Black Oystercatcher (Haematopus bachmani) foraging on
varnish clams (Nuttalia obscurata) in Nanaimo, BC
An undergraduate research project
By Emily Tranfield
Submitted in partial fulfillment of the requirements for the Bachelor of Science
Degree at Vancouver Island University, Nanaimo, British Columbia
April, 2014
Approved by:
_____________________________________________________________________
Research Advisor: Dr. Eric Demers, Biology Department, Vancouver Island University
I hereby grant permission for this research report to be bound and displayed in the Biology
Department at Vancouver Island University.
_____________________________________________________________________
1
ABSTRACT
The Black Oystercatcher (Haematopus bachmani) is a non-passerine shorebird found year-
round along the North American Pacific Coast. It lives on rocky coastlines and islands, and
forages in the intertidal zone. The rapid and very successful invasion of the varnish clam
(Nuttalia obscurata), a newly introduced species to British Columbia waters, has the capability
to alter pre-existing ecological interactions. No studies on Black Oystercatcher foraging on
varnish clams have previously been conducted. This study examined whether and to what extent
varnish clams represent a significant part of the diet of Black Oystercatchers and whether Black
Oystercatchers forage selectively on specific size classes of varnish clams. Study sites were
established at Piper’s Lagoon and Departure Bay Beach in Nanaimo, British Columbia.
Observations of foraging Black Oystercatchers were conducted from October 2013 to February
2014. Species consumed by Black Oystercatchers were recorded. The size of varnish clams eaten
by oystercatchers was measured to the nearest millimetre. Beach profiles of available prey
species composition and size were conducted at each site based on stratified random sampling
with 0.25-m2 quadrats. This study found that the varnish clam was an important food source for
the Black Oystercatcher. There was no obvious indication of size-selective foraging by Black
Oystercatchers in this study. However, there was a slight difference between the mean size of
available (larger) and eaten varnish clams at Piper’s Lagoon, but not at Departure Bay Beach.
Black Oystercatchers were also found to feed on varnish clams at a higher rate at Piper’s Lagoon
and throughout a broader tide range than at Departure Bay Beach. This research found that Black
Oystercatchers appear to exploit the varnish clams and may be able to obtain a substantial part of
their daily energy requirements from this invasive species.
2
INTRODUCTION
The Black Oystercatcher (Haematopus bachmani) is a non-passerine coastal shore bird that
can be found year-round along the North-American Pacific Coast from the Aleutian Islands and
Southern Alaska to central Baja California (Jehl, 1985). It lives on rocky coastlines and islands,
and forages on bivalve species amongst other species in the intertidal zone. It has a small
estimated global population size (8,900-11,000 individuals), contributing to its status as a species
of high conservation concern (Johnson et al., 2010). Until the 1980s, there was still confusion as
to whether the Black Oystercatcher (Haematopus bachmani) and the American Oystercatcher (H.
palliates) were a single species or two distinct species, as evidence of hybridization occurs where
these species overlap (Baja California) (Colwell, 2010; Jehl, 1985). They are now recognized as
two distinct species based on the prevalence of parental forms, assortative mating, and renewal
of stability after a period of dynamic change where hybridization occurs (Jehl, 1985).
Breeding Black Oystercatchers nest on offshore or remote islands almost exclusively, but
during the winter months, they relocate to more sheltered areas. During the winter, a portion of
the Black Oystercatcher population migrates from northern breeding areas to lower latitudes,
flying distances as far as 1,667 kilometers (Campbell et al., 1997; Johnson et al., 2010). Not all
Black Oystercatchers are considered migratory; most of them are considered to be year-round
residents (Purdy and Miller, 1988). If the year-round residents move at all, they only flock to
nearby areas that are better sheltered (Andres, 1994). When coastal maps of the British Columbia
coast are set into grids, 80% of the coastline has evidence of Black Oystercatcher nesting
grounds (Campbell et al., 1997). On Vancouver Island, close to Nanaimo, Pacific Rim National
Park is home to Black Oystercatchers that are believed to be non-migratory due to similar
Summer and Winter counts (P. Clarkson, unpubl. data; cited in Johnson et al., 2010). Black
3
Oystercatchers begin to form overwintering flocks from September to October, with their peak
group numbers being reached by late October to early November (Andres, 1994; Campbell et al.,
1997). Flock sizes range from a few to over 100 individuals (Campbell et al., 1997).
Shorebirds use a variety of methods to search for prey. The first method is the use of visual
cues to detect prey. Another method is to use tactile sensation to detect prey that are located
within intertidal sediments the surface (Colwell, 2010). The Black Oystercatcher uses both visual
hunting to glean prey from the surface and tactile sensation by probing with its bill for prey
buried in the substrate (Colwell, 2010). Oystercatchers have strong bills when compared to other
shorebirds (Colwell, 2010), enabling them to access the meat inside bivalves. Their bill can be
one of three possible morphs: pointed, chisel-shaped, or blunt (Butler and Kirbyson, 1979;
Colwell, 2010). Each bill morph enables the bird to best forage in a specific way: the birds with
pointed bills forage primarily for soft-bodied and thin-shelled prey; birds with chisel-shaped bills
forage primarily for hard-shelled bivalves with which they use a stabbing technique to cut the
bivalves’ abductor muscles, after which they then pry the shells open; and birds with blunt bills
forage primarily for hard-shelled bivalves with which they hammer at the shells to gain access to
the meat inside (Butler and Kirbyson, 1979; Rutten et al., 2006; Colwell, 2010). The blunt-
tipped-billed Black Oystercatcher still cuts the abductor muscle of bivalve prey, if possible, after
it has hammered through the bivalve’s shell. If successfully cutting the muscle is not possible,
the blunt-tipped-billed Black Oystercatcher may not be able to part the shell of larger bivalves,
and may either proceed to eat the meat through the hole or abandon the bivalve (Butler and
Kirbyson, 1979).
Varnish clams (Nuttalia obscurata) are a relatively newly introduced species to British
Columbia waters. They are thought to have been introduced in the early 1990’s as larvae in the
4
ballast water from trans-oceanic transport vessels (DFO, 1999). Successfully invading the British
Columbia shoreline very rapidly, they have spread over 500 km, have been recorded on over 100
beaches, and have shown capabilities of reaching densities of up to 800 m-2 (Dudas, 2005; Chan
and Bendell, 2013). This rapid invasion is due to several characteristics of the varnish clam such
as having a higher fecundity and reaching sexual maturity earlier than other bivalves in the area,
a lengthy planktonic phase which promotes dispersal, and its ability to exploit the upper
intertidal zone (Dudas, 2005; Dudas and Dower, 2006). Due to ocean currents and its planktonic
larval stage, the varnish clam is capable of spreading throughout its entire geographic zone in
only one reproductive period (Dudas, 2005).
The varnish clam primarily exploits the upper third of the intertidal zone, but when no other
clam species are present, the varnish clam ranges throughout the upper to the lower intertidal
zone (Dudas, 2005; Meacham, 2013). In coastal British Columbia, the varnish clam can be found
with two other main bivalve species: the native Pacific littleneck (Protothaca staminea) and the
introduced Japanese littleneck (Tapes philippinarum) (Dudas et al., 2005). Compared to these
littleneck species, the varnish clam has a thinner and flatter shell, allowing easier consumption
by predators (Dudas et al., 2005).
Predators may apply preferences when foraging. Preferential foraging results in prey species
composition and/or size deviating from random prey consumption (Chesson, 1978). This is
called selective foraging. Selective foragers should select prey that maximize their net gain of
calories. At first, it appears that the largest prey would be the best choice of prey in terms of
long-term caloric intake, but many factors may work against this. A predator may select a
smaller prey species or a different species altogether for reasons that include handling time,
consumption success, profitability, and predator avoidance (Dudas et al., 2005).
5
The “risky prey hypothesis” provides another explanation as to why a predator may
selectively forage for prey with smaller profitability. According to this hypothesis, the predator is
constrained by high risks that come with consuming larger prey (Rutten et al., 2006). An
example of this is that, despite the fact that larger bivalves can be expected to yield more energy
to oystercatchers than smaller bivalves, in the long term, net gain of calories may be diminished
with a diet of larger bivalves due to the fact that the risk of bill damage increases with prey size
(Rutten et al., 2006). A damaged bill would greatly reduce foraging ability of the oystercatcher.
Captive oystercatchers with bill damage had a 23% reduction in their food intake, and free-living
oystercatchers with bill damage were, on average, 14 g lighter in mass than oystercatchers
without bill damage (Rutten et al., 2006). Therefore, selectively foraging for smaller prey may
result in a larger long-term caloric intake for the predator than selectively foraging on larger
prey.
Invasions of non-indigenous species (NIS) are recognized as a threat to biodiversity through
competition and/or predation with indigenous species (Dudas et al., 2005). NIS potentially have
the ability to alter the structure of ecological interactions that already occur in an area (DFO,
1999; Dudas et al., 2005). Since the introduction and spread of the non-indigenous varnish clam
in the Black Oystercatcher’s range, there is little knowledge of the extent to which the diet of the
Black Oystercatcher may include the varnish clam. The Black Oystercatcher has been designated
as a species of high concern by both the Canadian and U.S. Shorebird Conservation Plans
(Donaldson et al., 2000; Brown et al., 2001). Black Oystercatchers have been the focus for
priority conservation action in part due to their small global population size, restricted range,
threats to preferred habitat, susceptibility to NIS invasions and lack of baseline data to assess
conservation status (Tessler et al., 2010). No studies of Black Oystercatcher foraging on varnish
6
clams have previously been conducted. Therefore, this study aimed to address the interaction of
the Black Oystercatcher with the non-indigenous and rapidly-spreading varnish clam.
OBJECTIVES
This study examined the foraging interactions of Black Oystercatchers on the introduced
varnish clam in Nanaimo, British Columbia. In particular, this study investigated whether and to
what extent varnish clams represent a significant part of the diet of the Black Oystercatcher. This
study also examined whether Black Oystercatchers forage selectively on specific size classes of
varnish clam.
MATERIALS AND METHODS
Study Sites
This study was conducted at Piper’s Lagoon and Departure Bay Beach in Nanaimo, British
Columbia (Figure 1), from October 2013 to February 2014. Black Oystercatchers were observed
at these sites during preliminary surveys from September to October 2013. Departure Bay Beach
is an approximately 1.9-km crescent-shaped beach running between Bay Street (north-western
end) and the Departure Bay BC Ferries terminal (south-eastern end). It is located in an urban
area with Departure Bay Road running alongside a substantial part of the beach. There is a
steeper gradient in the upper intertidal zone of the beach, where medium to coarse gravel is the
predominant substrate, and a very low gradient in the middle and lower intertidal zone, where
sand is the predominate substrate. People regularly walk the beach and dogs are frequently
present. The study site was located at the most northern end of the beach. Here, Departure Bay
Creek runs into Departure Bay. The study site was located on a small spit that extends from the
7
northern edge of Departure Bay Creek. Within the study site, the substrate was comprised of
gravel and sand.
Piper’s Lagoon is approximately 0.10 km2 in size and floods during a tidal height of
approximately 2.1 to 3.5 metres. Piper’s Lagoon is surrounded by land except for a small
opening through its northern shore which provides the tide access to the lagoon. The south and
east shore of the lagoon is a peninsula (40 m wide) that runs north-south and ends in a larger land
mass with many short hiking trails. The western shore of the lagoon is residential waterfront.
There is a small park along the very northwestern shore of the lagoon. The study site was located
on the western shore of the lagoon, south of the park and in front of the most northern residence.
The gradient of the shore of the lagoon is shallow in the upper intertidal zone and relatively flat
in the middle intertidal zone. The substrate within the study site was comprised of gravel in the
upper intertidal zone and sand in most of the lagoon.
Foraging Observations
Observations were conducted in 5-minute observational periods. Upon arrival at a study site,
at least one observational period was implemented whether Black Oystercatchers were present or
not. Within each observational period, the tidal elevation, air temperature, cloud cover, wind
direction and speed, and number and species of birds present were recorded. If no Black
Oystercatchers were present, the next study site would be assessed or observational periods may
continue at the one study site. If Black Oystercatchers were present, focal animal sampling
(Altman, 1974) was carried out to assess the number and species of prey items eaten by Black
Oystercatchers. Each 5 minute observation period was then followed by an instantaneous scan
sample to record the number of Black Oystercatchers present and the activity of each individual.
Observational periods with Black Oystercatchers present would continue until the Black
8
Oystercatchers left or for as long as possible for the observer. Oystercatchers were observed
using Nikon Monarch 10X42 binoculars and a Vortex Razor HD 85 mm spotting scope from a
minimum distance of 20 m. If more than one Black Oystercatcher was present, a random series
of the digits of the number pi (π) were used to select which bird to observe for each observational
period. If two Black Oystercatchers were present, a number from 0-4 and 5-9 represented the
bird to left and right, respectively. If three or more Black Oystercatchers were present, a number
from 1-3, 4-6, and 7-9 represented the bird to the left, centre, and right, respectively. The number
0 in this case represented the next number in the series of pi.
After the foraging Black Oystercatchers left the study site, the discarded shells of varnish
clams eaten by Black Oystercatchers were measured with Vernier calipers to the nearest
millimetre along the longest axis of the shells. The discarded shells were easily found due to part
of the abductor muscle usually still being attached on the inside of the shells, the shells still being
wet, and the shells only being pried apart and not cracked like the shells of the clams that
seagulls and crows ate (personal observation). The Black Oystercatchers foraged in the same
area, making it easy to keep track of discarded shell placement. After measurement of the
discarded shells, the shells were placed back on the beach as close as possible to where they were
found. These measurements of the discarded shells were used to calculate the mean size of clams
consumed by Black Oystercatchers.
Varnish Clam Distribution and Size
The availability of varnish clams to Black Oystercatchers was determined by sampling each
study site throughout the tide range in which Black Oystercatchers were observed feeding on
varnish clams. Sampling occurred in accordance Fisheries and Oceans Canada license to “Fish
for Experimental, Scientific, Educational or Public Display Purposes issued under Section 4 of
9
the Management of Contaminated Fisheries Regulations” (license number: XMCFR 14 2013). A
transect was run with a measuring tape laid out perpendicular to the edge of a dropping tide at
each study site. At 0.3-metre tidal interval determined with a tide table, two quadrats were placed
at a random location on each side of the transect and at the water’s edge. Random numbers were
used to determine how far from the transect the quadrats would be placed. The random numbers
potentially represented distances of 0.0 to 5.0 metres from the transect.
At Departure Bay Beach study site one tidal elevation was sampled with a pair of quadrats at a
tide height of 3.05 metres. At Piper’s Lagoon study site three tidal elevations were sampled in
pairs of quadrats at tidal heights of 3.05 metres, 3.35 metres, and 3.66 metres. The quadrats were
0.5 m × 0.5 m (0.25-m2) in area and each quadrat was dug to a depth of 0.1 m with the use of
hand trowels. This depth was chosen due to the fact that, despite the ability of the varnish clam to
bury to a depth of 0.3 m (Dudas et al., 2005), the maximum length of a Black Oystercatcher’s
bill is 8.1 cm (Marchant, 2010) and a depth of 0.1 m accounted for substrate roughness.
Through the beach profiling, the number and species of potential prey items available for the
Black Oystercatcher were recorded. All prey items other than varnish clams were immediately
returned back to where they were found after having been counted. The varnish clams, after
having been counted, were placed in plastic bags labelled for each quadrat. Each varnish clam
was then further washed to remove substrate, patted dry with paper towel, and measured with
Vernier calipers to the nearest millimetre along the longest axis of its shell.
Data Analysis
Welch’s t-test was used to assess if there was a significant difference in abundance of varnish
clams between the sites. The data on size of varnish clam eaten were pooled for each study site
due to limited sample sizes. The varnish clam size data were determined to be normally
10
distributed by visual assessment of histograms. The average size of available and eaten varnish
clams at each study site were compared with Welch’s t-test to assess size-selective foraging by
Black Oystercatchers. Significance was determined with α = 0.025, which included a Bonferroni
correction applied to account for multiple comparisons (Zar, 2010).
RESULTS
A total of 105 five-minute observational periods were conducted at the study sites (Table 1).
There were 76 and 29 observational periods conducted at Departure Bay Beach and Piper’s
Lagoon, respectively. Black Oystercatchers were observed were observed during 10 (13%) and
11 (38%) observational periods, respectively (Table 1). When Black Oystercatchers were
present, the median flock size at Departure Bay Beach and Piper’s Lagoon was four and one,
respectively, with flock sizes ranging from two to seven and one to two individuals, respectively.
Observational periods at Departure Bay Beach and Piper’s Lagoon occurred throughout similar
tidal ranges (2.4-4.5 metres, and 2.4-4.3 metres, respectively), and at both locations Black
Oystercatchers were observed foraging at similar tidal ranges (3.0-3.1 metres, and 3.1-3.7
metres, respectively) (Table 1). Of the total number of observational periods that occurred at
Departure Bay Beach and Piper’s Lagoon, Black Oystercatchers were observed consuming at
least one varnish clam in five (7%) and nine (31%) observational periods, respectively (Table 1).
The median number of varnish clams consumed per observational period by Black
Oystercatchers was zero and three at Departure Bay Beach and Piper’s Lagoon, respectively
(Table 1). Therefore, Black Oystercatchers were present more often at Piper’s Lagoon, where
they consumed varnish clams more often, and in greater number (Table 1).
Black Oystercatchers were observed foraging at the edge of the water, where they would walk
the waterline and probe the substrate with their bill to locate prey species. They either consumed
11
individual prey from where it was excavated or they wedged the edge of the bivalve beneath a
rock large enough to pin the shell before gaining access to the meat inside. Observations showed
that as many as three discarded varnish clam shells could be found stacked within each other and
wedged beneath a rock used to pin the shells. All discarded varnish clam shells were found intact
and with half-shells pried apart but still attached together. The majority of the meat was
consumed from each varnish clam eaten. Frequently, there were still traces of abductor muscle
attached to the discarded half-shells.
There was no significant difference between the mean size of available varnish clams and the
mean size of varnish clams eaten by Black Oystercatchers at Departure Bay Beach (t = 0.55; df =
38; P = 0.59) (Figure 2; Table 2). Interestingly, the largest and smallest sizes of available varnish
clams did not appear to be consumed by Black Oystercatchers at Departure Bay Beach (Figure
2). There was a nearly significant difference between the mean size of available varnish clams
and those eaten by Black Oystercatchers at Piper’s Lagoon (t = 2.28; df = 37; P = 0.03)
(Figure 2; Table 2), although the range of available and eaten varnish clams were similar. The
mean density of available varnish clams was slightly higher at Piper’s Lagoon than at Departure
Bay Beach (Table 2), but this difference was not significant (t = 0.17; df = 6; P = 0.87).
DISCUSSION
In this study, Black Oystercatchers were repeatedly observed to consume varnish clams in the
intertidal zone of study sites located at Departure Bay Beach and Piper’s Lagoon in Nanaimo,
British Columbia from October 2013 to 2014. Observed flock sizes were consistent with
previous research (Campbell et al., 1997). Departure Bay Beach and Piper’s Lagoon were
observed to have flock size medians of 4 (range: 2-7) and 1 (range: 1-2), respectively (Table 1).
Piper’s Lagoon appeared to be an especially important foraging site for Black Oystercatchers
12
since they consumed varnish clams more frequently and in greater quantity at this location than
at Departure Bay Beach (Table 1).
Observational periods at Departure Bay Beach and Piper’s Lagoon occurred throughout
similar tidal ranges (2.4-4.5 metres and 2.4-4.3 metres, respectively), and at both locations Black
Oystercatchers were observed foraging at similar tidal ranges (3.0-3.1 metres, and 3.1-3.7
metres, respectively). Black Oystercatchers foraged at tide heights that allowed them access to
the intertidal zone in which varnish clams are present. Previous research in British Columbia
indicates that at tidal heights greater than 2.5 metres the varnish clam is often the only bivalve
species observed (Dudas, 2005). Typically, there is a relationship between bivalves with longer
shell lengths being found at deeper burial depths, but the varnish clam does not display this
relationship clearly (Dudas, 2005). This permits the Black Oystercatcher to forage on larger
varnish clams than smaller size classes of other bivalve species from a shallow substrate depth,
presumably requiring less handling time and less energy expenditure. Furthermore, the varnish
clam may also require a shorter handling time than other bivalve species for the Black
Oystercatcher due to the compressed shape of the shell and presumably easier to grab
morphology.
Field observations revealed that after Black Oystercatchers successfully probed for a varnish
clam, their possible actions included wedging the edge of the varnish clam beneath a rock or
inside the shell of another previously eaten varnish clam. Presumably, this helped the Black
Oystercatcher gain access to the meat inside by allowing the rock to support the clam while
allowing the Black Oystercatcher to better chisel their way around the edge of the varnish clam
(Feare, 1971). Observations of Black Oystercatchers revealed that they opened the varnish clams
with a chiseling motion around the edge of the shell, presumably cutting the abductor muscle of
13
the varnish clam, and then pried the half-shells of the varnish clam apart with their bills. This is
consistent with the foraging methods of Black Oystercatchers with chisel-shaped bills (Butler
and Kirbyson, 1979; Colwell, 2010). The other two feeding modes of Black Oystercatchers with
pointed and blunt bill morphs were not observed.
There was no obvious indication of size-selective foraging by Black Oystercatchers in this
study. However, there was a slight difference between the mean sizes of available (larger) and
eaten varnish clams at Piper’s Lagoon, but not at Departure Bay Beach. A predator’s preference
for prey can be affected by the abundance of the prey (Hughes, 1979). According to basic
foraging theory, when prey are less abundant, a predator should expand its preferred diet to also
eat less profitable prey sizes, and when valuable prey are abundant, the predator should reject
less profitable prey sizes (Dudas et al., 2005). The mean density of available varnish clams was
slightly higher at Piper’s Lagoon than at Departure Bay Beach (Table 2). This difference was not
significant and likely not the main reason for the slight difference of mean prey size observed.
Black Oystercatchers were also found to feed on varnish clams at a higher rate at Piper’s Lagoon
and throughout a larger tide range. A longer tidal fluctuation allows the Black Oystercatcher
more time to forage, which would allow them to afford to selectively forage for a specific size of
prey (Colwell, 2010). The increased varnish clam consumption, slightly higher abundance of
available varnish clams, and larger tidal range spent foraging at Piper’s Lagoon than Departure
Bay Beach suggest possible selective foraging by Black Oystercatchers. Reasons for the Black
Oystercatcher to selectively forage for smaller sized varnish clams could include a possibly
shorter handling time, higher consumption rate and increased profitability (Dudas et al., 2005).
Further behavioural studies could be conducted to address this.
14
This study found that the varnish clam is now an important food source for the Black
Oystercatcher. When Black Oystercatchers were present at Departure Bay Beach and Piper’s
Lagoon, they were observed to forage on varnish clams during 50% and 82% of observation
periods, respectively. Black Oystercatchers appear to exploit the varnish clams and may be able
to obtain a substantial part of their daily energy requirements from this invasive species. The
contribution of varnish clam as part of daily food consumption is unknown and should be
evaluated. During this study, Black Oystercatchers were observed foraging at other locations
with rocky intertidal habitat. In these habitats, Black Oystercatchers were observed consuming
mussels and are likely to also consume limpets, barnacles and other bivalve species occasionally,
including oysters (Butler and Kirbyson, 1979). The indirect effects of this invasive subsidy on
marine food webs remain unknown and could be the subject of further studies.
ACKNOWLEDGMENTS
I would like to express my gratitude to Dr. Eric Demers, my research advisor, for his
guidance, enthusiasm and volunteer time in the field. Thank you to Dr. Liz Demattia and Dr.
Rosemarie Ganassin for their support as committee members, and to Nathan Hollenberg, Kim
Wetten and Stephanie Wetten for their volunteer time in the field.
15
LITERATURE CITED
Altmann, J. 1974. Observational study of behavior: sampling methods. Behaviour 49:227-267.
Andres, B.A. 1994. Year-round residency in northern populations of the Black Oystercatcher.
U.S. Fish and Wildlife Service, Migratory Bird Management. 10 p.
Brown, S., C. Hickey, B. Harrington, and R. Gill, eds. 2001. United States shorebird
conservation plan. 2nd ed. Manomet Center for Conservation Sciences, Manomet,
Massachusetts. 64 p.
Butler R.W. and J.W. Kirbyson 1979. Oyster predation by the Black Oystercatcher in British
Columbia. The Condor 81: 433-435.
Campbell R.W., N.K. Dawe and I. McTaggart-Cowan. 1997. Birds of British Columbia, volume
2: nonpasserines: diurnal birds of prey through woodpeckers. University of British Columbia
Press, Vancouver, BC. 131 p.
Chan K. and L.I. Bendell. 2013. Potential effects of an invasive bivalve, Nuttallia obscurata, on
select sediment attributes within the intertidal region of coastal British Columbia. Journal of
Experimental Marine Biology and Ecology 444: 66-72.
Chesson J. 1978. Measuring preference in selective predation. Ecology 59: 211-215.
Colwell M.A. 2010. Shorebird ecology, conservation, and management. University of California
Press, Berkeley. 344 p.
16
Department of Fisheries and Oceans (DFO). 1999. Varnish Clams. DFO Science Stock Status
Report C6-13 (1999).
Donaldson, G., C. Hyslop, G. Morrison, L. Dickson, and I. Davidson, Eds. 2000. Canadian
shorebird conservation plan. Canadian Wildlife Service, Ottawa, Ontario. 34 p.
Dudas S.E. 2005. Invasion dynamics of a non-indigenous bivalve, Nuttallia obscurata (Reeve
1857), in the northeast Pacific. Doctoral dissertation. University of Victoria, Canada. 158 p.
Dudas S.E., I.J. McGaw and J.F. Dower. 2005. Selective crab predation on native and introduced
bivalves in British Columbia. Journal of Experimental Marine Biology and Ecology 325: 8-
17.
Dudas S.E. and J.F. Dower. 2006. Reproductive ecology and dispersal potential of varnish clam
Nuttallia obscurata, a recent invader in the Northeast Pacific Ocean. Marine Ecology
Progress Series 320: 195-205.
Feare C.J. 1971. Predations of limpets and dogwhelks by oystercatchers. Bird Study 18: 121-129.
Hughes, R.N. 1979. Optimal diets under the energy maximization premise: the effects of
recognition time and learning. American Naturalist 113:209-221.
Jehl J.R., Jr. 1985. Hybridization and evolution of oystercatchers on the Pacific Coast of Baja
California. Ornithological Monographs 36: 484-504.
17
Johnson M., P. Clarkson, M.I. Goldstein, S.M. Haig, R.B. Lanctot, D.F. Tessler and D.
Zwiefelhofer. 2010. Seasonal movements, winter range use, and migratory connectivity of the
Black Oystercatcher. The Condor 112: 731-743.
Marchant J., P. Hayman and T. Prater. 2010. Shorebirds. A&C Black, London. 352 p.
Meacham, P. 2013. Washington Department of Fish and Wildlife. Retrieved Sept. 14, 2013 from
http://wdfw.wa.gov/ais/nuttalia_obscurata.
Purdy, M.A. and E.H. Miller. 1988. Time budget and parental behavior of breeding American
Black Oystercatchers (Haematopus bachmani) in British Columbia. Canadian Journal of
Zoology 66: 1742-1751.
Rutten A.L., K. Oosterbeek, B.J. Ens and S. Verhulst. 2006. Optimal foraging on perilous prey:
risk of bill damage reduces optimal prey size in oystercatchers. Behavioral Ecology 17: 297-
302.
Tessler, D.F., J.A. Johnson, B.A. Andres, S. Thomas and R.B. Lanctot. 2010. Black
Oystercatcher (Haematopus bachmani) conservation action plan. Version 1.1. International
Black Oystercatcher Working Group, Alaska Department of Fish and Game, U.S. Fish and
Wildlife Service, and Manomet Center for Conservation Sciences. 115 p.
Zar, J.H. 2010. Biostatistical analysis, 5th edition. Pearson, USA. 960 p.
18
Table 1. Summary of observations of Black Oystercatcher (BLOY) feeding on varnish clams (VC)
at Departure Bay Beach and Piper’s Lagoon during various dates from October 2013 to February
2014.
Parameter Departure Bay Beach Piper’s Lagoon
Number of observation periods 76 29
Periods with BLOY present 10 (13%) 11 (38%)
Median flock size (range) 4 (2 - 7) 1 (1 - 2)
Tidal range when BLOY present (m) 3.0 - 3.1 3.1 - 3.7
Periods with VC consumed 5 (7%) 9 (31%)
Median number of VC consumed per period (range) 0 (0 - 3) 3 (0 - 4)
19
Table 2. Descriptive statistics comparing available and eaten varnish clams at Departure Bay
Beach and Piper’s Lagoon during various dates from October 2013 to February 2014 (NS = not
significant; α = 0.025, with Bonferroni correction for multiple comparisons).
Parameter Departure Bay Beach Piper’s Lagoon
Mean length (mm; ± SD) and sample size of
available varnish clams
39.9 ± 6.3
n = 99
35.0 ± 3.7
n = 318
Mean length (mm; ± SD) and sample size of
varnish clams eaten by Black Oystercatchers
39.3 ± 3.9
n = 19
33.0 ± 4.8
n = 34
t-value 0.55 2.28
P-value 0.59NS 0.03NS
Mean density (m–2; ±SD) of available varnish
clams 198 ± 59 212 ± 177
20
500 metres
Piper’s Lagoon Departure Bay Beach
Google Earth Google Earth
500 metres
Figure 1. Aerial photographs of the Departure Bay Beach and Piper’s Lagoon study sites in
Nanaimo, British Columbia.
21
Figure 2. Size distribution of available(top) and eaten(bottom) varnish clams at Departure Bay
Beach(left) and Piper’s Lagoon(right) during various dates from October 2013 to February 2014.
0
5
10
15
20
25
2 8 14 20 26 32 38 44 50 56
Freq
uen
cy (
%)
Length (mm)
0
5
10
15
20
25
2 8 14 20 26 32 38 44 50 56
Freq
uen
cy (
%)
Length (mm)
0
5
10
15
20
25
2 8 14 20 26 32 38 44 50 56
Fre
qu
en
cy (
%)
Length (mm)
Departure Bay Beach
Av
ail
ab
le V
arn
ish
Cla
ms
Piper’s Lagoon
Ea
ten
Va
rnis
h C
lam
s
0
5
10
15
20
25
2 8 14 20 26 32 38 44 50 56
Freq
uen
cy (
%)
Length (mm)
n = 19 n = 34
n = 318 n = 99
