Yellow-Legged Gulls Along the River Thames in Essex M
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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 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 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 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/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 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 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 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.