Wintering Gulls in Worcestershire

By Peter Stewart

During the period January 1976 to December 1996 two independent gull groups, but with similar aims, collected data relating to the numbers, movements and migration of gulls feeding at landfill sites. Both groups' cannon-netted birds on landfill sites and fitted the birds with British Trust for Ornithology (BTO) supplied rings. For the Worcestershire and Warwickshire Wintering Gull Project (1976 - 1988), research was carried out on landfill sites in Worcestershire, Warwickshire, and parts of the West Midlands. Many individuals were involved in this project; too many to list here but included members of the Worcester Gull Group, Arden Ringing Group, and the Coleshill Ringing Group. Most of the findings from this project were published, including valuable data relating to the incidence of Salmonella in gulls. This involved cloacal swabbing and the collecting of faecal samples for microbiological examination. The Severn Estuary Gull Group (SEGG) netted birds on landfill sites in Gloucestershire, Hereford & Worcestershire during the period 1986 - 1996. This small group is still very much active, but confines its activities to landfill sites in Gloucestershire. This paper combines the results of the work carried out by both groups, for the County of Worcestershire, and for the period 1976 - 1996 only, and referred to hereafter as this study. It gives the number of birds caught and ringed and summaries of all known recoveries, controls and retraps. It also includes personal observations of the author who is a member of the Severn Estuary Gull Group, and has made a study of gulls at landfill sites and elsewhere for the last 18 years.

Part 1. The Black-headed Gull L. ridibundus

Foreign breeding birds heavily supplement the breeding population of the Black-headed Gull in Britain that winter in the Country. The total number of wintering birds that winter in Britain and Ireland is in the order of 3,000,000 birds, with some two thirds of foreign origin. It is the most commonly ringed gull in Britain, 330,000 from 1909 - 1998 (Clarke et al 2000), with some 17,000 recoveries, mainly abroad. This study has established that a large number of foreign birds winter in Worcestershire, and particularly birds from the Low Countries. Retraps have shown that many are faithful to this wintering area, both in the same year and subsequent years. There is also a strong tendency to use the same landfill site. One individual coloured ringed in Denmark in 1988, with a White Darvic inscribed U15, is a regular visitor to this area, and was last observed at Throckmorton in January of this year. Many birds ringed during the 1976 - 1988 project were being caught by the Severn Estuary Gull Group up to 18 years later. Those that do not stay in the area during winter tend to remain in the Midlands or move to coastal areas (Stewart 1995, 1996).

Number of birds caught

A total of 4345 Black-headed Gulls were caught at landfill sites in Worcestershire during this study. Of this total, 142 birds were already ringed, including 83 birds that were ringed elsewhere by other groups or individuals (controls), and 59 retraps of own birds.

Foreign controls

A total of 51 foreign ringed birds were controlled, 82% of these were ringed as pulli on nesting sites. The oldest bird controlled had been ringed as a pullus in June 1975 in Poland and caught at the ripe old age of 16 years 8 months in February 1992 at the Throckmorton landfill site. A summary of all controls is given in Table 1 and the geographical distribution of ringing sites for pulli controlled in the study area are shown in Figure 1.

Controls of British ringed birds

During the 1976 - 1988 project 22 BTO ringed birds were controlled, 45% of these had been ringed as pulli on nesting sites. The oldest control was 15 years 7 months and involved a bird ringed as a pullus in June 1966 at Powys in Wales, and controlled at Madeley Heath in January 1982. Seven of the 10 birds controlled by the Severn Estuary Gull Group (SEGG) had been ringed during the 1976 - 1988 project, including a bird that was ringed at the Evesham landfill site in January 1978 and caught at nearby Throckmorton in December 1996. Another individual, ringed in February 1979 at Throckmorton, was caught there again in December 1990. Of the three remaining birds controlled by SEGG only one was ringed as a pullus in Britain. The ringing sites for those ringed as pulli are shown in Figure 1.

Retraps

Site fidelity for this species was established during this study. A total of 59 birds were retrapped, 36 birds at the same site where ringed, seven in the same winter. Of those retrapped at another site, five were in the same winter.

Recoveries

Recoveries includes birds found and released by members of the public, rings read in the field, and recaptures by other ringers some distance from the ringing site. A total of 361 recoveries have been reported up to the end of 1998, a recovery rate of 8.6%. There were 199 recoveries abroad (Table 2), and 162 in Britain. The oldest recovery is 18 years 3 months. This bird, ringed at Madeley Heath in January 1980 and observed in Finland in May 1998, created a new longevity record for this species from BTO ringing at that time. However, the new holder of the longevity record is a bird that was ringed in March 1972 at a reservoir in Surrey and observed in Holland in June 1998, still very much alive 26 years 3 months after being ringed. While 85% of foreign recoveries involve dead or dying birds, or birds deliberately shot or killed by man, only 75% are recovered in Britain in similar circumstances. Man deliberately takes large numbers of birds in Finland and Sweden, in particular those involved in Mink farming (Kjeld Pedersen). The longest distance travelled is 2565 km, involving a bird that was ringed in January 1979 at Hallow, Worcester, and recovered dead in June 1982 near Solnechnogorsk, Moscow. This is the longest distance travelled by any gull ringed during this study. Figure 2 shows the geographical distribution of all foreign recoveries. Figure 3 shows that the life span of birds from ringing to recovery is 0-18 years, with an average life span of 4.6 years, with only 40% of the recoveries exceeding this figure. Thirty birds (8%) were recovered in the same year as ringed.

Table 1. Controls of foreign ringed Black-headed Gulls

* Estonia, Latvia & Lithuania

Table 2. Foreign recoveries of Black-headed Gulls by region and month of recovery.

Part 2. The Herring Gull L.argentatus

In addition to British breeding birds L.a.argenteus, this study established that birds from northern Norway and NW Russia L.a.argentatus, also winter in the study area (Stewart 1995). These large and darker mantled birds usually arrive in late October, occasionally in September, but are not observed in any numbers until late December and then depart again in February. The number of birds involved and the timing of arrival and departure is very much dependent on weather conditions. Birds showing the 'thayeri' wing tip patterns are also caught and observed (see below). A small number of immature/non-breeding northern birds are also observed at the landfill sites in summer.

Number of birds caught

Only 414 Herring Gulls were caught during this study, including two controls and four retraps. The two controls were adult birds of the British race, and neither originally ringed at their breeding sites. The first control was ringed at a landfill site near Strathclyde, Scotland in October 1979 and caught at Hollow, Worcester in January 1980, and the second bird was ringed at Barwell, Leicestershire in October 1980 and caught at Throckmorton in November 1987. Of the four birds retrapped, all had been ringed at another site.

In addition to the BTO metal ring, individually inscribed coloured Darvic rings are now being fitted to gulls, and in Europe many organisations only use Darvic rings. This has very much increased the recovery rate of gulls. From my own observations of colour-ringed gulls at the Throckmorton landfill site it has been established that Herring Gulls, ringed as pulli at nesting sites on the rooftops in Bristol, are frequent visitors with 43 sightings of 28 bird individually colour-ringed birds. A bird coloured ringed as a pullus in Weymouth, Dorset also visited Throckmorton in October 1997.

Recoveries

A total of 30 Herring Gull recoveries have been reported, a recovery rate of 5%. Eight of the recoveries were abroad, all involving L.a.argentatus, and 22 in Britain and Ireland. A bird, ringed at Madeley Heath in January 1980 and shot at a 'Garbage Dump' near Alta, Finnmark, Norway in May 1986, had travelled a distance of 2353 km. The geographical distributions of all recoveries are shown in Figures 4 & 5.

'Thayeri' wing tip patterns (Figures 6 & 7)

According to Barth (1969), 27% of Herring Gulls breeding in northern Scandinavia show the 'thayeri' wing tip pattern (Figures 6 & 7). It has been found in Herring Gulls from NW Russia; 27% (of 59) skins examined in the Zoological Museums of Leningrad and Moscow Universities (Coulson et al. 1984). Many bird are caught which show characters in their wing patterns typical of those shown by hybrid argenteus x argentatus in Iceland, but the subject is too complex and lengthy to deal with here.

Figure 6. The 'thayeri' pattern is usually characterised by the grey area on the inner web on the 9th primary extending uninterrupted by black pigment into the white mirror.

Figure 7. (a) Wing tip pattern of a Herring Gull caught in January 1992. This bird exhibited the 'thayeri' wing tip pattern typical of those from the breeding populations of northern Scandinavia and NW Russia. (b) Wing tip pattern typical of British Herring Gull L.a.argenteus. Wing tip patterns for both can be highly variable.

Part 3. The Yellow-legged Gull L. cachinnans

The Yellow-legged Gull L. cachinnans also occur in small numbers, most typical of the Mediterranean race michahellis, but occasionally birds showing characters typical of 'ponticus' (Black Sea-type) cachinnans are observed. Some authorities treat these 'Black Sea-types' as a full species, Caspian Gull L. cachinnans. Wing tip patterns for both types are shown in Figure 8. As with northern argentatus small numbers of immature/non-breeding michahellis are also present at the landfill sites in summer. A Yellow-legged Gull colour ringed as a pullus on Plane Island, Riou Archipelago, Marseille, and SE France in May 1999 was observed two months later at the Throckmorton landfill site. Birds ringed in Switzerland and Berlenga Island, Portugal have been caught in nearby Gloucestershire.

Figure 8. (a) Wing tip pattern of L.c. michahellis of unknown origin. (b) Wing tip pattern typical of ponticus (Black Sea-type) cachinnans. Like argentatus, wing tip patterns for both types can be highly variable.

Breeding

There is one instance of an adult male Yellow-legged Gull L.c. michahellis successfully breeding with a Lesser Black-backed Gull L.fuscus in the County. This was not an unexpected event as colonisation of a new area by gulls usually begins by hybridisation with closely related forms. In Germany and France the expansion of Yellow-legged Gulls also led to hybridisation with either Herring Gull or Lesser Black-backed Gull. However, in Holland only hybrid pairs have so far occurred (Van Swelm 1998).

To be continued.....