Yellow-legged Gulls along the River Thames in 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 rubbish- tips 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 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 power-station outfall was possibly linked to the marked increase in terns Sterna and gulls visiting these outfalls from 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 Yellow- legged Gulls visiting the inner Thames. Up to 60 frequented the large rubbish- tip at Stone, Essex, with smaller numbers (usually at low ride) using the wide expanse of foreshore mud at Mucking/ and . 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 any­ where 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 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 British Birds, vol. 88, no. 1, January 1995 n

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 Date Rainham Thurrock Mucking 8th June 0 0 2 15th June 0 0 1 22nd June 3 4 20 29th June 2 2 4 6th July 9 2 43 13th July 12 2 - 20th July 20 5" - 27th July 18 5 220 5th August 17 11 220 10th August 40 17 220 17th August 95 12 - 24th August 220 23 - 7th September 125 3 50 15th September 55 4 - 21st September 35 2 - 28th September 25 7 - 5th October 15 2 - 12th October 8 3 19th October 6 1 26th October 2 5 3rd November 2 5 10th November 1 5 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 Yellow- legged 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: 342- 343. 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