculated hatching success as the number of hatchlings
from the number of eggs in the successful clutch.
Chicks were considered fledged when they reached 35
days of age or were capable of flight (usually 38-40
days; Andres and Falxa 1995).
I performed statistical analyses with SAS 6.12 for
Windows. To prevent pseudoreplication in the statistical analyses, I calculated a mean reproductive success
measure for territories that were occupied during both
breeding seasons (n = 30), except for first clutch size,
which was reported for each season; the sample size
for first clutch size includes the number of first clutches located both years. Reproductive success data were
weighted by the number of estimates in multiple regression analyses and all proportional data were transformed using arcsine transformation (Zar 1984).
1 determined correlations among territory characteristics with Spearman's rank correlation tests. Stepwise
multiple regression models were used to investigate
the relationship between the dependent reproductive
success variables (first clutch size, hatching success,
fledging success, and productivity) and the independent territory characteristic variables. No multiple regression models produced more than one significant (P
< 0.05) predictor.
RESULTS
Mean territory length was 75 m (Table 1).
There was no significant correlation between
the length of shoreline defended and the intertidal slope of the territory (Table 2). Of the
38 territories, 20 were located at the edge of
Glaucous-winged Gull breeding colonies, and
most territories (28 of 34) had one or two
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THE WILSON BULLETIN * Vol. 113, No. 4, December 2001
406
TABLE 2. Correlation matrix (Spearman's r2 values) of Black Oystercatcher breeding territory (n = 38)
characteristics in the southern Gulf Islands, Strait of Georgia, British Columbia, 1996 and 1997.
Intertidal slope
Presence
Presence of
breeding gulls
Number of
neighboring
territories
0.04
-0.45*
0.56**
-0.18
-0.37*
0.19
-0.00
0.11
0.03
Length of shoreline
Intertidal slope
Distance to mainland
Distance to
mainland
-
0.40*
of breeding gulls
Indicates a signiticant correlation of P < 0.05.
** Indicates a significant correlation of P < 0.0001.
*
neighboring oystercatcher breeding pairs.
Both the number of neighboring oystercatcher
territories and the distance from the mainland
or nearest forested island were significantly
positively correlated with the presence of
Glaucous-winged Gulls (Table 2). Also, the
number of neighboring oystercatcher temtories was significantly negatively correlated
with intertidal slope, therefore territories along
shallow sloping shorelines had more neighboring pairs compared to steep sloping shoreline territories (Table 2).
The only significant predictor of first clutch
size, for both years, was the presence of
breeding Glaucous-winged Gulls (1996: n =
30, r2 = 0.14, F = 4.56, P = 0.042; 1997: n
= 31, r2 = 0.27, F = 10.90, P = 0.003). Mean
first clutch size was significantly smaller on
oystercatcher territories located on islands occupied by gulls than on islands without gulls
during 1996 (with gulls: 2.00 ? 0.07 SD, n =
19; without gulls: 2.50 + 0.50 SD, n = 11;
Wilcoxon test, Z = 1.94, P = 0.05) and during
1997 (with gulls: 1.70 ? 0.90 SD, n = 17;
without gulls: 2.60 + 0.70 SD, n = 14; Z =
2.80, P = 0.005). Mean replacement clutch
size on territories without gulls was 35%
greater than that on territories neighboring
Glaucous-winged Gull colonies, although the
difference was not statistically significant
(with gulls: 1.7 ? 0.5 SD, n = 9; without
1.0 SD, n = 7; Wilcoxon test, Z
gulls: 2.3
= 1.5, P - 0.10). The incidence of complete
clutch loss on territories near Glaucouswinged Gull colonies (25 of 48 clutches lost)
was similar to territories free from neighboring gulls (23 of 48 clutches lost). Replacement
clutches were laid on both gull and gull-free
territories (seven without gulls and nine with
gulls).
Hatching success was significantly and negatively related to intertidal slope of a territory
(Table 3, Fig. 1). This relationship was strongest among territories that hatched 21 chick
(Fig. 1). Thus, pairs occupying steep territories had significantly smaller brood sizes than
pairs on shallow territories (Table 3). Intertidal
slope had a significant negative linear relationship with hatching success, independent of
first clutch loss (n = 38, r2 = 0.12, F = 4.9,
P = 0.03). There was no significant difference
between mean intertidal slopes of territories
that successfully hatched young and territories
that experienced full clutch loss (hatched
young: 21.4 ? 10.4 SD, n = 32 successful
clutches; no hatched young; 18.5 ? 8.9 SD, n
= 36 depredated clutches; Wilcoxon test, Z =
1.2, P = 0.20).
Thirty of 64 hatchlings survived to 35 days
of age. There was no significant predictor of
fledging success or pair productivity, despite
the increased number of hatchlings on shallow
TABLE 3. Multiple regression models of the relationship between territory characteristics and reproductive
success of Black Oystercatcher breeding pairs in the Strait of Georgia, British Columbia, 1996 and 1997.
Dependent variable
n
Independent variable
r2
F
P
Hatching success
Brood size at hatching
Fledging success
Productivity
38
24
24
38
Intertidal slope
Intertidal slope
Intertidal slope
Intertidal slope
0.13
0.39
0.02
0.06
5.42
14.10
0.51
2.27
0.026
0.001
0.48
0.14
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407
TERRITORY QUALITY OF BLACK OYSTERCATCHERS
Hazlitt
2
1.0-
(D
_
(a)r=0.15P-0.015n=38
y -0.47 -0.009x
2
00
2
P = 0.001 n = 24
(b) r=.60
y =0.68 - 0.01x
c,
r)
S
0.8 -
co
0)C
aF)
0.6 0.6
a)
@0
S
15~
00
0)
0
20
*0 0 0
4
20
30
40
5
0
O
0
-
0-*1-1Z
0~
0 - ----------
Intertidalslope of the territory(degrees)
FIG. t. The relationship between Black Oystercatcher hatching success and the intertidal slope of the
breeding teffitory; (a) all breeding pairs and (b) only
pairs that hatched at least one chick. Data were collected during 1996 and 1997 in the southern Gulf Islands, British Columbia.
sloped territories compared
(Table 3, Fig. 2).
to steep territories
DISCUSSION
that the exposed area of inI hypothesized
tertidal beach at low tide, a surrogate measure
of food supply, would be an important feature
of an oystercatcher
breeding territory. I predicted that a negative relationship would exist
shoreline
between the length of monopolized
and the intertidal slope of the shoreline. Shallow sloping shorelines provide a larger area
of lower littoral zones during low tides, and
these zones contain a greater amount of inbiomass
eaten by oystercatchers
vertebrate
(Groves 1982). Moreover, access to some marine invertebrates and some larger size classes
foraging on
may be limited to oystercatchers
1982, Andres 1996).
steeper slopes (Groves
However, there was no significant relationship
shoreline
between the length of monopolized
and the intertidal slope of Black Oystercatcher
breeding territories in my study area, although
large numbers of territories were found along
shallow sloping shorelines.
High densities of
are
characteristically
breeding oystercatchers
associated with regions of large, rocky, intertidal reefs and shallow gravel beaches used for
10
20
30
40
50
Intertidal slope of the territory (degrees)
FIG. 2. No relationship was found between Black
Oystercatcher productivity and intertidal slope of the
breeding territories (n = 38) in the southern Gulf Islands, British Columbia, during 1996 and 1997. Black
circles represent a single data point, and black squares
represent multiple data points. Mean values are used
for pairs that bred in both years.
foraging (Hartwick 1974; Vermeer et al. 1992;
Andres and Falxa 1995; Andres 1996, 1998).
Breeding territories were associated closely
with breeding Glaucous-winged Gulls, and
both were located away from the mainland
and forested islands. The presence of nearby
nesting Glaucous-winged Gulls explained a
significant amount of variation in the first
clutch size of oystercatcher pairs. Pairs occupying territories along the periphery of Glaucous-winged Gull colonies had significantly
smaller first clutch sizes than pairs on territories without neighboring gulls. However, the
incidence of full clutch depredation and clutch
replacement did not differ significantly with
the presence of nesting Glaucous-winged
Gulls.
The close association between breeding
Glaucous-winged Gulls and Black Oystercatchers has been well documented, with both
species requiring similar breeding habitat and
refuges from terrestrial predators (Vermeer et
al. 1992, Andres and Falxa 1995). There is no
evidence that individual Black Oystercatchers
benefit from breeding near gulls in British Columbia (Vermeer et al. 1992, this study), although oystercatchers have higher hatching
success when associated with breeding terns
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408
THE WILSON BULLETIN * Vol. 113, No. 4, December 2001
and gulls in Alaska (B. A. Andres pers.
comm.). In this study, oystercatchers that bred
at the edge of gull colonies either laid a significantly smaller first clutch, or more likely,
these pairs experienced higher partial clutch
depredation during the laying period. Black
Oystercatcher eggs are incubated only 30% of
the time until clutch completion, and single
eggs lost from a clutch are not replaced (Purdy and Miller 1988). For the Eurasian Oystercatcher (H. ostralegus), 40% of the firstlaid eggs were lost before the second eggs
were laid (del Hoyo et al. 1996). Egg loss to
gulls or corvids during the laying period, before full incubation begins, is a probable explanation for smaller first clutch sizes on territories with nearby Glaucous-winged Gulls.
Intertidal slope of a territory was a strong,
significant predictor of hatching success, particularly among pairs that hatched at least one
chick. Steep-sloped territories were associated
with small, steep islets that usually lacked any
horizontal cover, such as logs or concealing
vegetation (Andres 1998). Evidence for nest
site selection near elevated rocks or logs by
oystercatchers suggests a need for nest protection from predators (Vermeer et al. 1989).
Perhaps predators can better detect eggs or
young chicks along steep-sloped territories
compared to shallow shorelines. However, territory slope had no relationship with the probability of full clutch loss during the incubation
period or fledging success.
Black Oystercatcher parents brood full time
while eggs are hatching and when chicks are
still in the nest scrape (Purdy and Miller
1988). The first two eggs hatch within a few
hours of each other and the chicks are mobile
soon after (Nol et al. 1984, Purdy and Miller
1988). The third egg often is still hatching or
has not yet hatched, and sometimes is abandoned by the parents (Nol et al. 1984, Hazlitt
1999, Hazlitt and Butler 2001). Nest scrapes
on steep territories may be more difficult for
foraging oystercatchers to keep in view to
watch for predators compared to shallowsloped territories, decreasing the level of vigilance of the scrape from the nonincubating
partner during hatching and chick brooding.
Oystercatcher eggs and chicks have low survival during the hatching period (Hazlitt and
Butler 2001), which supports the hypothesis
that this stage requires both parents to protect
mobile young and the contents of the nest
scrape.
None of the territory characteristics measured in this study significantly predicted
fledging success or productivity. Sometimes
oystercatchers use feeding areas outside of the
territory (Hartwick 1978; B. A. Andres pers.
comm.), which could be a factor. However,
provisioning behavior observed during this
study revealed that these forays were infrequent and are most likely function for self
feeding rather than provisioning young (Hazlitt 1999). Nol (1989) found that the size and
location of the intertidalforaging area relative
to the nest site was more important than the
actual density of prey during the chick-rearing
period in American Oystercatchers (H. palliates). Territorieswith large, nearby feeding areas fledged more young over a three-yearperiod compared to pairs on territories with
small, distant feeding areas. She proposed that
better American Oystercatcher territoriespermitted parents to watch for predators of the
young while foraging.
Research on the Eurasian Oystercatcher
also has shown that the distance of the territory to the closest intertidal foraging area,
ratherthan density of prey, plays a role in the
fecundity of parents (Ens et al. 1992). Pairs
nesting adjacent to the foraging beach led
their chicks into the foraging area where they
provisioned young at much higher rates than
pairs nesting farther away, who were forced
to deliver food items to chicks waiting in the
territory (Ens et al. 1992, Kersten 1996).
No surrogatemeasuresof food supply in this
study significantly predicted reproductivesuccess of breeding Black Oystercatchers.However intertidalslope was significantly and negatively related to hatching success and hence
the numberof chicks raised on a territory.Other studies of Black Oystercatchershave indicated that the distance and visibility of the foraging and provisioning area in relation to the
shoreline territorymay be related to provisioning rates and protection of young by parents
(Hartwick 1974). Since breeding territoriesin
the southern Gulf Islands are predominately
linear, with adjacentintertidalforaging areas, I
suggest that the slope of the intertidalfeeding
area may influence the ability of oystercatcher
parents to safeguard eggs and young from
predatorsduring the hatching period.
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Hazlitt - TERRITORY QUALITY OF BLACK OYSTERCATCHERS
ACKNOWLEDGMENTS
I thank D. Lissimore and J. E. Cotter for their help
with the fieldwork, and M. J. F Lemon, T M. Sullivan,
L. E Keller, D. C. Lank, C. M. Smith, and B. Sherman
for help with various aspects of the research. Permission to establish a field camp on Sidney Island was
granted by the British Columbia Ministry of Environment, Lands and Parks. I thank R. C. Ydenberg and
R. W. Butler for their comments on previous drafts of
this paper, and B. A. Andres and an anonymous reviewer for their review of the manuscript. Financial
support was provided by the Canadian Wildlife Service, Pacific and Yukon Region, the CWS/NSERC
Wildlife Ecology Research Group at Simon Fraser
Univ., and the John K. Cooper Foundation.
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