m. Moore Gull Predation on Nuttallina
Page 2
Introduction
Nuttallina californica (Reeve, 1847) is an inter-
tidal chiton often found in association with the mussel
Mytilus califorgianus (Conrad,1837), and the barnacle
Pollicipes polymerus (Sowerby, 1833) on californian rocky
shores (Ricketts & Calvin, 1952).
This report is about predation on N. californica by
the western gull, Larus occidintalis (Audubon, 1839).
The gulls are voracious and opportunistic feeders
(Hubbs et al., 1970). After feeding, the birds regurgitate
undigested material, particularly the bones, shells and
exoskeletons of their prey; consequently examination of
regurgitation pellets (hereafter "pellets") vields
information on the gulls' food choice.
Materials & Methods
The study was conducted from May 10 to June 1.
1974 at Granite Point, a precipitous rocky peninsula
located in the Point Lobos State Park Reserve (Monterey
County).
The eighteen western gulls observed during the study
appeared to have already established breeding territories.
They were studied during the beginning of their breeding
season (Harper, 1971; Tinbergen, 1953), and each gull was
seen daily near a specific nesting site. Breeding animals
restructed much of their foraging to the nearby rocky
intertidal zone and offshore waters. Alien gulls were rarely
seen in the study area.
M. Moore Gull Predation on Nuttallina
Page 3
Granite Point proved excellent for my study.
The gulls were rarely disturbed and the brooding sites
on bare rock were far enough'from tourist trails that
the pellets were not disturbed.
After the gulls were counted, accessible brooding
sites were marked and old pellets were collected and
Characterized. Each of the sites was then visited
daily and fresh pellets were collected.
Many of the old pellets had fallen apart and
their contents scattered, thus quantification was
difficult. Fresh pellet size was used to estimate
the number of old pellèts at a site. This uas done
by spreading the contents of fresh pellets to the
same density as that of the remains of old pellets
and comparing the areas covered.
The population of Nuttallina californica around
Granite Point was estimated by counting the nümber
of N. californica per square meter in quadrates every
10 meters along the shore. Areas of markedly differing
densities were thus defined and the numbers of N. californica
were found by multiplying the average density of N.
californica by the area size. The totals for each
area were added, which gave an estimate of the total
number of N. californica in the study area.
Finally, early morning activities of foraging
gulls were observed on three separate days.
Core
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Besults & Discussion
A total of 36 fresh pellets and an estimated
total of 65 old pellets were collected and analyzed.
Ihe results are shown in Table 1.
While plates of the barnacle Pollicipes polymerus
were the most common material found in the pellets.
an observation in accord with Martini (1966), over
a quarter of the pellets examined contained the plates
of at least one Nuttallina.
Of 24 old pellets containing Nuttallina, only
two had plates of three animals and none had more
than that. The four fresh pellets containing Nuttallina
had plates from 3, 8, 9, and 11 animals. A likely
explanation seems that one or several gulls
changed their feeding habits during the course
of the study, either by feeding in an area
containing a higher density of Nuttallina or by deliberately
seeking out Nuttallina in greater numbers.
The data indicate that gulls feeding in
the Pollicipes polymerus area concentrate most heavily
on P. polymerus.(Table 1). This might be explained
by the easy accessibility of P. polymerus to the gull
or by the gull actively seeking out the P. polymerus,
due to its taste or some other factor. In 22 of the
28 pellets containing Nuttallina californica, P. polymens
was also present. The gulls were likely eating the
N. californica and P. polymerus in proportion to
the numbers they were encountering.
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M. Moore Gull Predation on Nuttallina
The 43 pellets containing P. polymerus but no N. californica
may represent foraging in a zone with a high P. polymerus
to N. californica ratio.
The low number of Mytilus californianus shells
in the pellets is probably due to the gulls' habit
of breaking open the mussels and eating the flesh,
instead of ingesting the whole shell.
The high densities of M. californianus and Pollicipes
polymerus probably act as "predation-buffer" For
the N. californica as has been found for predation
on these organisms by starfish (Landenberger, 1968).
Isopods were found only in fresh pellets; if
left over night these lightweight pellets would be
swept away by wind. Thus, the isopods probably made
up more of the gulls diet than Table 1 indicates.
Barnacle shells, chiton plates, mussel shells, and
limpet shells appeared to be quantitatively recoverable
from month-old pellets.
The contents of the pellets indicate opportunistic
feeding of the gulls and indicate the gulls
often feed as a group, concentrating a days effort
on a habitat. On the morning of May 24 all the gulls
were observed to be feeding on schooling fish,
and all but one of the pellets that day and the next
contained only fish bones.
The effect of predation by the western gulls
on Nuttallina californica can be estimated by assuming
three things.
i. moore
Gull Predation on Nuttallina
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First, the foraging of the gulls is limited to the
general area around Granite Point, due to the gulls
breeding behavior and to the abundant food sources
around the point. The second assumption is that each
gull regurgitates one pellet per day. The final assumption
is that there is an average of 0.6 N. californica
per pellet. This is based on the overall ratio of
52 Nuttallina insthe 102 pellets found. The breeding
season of the western gull is roughly 90 days (Schreiber.1970:
linbergen, 1953), so the best estimate to be made
is as follous:
18 gulls x 1 pellet/day x O.6 chitonsx
90 days
Area
guii
pellet
sasing
972 Nuttallina
area--breeding
season
The number of Nuttallina in the area was estimated
at b,500, the average density being approximately
3 Nuttallina/meter-squared around the point; consequently
the gulls preyed on approximately 15% of the Nuttallina
around the point during the breeding season. This
estimate could be clearly be improved with the attainment
of more precise data; yet it serves to indicate the
possibility that western gull consumes a significant
proportion of Nuttallina in some localities.
Summary
In a study of the contents of western gulls' regurgitation
pellets, Pollicipes polymerus, the gooseneck barnacle,
was shown to be the most abundant material. The chiton
Page ?
M. Moore
allina
Gu.
Nuttallina californica was also found to occur in
the pellets in relatively high fregrency, indicating
that the western gull is a major predator on Nuttallina,
perhaps taking 15% of the population around the study
site per breeding season.
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M. Moore Gull Predation on Nuttallina
Acknouledgments
I wish to thank Alan Bakdridge, Judson Vandevere,
Anne Deßevoise, Bob Tenold, Chris Harrold, James Beck,
and Robin Burnett. In addition I am endebted to Point
Lobos State Park Reserve for allowing this study to be
carried out.
M. Koore
Gull Predation on N. californica
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Literture Cited
Harper, Charles A.
1971. Breeding biology of a small colony of western gulls
(Larus occidentalis wymani) in California. Condor 73: 337-341:
3 figs.
Hubbs, Carl L., Arthur L. Kelly and Conrad Limbaugh
1970.
Diversity in feeding by Brandt's cormorant near San Diego.
Calif. Fish Game 56 (3): 156-165
Landenberger, Donald E.
1968. Studies on selective feeding in the Pacific starfish
Pisaster in southern California. Ecology 49 (6): 1062-1075:
6 figs.
(5 July 1968)
Martini, Erland
1966. Otolithen in Gewöllen der Westmöve (Larus occidentalis).
Bonner Zool. Beitr. 3/4: 202-227
Ricketts, Edward F. and Jack Calvin
1952. Between Pacific tides. örd ed. Revised by Joel W. Hedgpeth.
xiii + 502 pp.; 134 figs.; 46 plts. Stanford, Calif. (Stanford
Univ. Press)
Schreiber, Ralph W.
1970.
Breeding biology of western gulls (Larus occidentalis) on
San Nicolas Island, California, 1968. Condor 72: 133-140; 2 figs.
Tinbergen, Niko
1953. The herring gull's world. xvi + 255 pp.; 58 figs.; 51 plts.
London, U.K. (Collins Clear-Type Press)
M. Moore Gull Fredation on N. californica
Table Caption
Table 1
the number of regurgitation pellets
Data on
in which individual animalswere found.
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I.. Moore Gull Predation on N. californica
Table 1
Number of Fellets in
Animal
which animal is found
—

65
Follicipes polymerus
Fish
43
Nuttallina californica
28
Limpets
16
Katharina
tunicata
Crab
Isopod
tilus
californianus
Mopalia muscosa
Urchin
Total Pellets = 102