Salmonella Carriage by Herring Gulls in the Clyde Area of Scotland in Relation to their Feeding Ecology
P. Monaghan, C. B. Shedden, K. Ensor, C. R. Fricker*
and R. W. A. Girdwood*
Department of Zoology, University of Glasgow, Glasgow G12 8QQ, and
*Department of Bacteriology, Stobhill General Hospital, Glasgow G21 3UW
(1) Between February 1982 and February 1984, 2985 herring gulls, captured
at refuse tips in the Clyde area, were examined for the presence of
salmonellae in their faeces; 9.2% of those examined in the breeding season
were found to be carrying salmonellae, and 9.8% during the non-breeding
season. The most common serotypes isolated were Salmonella virchow and
(2) The proportion of these herring gulls carrying salmonellae was significantly
positively correlated with the incidence of salmonellosis in the human
population in the same area at the same time, and presumably reflects the level
of environmental contamination.
(3) There were no statistically significant differences in carriage rates
between different age classes of herring gull, but during the non-breeding
season, there was a highly significant difference between the sexes. The rate
of female carriage at this time was more than double that of males, and
appears to reflect differences in their feeding ecology.
(4) A further 163 herring gulls obtained from breeding colonies were similarly
examined. These had a significantly higher carriage rate than those examined at
refuse tips at the same time, indicating that other kinds of feeding areas may
give rise to more frequent infection of gulls.
(5) There was no evidence that infection with salmonellae affected the health
of the gulls concerned. Such pathogen carriage by gulls, when coupled with
their considerable powers of dispersal, may give rise to potential public health
problems, particularly when gulls are roosting on potable water supplies.
In recent years, concern has been mounting over the role of Larus gulls in
the dissemination of human pathogens. In addition to their traditional
coastal haunts, these birds now feed at refuse tips and sewage outfalls, roost
at night on inland water reservoirs and by day on pastureland, and nest on
inhabited buildings. This brings them into proximity with man, his
pathogens, domestic animals, foodstuffs and water supplies. Moreover, the
present century has seen considerable population increases in all five species
of gull which commonly breed in Britain. In particular, the herring gull
(Larus argentatus Pontoppidan) has been doubling in numbers every 6 years for
much of this century (Chabrzyk & Coulson 1976), and its present population in
Britain is in excess of two million. This change in status is reflected in the
increased incidence of urban nesting herring gulls and the increasing numbers
found inland (Hickling 1977; Monaghan & Coulson 1977). In view of these
population changes, such problems as these birds create are likely to increase,
and, coupled with their feeding habits and considerable powers of dispersal, it
is not surprising that they have been implicated in the spread of infection
(Parsons & Duncan 1978; Monaghan 1980; Coulson, Butterfield & Thomas 1983).
Numerous salmonella serotypes have been isolated from the faecal material
of British gulls (e.g. Williams, Richards & Lewis 1976; Johnston, Maclachlan
& Hopkins 1979; Fenlon 1981; Reilly et al. 1981; Benton et al. 1983;
Butterfield et al. 1983). These bacteria are among the causative agents of
gastro-intestinal infections in man and domestic animals; over two and a half
thousand cases of such infection in humans are reported annually in Scotland,
which is likely to represent only a small fraction of the number occurring
Since these birds have been implicated in the spread of infection to man
and his domestic animals (e.g. Jones, Smith & Watson 1978; Johnston, Maclachlan
& Hopkins 1979) it is clearly important that we have a realistic picture of
the proportion of gulls carrying salmonellae, and the extent to which this
relates to their feeding ecology. This information, coupled with an
understanding of the movement patterns and behaviour of the birds, will enable
their role in the dissemination of these pathogens to be assessed, and the
possible hazards which they pose to public health to be evaluated. This paper
presents the results of a detailed study of the proportion of herring gulls of
known age and sex carrying salmonellae in the Clyde area of Scotland. The
results reported here form part of a wider study encompassing all of Scotland,
details of which are reported elsewhere (Girdwood et al. in press).
It was essential to this study that gulls were caught in large numbers at
different localities. This was done by cannon-netting flocks of gulls at
feeding sites, predominantly refuse tips, with up to 350 gulls being caught at
one time. Over 8000 herring gulls were caught at eight refuse tips in the
Clyde area (Fig. 1) by this method between October 1978 and February
1984. During the period February 1982 to February 1984, 2985 of these
herring gulls were examined for the presence of salmonellae in their faeces.
In addition, a further 163 herring gulls, obtained dead from culls during
1982 and 1983 at the breeding colony on Horse Island in the Firth of Clyde
(carried out by the Royal Society for the Protection of Birds during control
measures), were examined for the presence of salmonellae in the gastro-
In culled gulls the entire gut was cultured for the isolation of salmonellae,
whereas with captured birds faecal samples were obtained by cloacal washing.
This latter procedure identifies 15-20% fewer carriers than does the gut
culture (Girdwood et al. 1985). Data from culled birds are treated separately
in the analyses. Full details of the microbiological methods used in the
isolation of salmonellae from this material are given in Fricker, Girdwood
& Munro (1983) and Fricker & Girdwood (1984). The nomenclature used for the
Salmonella spp. isolated follows that recommended by the WHO Collaborating
Centre for Reference and Research on Salmonella (Anon. 1984).
The captured birds were aged in the hand up to the fifth year of life by
plumage characteristics (see Grant (1982) for details) and divided into first
years, intermediates (2-4 years) and adults (5th year and older) for the
purposes of analyses. Birds aged in the field were divided, for ease of
identification, into immatures (lst and 2nd years, i.e. basically brown birds)
and 3+ years (i.e. birds showing clear signs of adult plumage). Salmonella
carriage by herring gulls is considered here in relation to both the Clyde area
breeding population of these birds and the wintering population. Throughout
this paper the breeding season is taken as April to July, and the non-breeding
or wintering period as October to February.
FIG. 1. The Clyde area to which this study refers. The locations of the eight
refuse tips at which gulls were caught (circles) and of Horse Island (triangle),
the breeding colony from which the culled birds were obtained, are shown. The
dotted line ( ---- ) indicates the area within which all nocturnally roosting
herring gulls, and those feeding at refuse tips, were counted.
Standard biometric measurements (see Coulson et al. (1984) for details) were
taken from 61% of the captured birds. Live gulls from which biometric
measurements were taken were sexed using the combined head and bill
measurement which discriminates between the sexes with an accuracy of c. 95%
(Coulson et al. 1983a); dead gulls were sexed by dissection. The live
birds were released shortly after capture, having been individually marked
using a combination of dye marking and colour ringing; this permitted
subsequent field observations of relevant movements and behaviour of
individuals. The birds were also given a numbered metal ring, as a result of
which recoveries of dead birds were reported to us through the British Trust
for Ornithology Ringing Scheme.
Counts of birds feeding within the study area were made from inside a
vehicle, using binoculars; roosting birds were counted either as they came
into roosts at dusk, or after dusk using an image intensifier. In order to
estimate the proportion of herring gulls present in the area feeding at refuse
tips, regular weekly visits were made to all refuse tips and all nocturnal
roosting sites within a delimited area (Fig. 1) throughout one winter from
November to January, and the number of herring gulls counted. In addition,
herring gulls feeding at the refuse tip at Helensburgh were observed all day on
each of five consecutive weekdays once per month for 12 months. These data are
used here to give an estimate of the extent to which individual herring gulls
used a single refuse tip at different times of year. The other tips were
visited in a regular monthly circuit. All colour-ringed herring gulls present
on each visit to a tip were recorded. Movements of herring gulls were therefore
documented by means of repeated sightings of individually marked birds and
recoveries of dead birds by ourselves, co-workers and members of the public.
Salmonella carriage by herring gulls in the Clyde area
The overall incidence of salmonellae found in herring gulls caught at refuse
tips during the breeding season was 9.2% (588 birds examined) and 9.8% during
the non-breeding season (1433 birds examined).
TABLE 1. The salmonella serotypes isolated from herring gulls in the Clyde area.
The number of positives of each serotype is given, plus the percentage of the
total number of gulls (3148) examined which carried each serotype
Serotype Number of isolates Per cent of herring gulls examined
agona 5 0.16
anatum 2 0.06
braendenburg 1 0.03
bredeney 26 0.83
hvittingfoss 1 0.03
haardt 4 0.13
hadar 6 0.19
heidelberg 2 0.06
indiana 4 0.13
infantis 10 0.32
java 1 0.03
livingstone 4 0.13
mbandaka 10 0.32
montevideo 7 0.22
newport 12 0.38
panama 1 0.03
saint-paul 2 0.06
schwarzengrund 1 0.03
stanley 9 0.29
typhimurium 90 2.86
virchow 186 5.91
4,12:d- 1 0.03
6,7: r- 1 0.03
Table l lists the twenty-three salmonella serotypes isolated from the 317
positive faecal samples from herring gulls examined during this study. The
most common serotypes were Salmonella virchow and S. typhimurium, which
accounted for 48 and 23%, respectively, of the total number of salmonella
isolations in the Clyde area. The proportion of herring gulls caught at refuse
tips in the Clyde which were positive for salmonellae is strongly correlated
with the incidence of salmonellosis in the human population in the same area
at the same time (Fig. 2). Using gulls from which biometric measurements were
taken, and which could therefore be sexed, Table 2 shows the proportions of
herring gulls of each age and sex class, caught at the tips, which were found
to be carrying salmonellae at different times of year. Within a season, there
were no significant differences in the carriage rates between the age classes
for either males or females. When these data were examined in relation to
seasonal differences, there was a highly significant difference in the
carriage rates of females (X2 = 14.23, d.f. = 1, P < 0.001, all age classes
combined), the carriage rates during the non-breeding period (22%) being
three times that recorded during the breeding season (7%). There was no
significant difference in the carriage rates of males, which remained around
10% throughout the year (overall value for all age classes combined). The
incidence of salmonella carriage in the tip-caught birds did not differ
between the sexes during the breeding season. During the non-breeding
season however, the difference between the sexes was significant (Table 2,
X2 = 8.03, d.f. = 1, P < 0.01, all age classes combined); the rate of female
carriage (22%) was more than double that of males (10%). Table 3 gives the
carriage rates of adult breeding herring gulls culled at Horse Island during
the incubation period in May. As found for adults caught at refuse tips
during the same period, there was no difference between the sexes in the
carriage rate; however, the carriage rates in the culled birds were
considerably higher than those found at refuse tips (Tables 2 & 3).
FIG. 2. The proportion of herring gulls (%) per 4-weekly period which were
found to be infected with salmonellae, and the number of human cases
during the same periods in the same area. (Data from humans supplied by
the Communicable Diseases (Scotland) Unit, Ruchill Hospital). The two are
significantly correlated (r= 0.78, d.f. = 9, P < 0.01). The line drawn was
fitted by least squares regression (y = 0.074x + 5.44).
TABLE 2. The proportion of male and female herring gulls caught at refuse
tips in the Clyde area during the breeding and non-breeding seasons, which
were found to be +ve for salmonella carriage
Period Age (year) Number +ve Number -ve Percent +ve
April-July Adult 6 67 8.2
2-4 7 64 9.9
1st 8 32 20.0
October-February Adult 10 111 8.0
2-4 4 33 10.8
1st 7 36 16.3
April-July Adult 6 57 9.5
2-4 3 63 4.6
1st 1 26 3.7
October-February Adult 18 73 20.0
2-4 9 21 30.0
1st 8 31 20.5
Even when the Horse Island data are corrected to allow for the difference in
isolation rates between the two sampling methods (Table 3), the difference
between the adult herring gulls caught at colonies and those adults caught
at tips remains highly significant for both sexes (males X2 = 9.13, d.f. = 1,
P < 0.01; females X2 = 10.97, d.f. = 1, P < 0.001). Body weights of gulls
positive for salmonellae were compared with those that were negative, taking
account of differences in body size and seasonal weight changes (Coulson et al.
1983b). There was no indication of any loss of body condition in those gulls
found to be carrying salmonellae. Recovery rates of dead birds were also
examined, and there was no evidence to suggest that herring gulls which carried
salmonellae had higher mortality rates, or travelled shorter or longer
distances, than those which did not.
TABLE 3. The salmonella carriage rates of breeding herring gulls obtained
from culls on Horse Island in the Firth of Clyde in 1982 and 1983. The
figures in parentheses represent the proportion of positives and negatives
corrected to take account of differences in isolation techniques between
these birds and live birds caught at tips during the same period; this enables
comparisons between the two groups
Number Number Per cent Corrected
+ve -ve +ve per cent +ve
Males 18 (14) 31 (35) 36.7 (28.6)
Females 30 (25) 46 (51) 39.5 (32.9)
The use of refuse tips by herring gulls in the Clyde area
Greater numbers of herring gulls are found at the refuse tips in winter than
in summer, as can be seen from Fig. 3, which shows the mean number
present at a refuse tip near Helensburgh in the Clyde area throughout the
year. Other tips in the area show the same pattern, with large numbers,
particularly of adults present in the winter months (Shedden, 1983). Data
based on the weekly observations at Helensburgh tip showed that individual
birds were more likely to be present at this particular site on more than 1
day per week in winter than in the breeding season (Table 4); there was no
difference between age or sex classes in this respect.
FIG. 3. The mean (+ 1 S.E.) number of herring gulls present at Helensburgh
refuse tip throughout the year. (RED), the numbers of 1st + 2nd years;
(BLUE), the older age classes.
The counts of the numbers of herring gulls roosting in the area shown in
Fig. 1, in comparison with the numbers counted feeding at refuse tips in the
same area showed that, on average, 77% of the herring gulls roosting in the
area were to be found feeding at refuse tips on weekdays during the winter
period (Table 5). While there is clearly not a closed population roosting and
feeding in this area, these data demonstrate that refuse tips are a major food
supply for herring gulls wintering in the Clyde area.
TABLE 4. The number of individually marked herring gulls present on I
day per week and more than I day per week at Helensburgh refuse tip,
during the breeding and non-breeding seasons. A greater proportion of the
birds was present at the tip more than I day per week outside the breeding
season (X2 = 4.9, d.f. = 1. p < 0.05)
Period Number present 1 day week-1 Number present > 1 day week-1
April-July 124 (62.7%) 74 (37.3%)
October-February 162 (52.3%) 148 (47.7%)
TABLE 5. The number of herring gulls roosting overnight within a
delimited area (see Fig. 1) and the number within this area found feeding at
refuse tips. The proportion of the roosting population feeding at tips is also
shown (see Shedden (1983) for further details)
November December January
Number roosting 8500 13950 10000
Number feeding at tips 7250 8330 8660
Per cent feeding at tips 85 60 86
Difference between the sexes in the use of refuse tips
Of 1520 adult herring gulls caught at tips in the Clyde area during the non-
breeding season, 64% were female; similarly 64% of 464 adult herring gulls
caught in the same area during the breeding season were female. Amongst
the younger age classes, females predominated at tips during the non-
breeding season (61% of 790 1-year-olds and 58% of 566 2-4-year-olds),
but during the breeding season the sex ratio in these age classes was almost
equal (48% of 190 1st year and 47% of 382 2-4-year-olds were female). As
stated above (Table 4), both sexes visited a particular tip more often in
winter than in summer, and a greater number of herring gulls was present at
tips on any one day in winter (Fig. 3). The high proportion of females found
at tips could be due to fewer males than females being present in the Clyde
area overall, or to males using other kinds of feeding site. Observations on
the sex ratio of herring gulls at different types of feeding site support the
latter suggestion. Of twenty-four adult herring gulls caught behind trawlers
in the Clyde area during the non-breeding season in 1981, sixteen (67%)
were male, which differs significantly from the sex ratio in catches at tips in
the Clyde area at the same time of year X2 = 8.32, d.f. = 1, P < 0.01).
Herring gulls do move regularly between different kinds of feeding sites,
and our sightings of birds marked at tips also indicate that males are more
likely to follow fishing boats than females. Three times as many marked
males as females were subsequently seen feeding around fishing boats
(twenty-one males and seven females), a highly significant difference from
the proportion expected based on the numbers marked (3101, of which
47% were male; X2 = 9.07, d.f. = 1, P < 0.01). It thus appears that,
while refuse tips are clearly utilized extensively by herring gulls in the
Clyde area, particularly in winter, male herring gulls use them
comparatively less often than females.
Dispersal of herring gulls
Clearly, a comparatively high proportion of the herring gulls in the Clyde area
carry salmonellae. During the winter period, between twenty-five and thirty
thousand herring gulls roost overnight in the area, and the breeding population
is of the order of five and a half to six thousand. Herring gulls are highly
mobile and undertake both seasonal movements to and from the breeding colonies
in spring and autumn, respectively, and day-to-day movements to and from the
feeding sites. A proportion of the breeding population in the Clyde area also
overwinters in that area, while others move south at the end of the breeding
season. There is a corresponding movement of herring gulls into the Clyde
area in winter from the breeding colonies to the north and west. The
locations of these colonies are shown in figure 4a. The range of movements of
younger birds, which are not tied to breeding colonies, is shown in figure 4b.
These data demonstrate the extent of the movements of which these birds are
capable, involving distances of hundreds of kilometresin a short time. Our
sightings records show both immatures and adults moving over 250 km in less
than a week, clearly minimal estimates.
The data presented in this paper show that a high proportion of herring gulls
feeding at tips in the Clyde area (c. 10%) are carrying salmonellae. The
carriage rate in the Clyde area is generally higher than that which we have
found elsewhere in Scotland using the same methods, and such regional
differences appear to be related to human population density (Girdwood et
al. in press). Furthermore, the salmonella serotypes present in gulls and
humans are similar, with S. typhimurium and S. virchow being amongst the most
common in both groups, and these serotypes are also those most commonly
isloated from environmental sources (Table 1; Anon. 1982, 1983). Since gulls
typically carry only small numbers of salmonellae (Fenlon 1981; Girdwood et
al. in press), it is unlikely that they infect humans directly. These data
suggest that the close association which now occurs between gulls and man
gives rise to the infection of gulls with human pathogens; this could result
either from ingestion of infected material at feeding sites, or the drinking
of polluted water while roosting on contaminated estuaries. It is likely that,
when the degree of contamination of the environment with salmonellae increases,
the rate of carriage of these pathogens by gulls increases.
Fig. 4.(a) Locations where marked adult herring gulls, caught in the Clyde area
outside of the breeding season, were found to breed. (b) The locations where
marked immature herring gulls (<4 years old), caught in the Clyde area, have
been seen or found dead. Birds have also been recovered in North Norway,
Denmark and Belgium.
The large number of herring gulls feeding in the Clyde area, particularly
in winter, make considerable use of food sources supplied from human waste,
such as refuse tips and sewage outfalls, which are likely to be contaminated
with salmonellae (Durrant & Beatson 1981; Fenlon 1983; Fricker 1984). They
appear to be more dependent on these sources than are gulls elsewhere in
Scotland since there is no large supply of fish material, their traditional
diet, available at fishing ports or around fishing fleets in the vicinity.
Landings of fish species from which offal and discards are produced (mainly
cod, haddock and whiting) are very small in the Clyde area compared with the
North Sea fishing ports on the east coast of Scotland (Fisheries Statistics
Unit, D.A.F.S., personal communication). This study has shown that many more
herring gulls are present at Clyde tips in winter than in summer, as has been
found in other areas (Horton, Brough & Rochard 1983). More than three quarters
of the herring gull population wintering in the Clyde area is found feeding at
refuse tips. It is more difficult to estimate the proportion feeding at tips
during the breeding season. The total population cannot be estimated from
roost counts since the adults, plus an unknown proportion of younger birds,
roost at the breeding colonies. In addition, counts of birds at tips are less
reliable because breeding birds also spend less time loafing at tips, making
only short foraging trips to and from the nesting site.
The association between the prevalence of salmonella carriage by gulls and
their feeding ecology is further demonstrated by the difference between the
sexes in both these respects. Other studies (Spaans 1971; Kihlman & Larsson
1974) have found a preference in herring gulls for the more traditional kinds
of feeding areas, rather than refuse tips; the latter are used most when the
weather is severe. What food resources are available to herring gulls from the
fishing industry in the Clyde appear to be preferentially used by adult males,
and the majority of herring gulls present at tips in the area are female.
However, the male herring gulls present at tips dominate the females and, when
competition is severe, as in winter, males monopolize the best feeding areas
of freshly dumped refuse (Monaghan 1980; Greig, Coulson & Monaghan in press).
Females tend to concentrate on secondary areas of older refuse, and thus will
have a higher intake of more putrid food, which may contribute to their
increased salmonella carriage in winter. Furthermore, since the availability
of alternative food sources is reduced in hard weather, and there are also
more birds in the area in winter, the dependence of females on feeding areas
such as tips is likely to be greatest at this time.
No statistically significant differences were found between age classes
in this study, though other workers have found that elsewhere 1st year herring
gulls are more likely to carry salmonellae than adults, possibly due to their
increased use of sewage outfalls for feeding (McDonald & Brown 1974,
Butterfield et al. 1983). Individual herring gulls show a degree of
specialization in feeding site, particularly during the breeding season (Davis
1975; Sibly & McCleery 1983). Thus samples of herring gulls obtained from
culls at breeding sites include birds which have feeding specializations other
than refuse tips, such as foraging on the coast, at sea or at sewage outfalls.
The high rate of salmonella carriage found in adult herring gulls obtained at
breeding colonies, in comparison with those adults obtained at refuse tips at
the same time (the majority of which will also be breeding birds), suggests
that there may be another source of contamination of gulls other than tips.
High incidences of salmonellae have been recorded in gull droppings collected
around sewage outfalls (Fenlon 1981, 1983; Fricker 1984) and it is likely
that these feeding sites contribute to the higher incidence found in gulls at
the Horse Island colony, since, in summer, large numbers of birds from this
site are known to frequent sewage outfalls on the nearby Ayrshire coast and
also follow sewage sludge boats dumping in the Firth of Clyde (N. Metcalfe
There was no evidence in this study that contamination with salmonellae
affected the health of the gulls concerned, a similar result to that found
elsewhere (Butterfield et al. 1983). Experimental studies with captive
gulls have shown that salmonella carriage is generally passive, lasting only a
few days and that the gulls are not actively infected (Girdwood et al. in
press). It is unlikely that ingestion of contaminated gull faeces on
pastureland would lead directly to salmonella infection in livestock, other
than in animals rendered susceptible through illness or stress (Spence &
Westwood 1978). However, numerous salmonella outbreaks amongst livestock in
Scotland have been attributed to infections having been brought into the area
by gulls (Reilly et al. 1981). The dispersal powers of gulls are considerable
and, particularly during spring and autumn, they undertake rapid and
comparatively long range migrations to and from the breeding areas. They could
therefore be involved in the dissemination of pathogens over considerable
distances within Britain.
The hazards arising from the pollution of potable water supplies by
roosting gulls, while potentially more serious, are usually offset by the fact
that the water is disinfected before distribution to the public. Gulls feeding
at coastal sewage outfalls will mostly roost on the coast rather than on inland
reservoirs. However, the distribution of herring gulls inland in winter is
greatly influenced by the distribution of refuse tips, and gulls feeding at
inland refuse tips tend to use adjacent water storage reservoirs as night
roosts (Benton et al. 1983; Horton, Brough & Rochard 1983; Shedden
1983). In some areas of Scotland, rural water supplies receive no disinfection.
In this situation, if there is a refuse tip nearby, every effort should be
made to safeguard the purity of the supply by preventing gulls from roosting
on the water at night, or bathing in it by day. This is especially true in
winter since the numbers roosting nocturnally on water are greatest at this
time of year.
We thank the District Authorities in Strathclyde Region for access to refuse
tips, all those who assisted in catching and sighting gulls, the
Communicable Diseases Scotland Unit, Ruchill Hospital for providing
comparative data on humans and D. Munro for much assistance in the
laboratory. Thanks also to N. Metcalfe, J. C. Coulson and J. Butterfield for
comments on an earlier draft of the manuscript. This work was supported by
a grant from the Scottish Home and Health Department.
Anon. (1982). Communicable Diseases in Scotland. Annual Summary of
Salmonellosis 1982. CDS Unit, Ruchill Hospital, Glasgow.
Anon. (1983). Communicable Diseases in Scotland. Annual Summary of
Salmonellosis 1983. CDS Unit, Ruchill Hospital, Glasgow.
Anon. (1984). Antigenic Formulae of the Salmonella. WHO Collaborating
Centre for Reference and Research on Salmonella, Institut Pasteur, Paris.
Benton, C., Khan, F., Monaghan, P., Richards, W. N. & Shedden, C. B.
(1983). The contamination of a major water supply by gulls
(Larus sp.). A study of the problem and remedial action taken.
Water Research. 17, 789-798.
Butterfield, J., Coulson, J. C., Kearsey, S. V., Monaghan, P., McCoy, J. H.
& Spain, G. E. (1983). The herring gull Larus argentatus as a
carrier of salmonella. Journal of Hygiene, Cambridge, 91, 429-436.
Chabrzyk, G. & Coulson, J. C. (1976). Survival and recruitment in the
herring gull Larus argentatus. Journal of Animal Ecology,
Coulson, J. C., Butterfield, J. & Thomas, C. (1983). The herring gull Larus
argentatus as a likely transmitting agent of Salmonella montevideo
to sheep and cattle. Journal of Hygiene, Cambridge, 91, 437-443.
Coulson, J. C., Thomas, C. S., Butterfield, J. E. L., Duncan, N., Monaghan,
P. & Shedden, C. (1983a). The use of head and bill length to sex live gulls
(Laridae). Ibis, 12S, 549-557
Coulson, J. C., Monaghan, P., Butterfield, J. E. L., Duncan, N., Shedden, C.
& Thomas, C. S. (1983b). Seasonal changes in the Herring Gull in Britain:
weight, moult and mortality. Ardea, 7, 235-244.
Coulson, J. C., Monaghan, P., Butterfield, J. E. L., Duncan, N., Ensor, K.,
Shedden, C. & Thomas, C. S. (1984). Scandinavian Herring Gulls wintering
in Britain. Ornis Scandinavica, 15, 79-88.
Davis, J. W. F. (197S). Specialization in feeding location by herring gulls.
Journal of Animal Ecology, 44, 795-804.
Durrant, D. S. & Beatson, S. H. (1981). Salmonellae isolated from domestic
meat waste. Journal of Hygiene, Cambridge, 86, 259-264.
Fenlon, D. R. (1981). Seagulls (Larus spp.) as vectors of
salmonellae: an investigation into the range of serotypes and numbers of
salmonellae in gull faeces. Journal of Hygiene, Cambridge, 86,195-202.
Fenlon, D. R. (1983). A comparison of salmonella serotypes found in the
faeces of gulls feeding at a sewage works with serotypes present in the
sewage. Journal of Hygiene, Cambridge, 91, 47-52.
Fricker, C. R. (1984). A note on salmonella excretion in the black-headed
gull (Larus ridibundus) feeding at sewage treatment works
Journal of Applied Bacteriology, 56, 499-502.
Fricker, C. R. & Girdwood, R. W. A. (1984). The effect of the use of
different selective media on the ability to recover salmonellae from seagull
faeces Journal of Hygiene, Cambridge, 93, 35-42.
Fricker, C. R., Girdwood, R. W. A. & Munro, D. (1983). A comparison of
enrichment media for the isolation of salmonellae from seagull cloacal
swabs. Journal of Hygiene, Cambridge, 91, 53-58.
Girdwood, R. W. A., Fricker, C. R., Munro, D., Shedden, C. B. &
Monaghan, P. (in press). The incidence and significance of salmonella
carriage by gulls (Larus spp.) in Scotland. Journal of Hygiene.
Greig, S. A., Coulson, J. C. & Monaghan, P. (in press). Feeding strategies
of male and female herring gulls Behaviour.
Grant, P. J. (1982). Gulls: A Guide to Identification. T & A. D. Poyser Ltd.,
Hickling, R. A. O. (1977). The inland wintering of gulls in England
& Wales. Bird Study, 24, 79-88.
Horton, N., Brough, T. & Rochard, J. B. A. (1983). The importance of
refuse tips to gulls wintering in an inland area of south-east England.
Journal of Applied Ecology. 20, 751-765
Johnston, W. S., Maclachlan, G. K. & Hopkins, G. F. (1979). The possible
involvement of seagulls (Larus sp.) in the transmission of
salmonella in dairy cattle. Veterinary Record, 105, 526-527.
Jones, F., Smith, P. & Watson, D. C. (1978). Pollution of a water supply
catchment by breeding gulls and the potential environmental health
implications Journal of Institute of Water Engineers & Scientists, 32,
Kihlman, J. & Larsson, L. (1974). On the importance of refuse dumps as a
food source of wintering Herring Gulls Larus argentatus Pont.
Ornis Scandinavica, 5, 63-70.
MacDonald, J. W. & Brown, P. D. (1974). Salmonella infection in wild
birds in Britain. Veterinary Record, 94, 321-322.
Monaghan, P. (1980). Dominance and dispersal between feeding sites in the
Herring Gull (Larus argentatus). Animal Behaviour, 28, 521-527.
Monaghan, P. & Coulson, J. C. (1977). The status of large gulls nesting on
buildings. Bird Study, 24, 89-104.
Parsons, J. & Duncan, N. (1978). Recoveries and dispersal of Herring Gulls
from the Isle of May. Journal of Animal Ecologv, 47, 933-1005
Reilly, W. J., Forbes, G. 1., Patterson, G. M. & Sharp, J. C. M. (1981).
Human and animal salmonellosis in Scotland associated with environmental
contamination, 1973-74. Veterinary Record, 108, 553-555.
Shedden, C. B. (1983). Feeding and roosting behaviour of gulls:
implications for water storage. Ph.D. thesis. University of Glasgow.
Sibley, R. M. & McCleery, R. H. (1983). Increase in weight of herring gulls
while feeding. Journal of Animal Ecology, 52, 35-50
Spaans, A. L. (1971). On the feeding ecology of the Herring Gull
Larus argentatus Pont. in the northern part of the Netherlands.
Ardea. 59, 73-188.
Spence, J. B. & Westwood, A. (1978). Salmonella agona
infection in sheep. Veterinary Record, 102, 332-336
Vernon, E. (1977). Food Poisoning in England and Wales 1973-75. Public
Health, London, 81, 225-235.
Williams, B. M., Richards, P. W. & Lewis, J. (1976). Salmonella infection
in the herring gull (Larus argentatus). Veterinary Record. 98,
Check out my Home Page