Yellow-legged Gulls along the
River Thames in Essex
M. K. Dennis
The improvement in water quality of the River Thames has led to an increase in
the diversity and numbers of gulls Lams, waders and wildfowl using the river in
recent years (Dennis 1992). This, coupled with the creation of several large rubbishtips in Thames-side Essex and Kent, has encouraged large numbers of gulls into
the area, particularly in autumn and winter. In the last decade, however, many gulls
have also been present throughout the summer, with a marked increase in the
number of Yellow-legged Gulls L. cachinnans since the mid 1970s. The present
paper documents this increase along the Thames and examines the behaviour and
movements of this now-regular late-summer visitor to southeast England. For
convenience, the outer Thames estuary as defined here includes all Essex sites east
of Mucking Flats, while the inner Thames includes those to the west.
Pre-1980s
The first indication that Yellow-legged Gulls were becoming more regular visitors
to Essex came in the early 1970s, mainly as a result of improved observer coverage
(Melville 1991). Regular monitoring of gull populations in the outer Thames
estuary in Essex revealed up to 12 Yellow-legged Gulls in August and September
1973 and 1974; these usually arrived in early July and remained to mid October,
a few staying into the winter (Melville 1991). Very few were seen elsewhere along
the river at that time, and none at any of the well-watched sites along the inner
Thames. The few London-area records in the early to mid 1970s were considered
to involve probable hybrid Herring L, argentatus X Lesser Black-backed Gulls L.
fuscus, although recent research has shown such mixed pairings to be very rare
(Yesou 1991). Detailed counts were lacking in the late 1970s, but small numbers
continued to be reported in most years until the end of the decade (Cox 1984),
mainly associated with increased numbers of larger gulls visiting big rubbish-tips
that had been established in south Essex in the late 1960s.
8
{Brit. Birds 88: 8-14, January 1995]
British Birds, vol. 88, no. 1, January 1995
9
Increase since 1980s
Signs of an upsurge became apparent in the early 1980s, with an increase in
sightings from the inner Thames. This spread to such sites as the West Thurrock
power-station outfall was possibly linked to the marked increase in terns Sterna
and gulls visiting these outfalls from Tilbury to Littlebrook in the summer
(Dennis 1992). Numbers remained small, however, and, although there were
scattered winter and early-spring reports, most records were from the typical
summer period.
In the summer of 1986, there was a marked increase in the numbers of Yellowlegged Gulls visiting the inner Thames. Up to 60 frequented the large rubbishtip at Stone, Essex, with smaller numbers (usually at low ride) using the wide
expanse of foreshore mud at Mucking/East Tilbury and West Thurrock. During
the next three summers, the flock at Stone continued to grow, peaking at 135 in
1989, while up to 100 were present at Mucking, with smaller numbers on the tips
at Rainham Marsh and East Tilbury. An inner-Thames population exceeding 150
adults was present in the summer of 1989 (fig. 1). As only the adults were
counted at that time, the true population was probably much greater. The gulls
tended to feed on the rubbish-tips and roosted on the Thames foreshore or on
exposed dry wasteland nearby.
Fig. 1. Annual numbers of Yellow-legged Gulls Lams cachiniians on the inner Thames in
Essex, 1978-92.
In 1990, when tipping at Stone ceased, Yellow-legged Gull numbers were
much lower along the inner Thames except at Mucking, where they were able to
utilise the tips in the East Tilbury area and to roost on the Mucking foreshore.
In 1991, the distribution shifted to Rainham Marsh, where a large tip had been
established on former silt-lagoons in the late 1980s. There was a slow build-up
during July and early August, reaching a peak of about 220 by the end of August
(fig. 2). This was by far the largest total of Yellow-legged Gulls recorded anywhere in Essex or London. About 75% were adults, immatures tending to arrive
in August (see below), but a flock of 220 adults was present along the Mucking
foreshore in late August, again the largest recorded there (fig. 3), and, as only
adults were counted, the true figure could have been nearly 300. Allowing for
possible duplication between the Rainham and Mucking figures, the inner-
10
Yellow-legged Gulls in Essex
Fig. 2. Timing of autumn occurrences of Yellow-legged Gulls Larus cachinnans
(histogram) and Herring Gulls L. argentatus (graph) at Rainham Marsh, Essex,
in 1991. Note peaks of former in the last week of August and of the latter in early to
mid October.
Fig. 3. Annual numbers of Yellow-legged Gulls Lams cachinnans at Mucking, Essex, 1982-92.
Thames population of Yellow-legged Gulls probably exceeded 400 individuals in
1991 (fig. 1). Very few were present elsewhere on the Essex side of the estuary,
but these included five immatures trapped at Pitsea in late July out of a total of
at least eight present (since very few were reported subsequently, they were
probably moving upriver to join the increasing flocks at Rainham and Mucking).
Small numbers continued to be reported along the inner Thames throughout the
autumn and early winter; some moved inland in Essex, to tips in the Aveley area
and to Hanningfield and Lea Valley reservoirs, while others wandered north along
the coast to the mouth of the River Colne.
Distribution during the year
In 1991, regular weekly counts were made of Yellow-legged Gulls at Rainham,
so as to establish the pattern of occurrence during the year (table 1 & fig. 2).
Occasional individuals were recorded during the first six months (particularly in
winter), but it was not until mid June that the first returning adults were present.
A slow increase took place during July, with at least 20 adults by the end of the
n
British Birds, vol. 88, no. 1, January 1995
Table 1. Counts of Yellow-legged Gulls Lams cachinnans at three sites along the Essex
shore of the inner River Thames, 1991. - = no count.
West
Mucking
Date
Rainham
Thurrock
8th June
15th June
22nd June
29th June
6th July
13th July
20th July
27th July
5th August
10th August
17th August
24th August
7th September
15th September
21st September
28th September
5th October
12th October
19th October
26th October
3rd November
10th November
0
0
3
2
9
12
20
18
17
40
95
220
125
55
35
25
15
8
6
2
2
1
0
0
4
2
2
2
5"
5
11
17
12
23
3
4
2
7
2
3
1
5
5
5
2
1
20
4
43
220
220
220
50
-
month, the rate of increase gathering pace from the end of the second week of
August, augmented by the first sub-adults and juveniles, to reach a peak of at least
220 at the end of the month. By the end of the first week of September, the flock
size had fallen by about half, as first the immatures and then the adults began to
disperse. A gradual decline followed throughout September and October, with
just a few remaining in the heart of the tip until the end of the year. This pattern
was repeated at Mucking in 1991, but with a slightly earlier peak (mid August)
and an earlier dispersal (September). At West Thurrock, the peak was also in the
last week of August, but a few (probably dispersing from Rainham or Mucking)
remained well into November.
As the Rainham gulls dispersed, small numbers appeared inland, where a few
remained well into winter. Yellow-legged Gulls also moved upriver as far as Barn
Elms and Staines reservoirs.
The same pattern was repeated in 1992 at Rainham, with up to 200 present
during the second half of August; the largest flock was made up of 80% adults,
immatures generally arriving during the first half of August. In contrast to 1991,
a large flock of 105 used the foreshore at Swanscombe, Kent, for roosting, usually
when the Rainham tip was not in operation. At Mucking, a peak of 200 was
recorded in August and, while there was probably some interchange with West
Thurrock (fig. 4) and Rainham, the total inner-Thames population in 1992 was
of over 350 individuals. There was also a wider autumn dispersal, with some
recorded along the Thames into London and up the Lea Valley to Nazeing. Up
to seven at Barn Elms and four at Nazeing Gravel-pit were apparently wintering:
unprecedented numbers for London at this season. Small numbers were also
recorded along the southeast Essex coast.
12
Yellow-legged Gulls in Essex
Fig. 4. Combined numbers of Yellow-legged Gulls Lams cackinnans at Rainham (black)
and West Thurrock (white), Essex, in 1991. Note peak in last week of August.
In 1994, there was an increasing tendency for adult Yellow-legged Gulls to
occur inland in southwest Essex. These individuals associated mainly with Lesser
Black-backed Gulls at inland rubbish-tips and on arable fields.
Most records and counts have involved adults, suggesting that adult Yellowlegged Gulls along the Thames move into the area from mid June onwards;
numbers build at favoured feeding sites and reach a peak in mid August, with a
gradual dispersal in September. Immatures do not appear until the second half of
July, and reach a peak towards the end of August; most appear to move out of
the area during the latter half of September, with very few present in autumn.
The 1991 counts at Rainham also revealed a scarcity of Herring Gulls during
the summer (fig. 2). Very few were seen during late August and early September,
after which numbers began to increase. The main influx occurred in early
October, with a fairly constant population thereafter of about 100 individuals.
The summer movements of Lesser Black-backed Gulls along the inner Thames
show an earlier arrival, in early June, with maximum numbers from late July to
early September. The main arrival of Yellow-legged Gulls therefore fits neatly in
between those of the other two species.
Behaviour
At Rainham Marsh, Yellow-legged Gulls form a compact flock in the heart of the
rubbish-tip or on the foreshore, keeping well apart from the other larger gulls. At
West Thurrock and Mucking, they tend to keep apart from Lesser Black-backed
Gulls, forming distinct groups on the foreshore mud. Adults and immatures
tend to form separate flocks, and spend much of their time apparently loafing
either on the tip or on the River Thames. The immature near-adults, with a
typically pale head, recall smaller versions of immature Great Black-backed Gulls
L. marinus.
It is probably significant that the areas with the largest Yellow-legged Gull
population are those with the large rubbish-tips and expanses of low-tide mud
along the river. The ones at Rainham usually remain on the tip all day, but when
disturbed by tipping activities they sometimes move the short distance on to the
foreshore. At West Thurrock, they spend most of their time on the foreshore
British Birds, vol. 88, no. 1, January 1995
13
mud, moving to nearby wasteground only at high tide. The smaller numbers
present in autumn and winter also keep very much to themselves within the tip
at Rainham and do not really mix with the other large gulls at any season.
Origin of British individuals
Recent studies in Belgium have given some indication of the reasons for this
sudden increase in Yellow-legged Gulls in Britain and of their origins (De Mesel
1990). Since the early 1980s, there has been a marked increase in Yellow-legged
Gull numbers in late summer along the North Sea coast of Belgium. During
1982-86, about 600 were ringed, mainly in July-September. The ringing
programme revealed that the main influx occurred from late July to mid August,
involving mainly individuals of the race michahellis from islands in the West
Mediterranean. When all known recoveries of southern Yellow-legged Gulls in
northwest Europe were combined, this showed a northward summer migration
through inland France to the coasts of northwest Europe. Colour-ringing also
indicated that juveniles were returning to the same sites in successive years. This
was presumably to capitalise on optimum feeding and roosting sites in order to
carry out moult. Most adults complete their moult by the end of October, which
is when most leave Belgium (De Mesel 1990).
These results fit in well with the picture of Yellow-legged Gulls along the inner
Thames, particularly with regard to their arrival and dispersal. It suggests that the
gulls move up through France from the Mediterranean in June and early July,
reaching the southern North Sea shores in mid July. While large numbers remain
in Belgium and France to moult, increasing numbers are now moving across the
North Sea into the Thames estuary, where they have exploited suitable roosting
and feeding sites, such as at Mucking, Stone and Rainham. Given that the main
increase in Belgium took place from the early 1980s, it correlates well with the
marked increase noted along the Thames from 1986 onwards. What remains a
mystery is the origin of those on the outer Thames estuary in the early 1970s,
well before the recent increase along the southern North Sea coasts of Britain and
Belgium.
The Yellow-legged Gull has, in less than ten years, changed from being a
scarce vagrant to become a regular late-summer and scarce winter visitor to the
River Thames in Essex. This is due in part to a rapid expansion and range
extension of the breeding area to the Atlantic coast of France (Yesou 1991) and
a corresponding increase along the southern North Sea coasts of Europe (De
Mesel 1990). Further expansion of Thames-side gulls to rubbish-tips and
reservoirs inland in Essex and London is already leading to increased numbers in
summer and winter. With small numbers remaining all year, it is interesting to
speculate as to whether this species may soon breed at one of the suitable sites
along the lower Thames in Kent or Essex.
Acknowledgments
My thanks go to Mark Hardwick for comments at the draft stage and for the production of the
graphs. Thanks are also due to Phil Vines, Tony Gray, Alan Bell, Ken Barrett, Stuart Pickering,
John Miller, Andy Kane, Norman Inguis, Nigel Wheatley, Andrew Moon, Mark Hardwick and
the late Rupert Hastings for providing many of the records and many comments over the years.
I should also like to thank Nik Roothaert and Dirk De Mesel for help with the Belgian data, and
14
Ycllow-lcggcd Gulls in Essex
Cleanaway Ltd for allowing access to the Rainham Marsh tip. Finally, I thank Tim Sharrock for
his encouragement to complete this paper.
Summary
Numbers of Yellow-legged Gulls Lams cachinnans recorded along the Thames in Essex have
increased markedly since the mid 1980s. This increase is described, together with the seasonality
of occurrence and the behaviour of the gulls. Reasons for this species' change in status in mis
area, from scarce vagrant to regular late-summer visitor and scarce winterer, are briefly discussed;
an annual northward movement takes place in June-July from West Mediterranean breeding sites.
References
Cox, S. 1984. A New Guide to the Birds of Essex. Essex Birdwatching Society.
Dl- MHSIU., D. 1990. Yellow-legged Gulls in Belgium 1982-1986. Le Gerfaut 80: 25-56.
DHNNIS, M. K. 1992. The birdlife of the Inner Thames—the rebirth continues. London
Bird Report 56: 157-174.
MHI.VIU.K, D. S. 1991. Yellow-legged Herring Gulls in Essex, 1973-74. Brit. Birds 84: 342343.
YHSOU, P. 1991. The syrnpatric breeding of Lams fuscus, Lams cachinnans and Lams
argetuatus in western France. Ibis 113: 256-263.
M. K. Dennis, 173 Collier Row Lane, Romford,
Essex RMS 3ED