Hahn
Oystercatcher impact on limpets
Abstract:
The diet of Black Oystercatchers (Haematopus bachmani Audubon)
studied at Pacific Grove, California, included many limpets. In
this study, the birds usually took limpets clearly exposed to view,
not those hidden beneath larger leafy algae. Limpets on very steep
slopes were only attacked if the bird could do so while standing on
an adjacent gentle slope. Most of the limpets eaten were Collisella
pelta and C. digitalis; virtually none were C. scabra. This was in
striking contrast to the overwhelming abundance of C. scabra living
on foraging sites. Evidence is presented indicating that selective
predation by oystercatchers is important in the localization of
C. pelta under large leafy algae (animals in plain view at low tide
are selectively preyed upon), and in the separation of the habitat
niches of C. scabra and C. digitalis. C. digitalis survives best on
steep surfaces inaccessible to foraging oystercatchers; C. scabra,
largely not subject to oystercatcher predation, is more abundant on
horizontal surfaces where competition from other grazing limpets may
be less.
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Oystercatcher impact on limpets
Introduction:
The Black Oystercatcher, Haematopus bachmani Audubon, is a
shore bird which frequents the rocky parts of the west coast of
North America. It ranges from Alaska to Baja California, where it
is replaced by the black and white American Oystercatcher (A. 0. U.
Checklist, 1957). It is a year round resident throughout its range,
though uncommon (Stout, 1967).
Work by Webster (1941), Hartwick (1976), and Helbing (1976) in-
dicates that, while mussels are a major food, large numbers of lim¬
pets are also consumed by oystercatchers. It follows that they are
potentially capable of exerting considerable pressure on limpet pop¬
ulations in their foraging areas. The purpose of this study was to
investigate oystercatcher feeding activities and feeding behavior,
paying special attention to how they might affect the size-class
structure and spacial distribution of limpet populations. In par-
ticular, I looked for links between activities of oystercatchers and
the following well-documented observations on west coast'Acmaeid
distributions. Collisella digitalis (Rathke, 1833) is dominant on
steeply sloping substrates (Haven, 1971). Collisella pelta (Rathke,
1833) is commonly found under macroscopic algae (Jobe, 1968; Craig,
1968). Collisella scabra (Gould, 1846) is most common on horizon-
tal substrates exposed to sun (Haven, 1971), and also exhibits pre¬
cise homing behavior which results in a very close fit between shell
margin and home site substrate contour (e.g., Hewatt, 1910; Suther-
land, 1970).
Study Site and Methods
Fig 1 shows the study site on the southern shore of Montere
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Oystercatcher impact on limpets
Bay, adjacent to the hopkins Marine Station of Stanford University.
A single pair of Black Oystercatchers nested at the site indicated
in the spring of 1982. Their defended feeding territory extended
over approximately one half mile of coast. The great majority of
the data presented in this study results from observations on this
pair and their foraging areas over the period April 20 through May
28, 1982. The birds studied were accustomed to human activity,
obviating the need for blinds. Direct observations were made mostly
with a 20X spotting telescope, occasionally with 8 x 30 binoculars.
A portable tape recorder was used to record observations of feeding
activity.
One particular foraging site (henseforth called foraging site 1)
was exceptionally favorable, allowing a clear, close-up view of
foraging birds and the limpet prey under attack. It was possible to
record accurately both the degree of concealment under algae of the
prey, and the angle of the slope to which it was attached. In ad-
dition, this site permitted collection of most of the shells of lim¬
pets consumed, since surf did not wash many away. The site consis¬
ted of two very large rocks side by side which were identical in
terms of exposure to surf, and which supported similar animals and
algae. On a single date (May 10, 1982), the birds were observed
feeding at this site, their behavior was recorded in detail, and
a fairly large number of prey item shells were collected. At a
later date the population of living limpets at this site was sur-
veyed for comparison with the data collected on May 10. In the
interim period, a few more shells of prey were also collected from
the site. To determine the foods available at this site, all lim-
pets on the generally horizontal parts of one rock were counted
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Oystercatcher impact on limpets
except for C. scabra whose population was so large that its numbers
were estimated by the use of twenty-three haphazardly placed 25cm x
25cm quadrats. Two transects surveyed on the second rock verified
that limpet distribution there was very similar to that found on
the first rock.
Results:
Detailed descriptions of limpet foraging by oystercatchers,
including methods of removal from rock and extraction of edible
parts from the shell, are given by Webster (1941) and Helbing
(1976). My observations confirm theirs, without substantial mod-
ifications, and are not repeated here.
Limpet availability on foraging site 1:
Results of the limpet survey on foraging site 1 appear in fig
2. Collisella pelta found living on the foraging site (after feed-
ing be the oystercatchers was observed) were mostly hidden under
leafy algae. C. scabra and C. digitalis were mostly located in the
open. Of these three species, C. scabra was by far the most abun-
dant, and C. pelta was actually scarce.
Limpets eaten:
Foraging oystercatchers tend to deshell their limpet prey on
rocks above water level. Consequently, shells of freshly eaten lim-
pets can often be collected following observation of foraging.
Newly cleaned shells can easily be recognized by the fact that they
often have small bits of fresh flesh still attached, and almost
always they are lying upside down on top of rocks or algae. Only
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Oystercatcher impact on limpets
fresh shells were collected.
Shells from fresh prey provided the data in figure 3. Shells
collected from foraging site 1 on May 10, as well as all shells ever
collected at site 1, and shells collected at all sites, are plotted
separately for comparison. Throughout, C. pelta and C. digitalis
were selected frequently, while C. scabra were not. Also, limpets
between 10 and 25 millimeters in shell length were taken much more
often than those of other sizes.
Many more limpets were eaten during the total study period
than is suggested by the bulk data in fig 3. In most foraging sit-
uations it is impossible to collect many shells, since surge fre¬
quently washes freshly cleaned shells off the rocks before they can
be collected. In some cases the birds were foraging in places like
small rock islands that were inaccessible to me. In reality the
pair commonly consumed sixty or more limpets in a 15-30 minute for¬
aging period, but I was seldom able to collect more than a dozen
shells. Foraging site 1on May 10 was a valuable exception.
Visibility of limpets attacked:
The oystercatchers studied tended to take almost exclusively
limpets they could easily see, although they also neglect many
plainly visible limpets, notably C. scabra. Clearly, visibility
is not adequate to induce an attack. Figure I shows data collect-
ed at foraging site 1 on May 10, as well as general data from all
sites for comparison, with particular attention to the degree of con¬
cealment of the prey under attack. In many instances the limpet
attacked was plainly visible through the telescope on a patch of
bare rock. For attacks in which the prey was not plainly visible,
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Oystercatcher impact on limpets
the behavior of the bird was considered. If the bird directed the
usual conspicuous visually oriented strike at the limpet, then it
was recorded as 'in open' if the attack was within a stretch of
bare rock, and 'partially hidden' if directed alongside a clump of
algae. If the bird abtained the limpet by probing blindly in algae,
it was recorded as 'hidden'. All questionable cases were discarded.
I never observed the birds turning algae aside to uncover hidden
limpets. Clearly, limpets under algae were relatively safe from
oystercatchers in this study.
Slope preference of foraging oystercatchers:
Fig 5 presents data indicating that the birds seldom took lim-
pets which could not be reached from a slope of less than 15 degrees.
(Slopes were estimated through the telescope, and any which looked
very close to 15 degrees were considered greater than 15 degrées.)
General data collected throughout the birds' territory at various
times is again presented alongside the foraging site 1 data for
comparison. It is of interest that foraging site 1 provided ample
opportunity for the oystercatchers to try to get limpets on steer
slopes out of reach of gentle slopes. Limpets high on very steep
slopes (but not so high as to be in reach of a bird leaning over
the top) are effectively exempt from oystercatcher predation.
These birds were never seen trying to capture limpets while stand-
ing on slopes exceeding about 60 degrees.
Comparison of diet with availability:
Fig 6 directly compares availability of limpets in the most
commonly eaten size classes on foraging site 1 with actual consump-
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Oystercatcher impact on limpets
tion by the birds. They had a striking tendency to overlook C.
scabra, in spite of its overwhelming abundance in the 10 to 20
millimeter range, and to select instead the relatively rare C.
pelta and C. digitalis. Figs 6B and C are directly comparable to
6A. 6D is only intended to show that the general trends in limpet
choice by the birds are reflected in the particular data from site
The fact that Collisella digitalis are consumed in fair numbers
and C. scabra all but avoided emphasizes the question of whether
oystercatcher foraging activities might be in part responsible for
the localization of C. digitalis on vertical slopes, and the dom-
inance of C. scabra on exposed horizontal areas. Fig 7 depicts
measurements made on two transects on foraging site 1 which indi-
cate that the birds may indeed be a factor in this phenomenon. C.
digitalis is the dominant species on the vertical slope out of reach
of the oystercatchers, whereas C. scabra dominates in regions ac-
cessible to the birds, even on vertical slopes. An interesting
fact is that during observation of foraging site 1 the oyster-
catchers were observed to remove and eat several limpets (almost
certainly C. digitalis) from the lower region of the vertical face
shown in fig 7.
Discussion and Conclusions:
Data presented in figs 4 and 5 clearly demonstrate that lim-
pets out of reach on steep slopes and limpets hidden well under mac-
roalgae are quite safe from the oystercatchers in this study, since
the birds rarely attack such limpets. Other observers have also
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Oystercatcher impact on limpets
noted the visual nature of oystercatcher foraging (Helbing, 1976).
Although Hartwick (1976) mentions that they turn aside algae to find
what is underneath, I never observed such behavior. Nothing spe-
cific has been noted with respect to slope in other studies.
The observations concerning visibility and slope of prey are
most meaningful when related to diet composition and availability
(figs 2 and 3). As to prey visibility, the birds indeed may be re-
sponsible for the fact that most of the larger C. pelta one finds
are hidden under algae (like Pelvetia). The birds prefer C. pelta;
they take mostly limpets out in the open; and the C. pelta we find
are hidden.
It is striking that C. scabra is almost absent from the diet of
the oystercatcher, in spite of its abundance in acceptable size clas-
ses in the open (fig 6). Other researchers have found a similar
scarcity of C. scabra in the diet. Legg (1954) reported four C.
scabra fed to young oystercatchers at Point Lobos, California, in
a total sample of 111 limpets, and Helbing (1976) noted only three
C. scabra in a sample of 2360 limpets taken by oystercatchers in
Humboldt County, California. No information was provided on avail-
ability.
Helbing (1976) cites C. digitalis as the main item in the win-
ter diet of his birds. In my sample, C. digitalis was the second
most frequently consumed limpet (fig 2). The data in fig 7 suggests
that C. scabra may be abundant on horizontal surfaces partly be-
cause it is relatively immune to oystercatcher predation; C. digi¬
talis on the other hand are selectively removed from areas acces-
sible to oystercatchers, consequently most animals appear on ver-
tical surfaces out of oystercatcher reach.
10
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Oystercatcher impact on limpets
Why do the birds avoid C. scabra? While many C. scabra are
too small for oystercatchers, fig 6 demonstrates that many others
are not. Perhaps C. scabra are in general too low in profile to
induce attack. The order of decreasing limpet prey preference in
fig 3 is also approximately the order of decreasing species shell
height. Collisella pelta, Fissurella volcano, and C. digitalis
all have fairly high shell apices as compared to C. limatula and
C. scabra, which are scarce in the diet. Another possibility is
that the homing behavior of C. scabra may decrease its vulner-
ability to oystercatcher predation. Animals on their indented
'home scars' have even lower profiles than the same animals away
from home. Further, limpets at home fit tightly to the substrate
and may be more difficult to dislodge. Wells (1980) found that
C. scabra placed in aquaria with predatory octopi were less frequent.
ly captured when they had access to their home site than when on a
strange rock lacking their home site, suggesting that homing be-
havior may reduce predation. With regard to oystercatcher preda-
tion, the birds feed mainly at low tide (Helbing, 1976), precisely
when C. scabra are at their home sites. The only C. scabra which
I actually observed being captured (I ran down and retrieved the
shell as soon as the bird had eaten it, since I suspected that it
was a C. scabra), was taken near high tide when the capture site
was partially awash and the limpets were probably moving about
foraging instead of attached to their home sites (Brandt, 1950).
Whatever the reason for neglect of C. scabra and attention to
C. digitalis by oystercatchers, the birds represent a potential
cause for the differential distribution of the two species on hore-
zontal and vertical rock surfaces. Fig 7 shows another phenomenon;
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Oystercatcher impact on limpets
areas out of oystercatcher reach, occupied by C. digitalis, are
deficient in C. scabra. A possible explanation for this is the
finding by Haven (1973) that the two limpet species compete for the
same food supply, and that in mixed populations C. digitalis is the
superior competitor. In areas with high populations of C. digitalis,
C. scabra are less numerous and smaller. Collisella digitalis
dominates on steep slopes out of reach of oystercatchers, C. scabra
dominates horizontal rock faces, but it also dominates on vertical
faces in reach of oystercatchers.
One difference between my observations and those of Hartwick
(1976) is that Hartwick saw oystercatchers in British Columbia moving
algae aside in their search for prey. I did not, and the discrepan-
cy warrents further study. Different birds, or even different
groups of birds, may practice different methods of hunting. Indi-
vidual European Oystercatchers practice either the stabbing or ham-
mering technique of opening mussels, but not both (Norton-Griffiths,
1967). It is also possible that at lower densities of limpets ex-
posed to view or covered with algae, the birds may emphasize dif-
ferent hunting methods.
In conclusion, it is clear that the striking preferences ex¬
hibited by oystercatchers for certain species of limpets combined
with the limitations posed by their foraging methods may have im-
portant effects on the distribution and relative abundance of lim-
pet species in their foraging areas.
C
Acknowledgements
My sincere thanks go to my advisor Donald P. Abbott for his
endless ideas, time, suggestions, interest, and candid criticism,
without which this project would never have left the ground. I
also thank Charles Baxter for his assistance in identifying numer-
ous smelly limpet shells, as well as for his interest and en-
couragement.
Literature Cited
1. A.O.U. Checklist of American Birds, American Ornithologists'
Union, 1957.
Brandt, D. H.: A quantitative study of the homing behavior of
2.
the limpet Acmaea scabra. Spec. Prob. Rep. (Unpublished).
Zoology, University of California, Berkeley (1950).
3. Craig, P. S.: The activity pattern and food habits of the limpet
Acmaea pelta. Veliger 11 (Suppl.): 13-19 (1968).
Hartwick, E. B.: Foraging strategy of the Black Oystercatcher
1.
(Haematopus bachmani Audubon). Can. J. Zool., 51(2): 142-155
(1976).
5. Haven, S. B.: Niche differences in the intertidal gastropods
A. scabra and A. digitalis (Gastropoda) in central California.
Veliger, 13: 231-248 (1971).
----Competition for food between the intertidal gastropods
Acmaea scabra and Acmaea digitalis. Ecology, 54: 143-151 (1973).
Helbing, G. L.: Maintenance activities of the Black Oysterca-
6.
tcher, Haematopus bachmani Audubon, in Northwestern California.
Unpublished Master's thesis (1976).
Hewatt, W. G.: Observations on the homing limpet Acmaea scabra
7.
Gould. Amer. Midl. Natur., 24: 205-208 (1940).
Jobe, A.: A study of morphological variation in the limpet Acmaea
8.
pelta. Veliger 11 (Suppl.): 69-72 (1968).
Legg, K.: Nesting and feeding of the Black Oystercatcher near
Monterey, California. Condor, 56: 359-360 (1954).
10. Norton-Griffiths, M.: Some ecological aspects of the feeding be-
havior of the oyster catcher Haematopus ostralegus on the edible
mussel Mytilus edulis. Ibis, 109: 112-124 (1967).
11. Stout, G. D.: The shore birds of North America. 270 pp. New
York: Viking Press 1967.
12. Sutherland, J. P.: Dynamics of high and low populations of the
limpet Acmaea scabra (Gould). Ecol. Mongr. 10: 169-188 (1970).
13. Webster, J. D.: Feeding habits of the Black Oystercatcher.
Condor 56: 359-360 (1911).
11. Wells, R. A.: Activity pattern as a mechanism of predator a-
voidance in two species of Acmaeid limpet. J. Exp. Mar. Biol.
Ecol. 18: 151-168 (1980).
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Captions for Figures:
Figure 1. Map of the study site, showing the defended feeding ter-
rittory of the oystercatchers studied.
Species, sizes, and visibility of living limpets on for-
Figure 2.
aging site 1 after foraging was oblserved. Small num-
bers over each bar are the actual numbers of limpets.
Species and sizes of limpets eaten, (A) at all foraging
Figure 3.
sites, (B) at foraging site 1 on several days, and (C)
at foraging site 1 on May 10. Numbers in each column
are total numbers of each species in that column.
Visibility of limpets attacked, (A) throughout the ter-
Figure 1.
ritory, and (B) at foraging site 1 on May 10. Numbers
in bars show actual number of limpets attacked.
Slope preference shown by birds foraging, (A) throughout
Figure 5.
the territory, and (B) at foraging site 1 on May 10.
Numbers in bars show actual numbers of limpets attacked.
(A) Availability of each limpet species in each of three
Figure 6.
commonly attacked size ranges is compared with, (B) lim-
pets eaten in those size ranges at foraging site 1 on May
10. (C) with all limpets eaten at foraging site 1, and
(D) with limpets eaten at all foraging sites. Numbers in
each section are the total number of limpets in that sec-
tion.
Comparison of the occurrence of C. digitalis and C. scabr:
Figure 7.
in and out of reach of foraging oystercatchers. Birds
illustrate which parts of the rock face were in reach.
Bars represent average number of limpets of each species
per pair of corresponding quadrats (.0625 meters square,
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on parallel transects.