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  • 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.


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    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.

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    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

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    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

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    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


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