Gulls. Professor John C. Coulson

abandonment of the Ravenglass colony probably involved a movement of Black-headed Gulls to several other colonies, possibly those in the Ribble Estuary and Sunbiggin Tarn, because no new or established colonies were nearer.

      FIG 20. The numbers of nests (pairs) of Black-headed Gulls nesting at Ravenglass in 1939–90.

      The history of the Ravenglass colony and other similar sites illustrates two important points. First, egg collecting over many years that was well organised and had an early cut-off had no adverse effects on the colony and did not lead to its desertion. Second, the creation of a nature reserve without effective predator management does not prevent colony desertion; indeed, in recent years there have been other instances of colonies within nature reserves disappearing.

Sunbiggin Tarn colony

      The history of the large inland Black-headed Gull colony at Sunbiggin Tarn, near Orton in Cumbria, parallels that of Ravenglass. In 1891, a thousand eggs were said to have been collected from the edge of the colony. The site was estimated to contain 400 pairs in 1938, 1,200 pairs in 1958 and 2,500–3,000 pairs in 1973. The colony continued to grow and reached about 12,500 pairs in 1988 (see below), maintaining similar numbers in 1989, when Foxes were seen. Numbers of gulls then showed a noticeable decline in 1990, and only a few pairs bred in 1991. The colony persisted with a few pairs, and by 2000 had increased to 1,200 pairs, but by 2010 gulls had ceased nesting at the site altogether (Fig. 21).

Direct counting of all nests proved to be impossible at the large Sunbiggin Tarn colony because of the boggy areas within the tarn and the high vegetation, which would be damaged by trampling during a search for nests. A count of the colony was not obtained for the national census in 1985–87, and in 1988 a novel method was used to measure its size. This involved capturing a large sample of the breeding adults in late May and dyeing their tails yellow. A total of 1,010 adults were captured and marked during two weeks by attracting them with food placed about 1–2 km from the colony and then cannon netting them. When marking was completed, the proportion of dyed birds in the colony was obtained by counts of 4,540 adults passing in and out of the site along flight lines. Of these, 186 – or one in 24.4 – had yellow tails, so multiplying the total number dyed by this proportion (24.4) gave a measure of the total number of breeding birds in the colony. This produced an estimate of 24,644 adults (rounded off to 12,500 pairs), identifying it as the largest inland colony in Britain at the time. Unfortunately, Clare Lloyd et al. (1991), in their book reporting the status of seabirds in Britain and Ireland in 1985–88, erroneously recorded this census as 25,000 pairs of Black-headed Gull, not 25,000 individuals.

      FIG 21. Part of the large Black-headed Gull colony at Sunbiggin Tarn, Cumbria, during an exceptionally dry spring in 1984. (John Coulson)

      Part of the colony at Sunbiggin Tarn could be viewed by people from the nearby road, and they often fed the gulls, which made attracting and capturing the birds in cannon nets much easier. The public’s interest in the well-being and size of this colony was considerable, and not without humour. The investigators netting adult birds had to explain to onlookers what they were doing and that there would be an explosion when the cannons were discharged. On two occasions, the observers remarked that they were puzzled because they could not find a gull in their field guides with a yellow tail, and asked whether this was a rare species! Members of the public often also liked to guess how many gulls were nesting, generally coming up with figures that were widely lower than the actual numbers calculated – most likely because the extent of the colony could not be viewed from a single vantage point. Incidental to the main study, adults with yellow tails were seen feeding in the Lake District and several places on the river Eden up to 40 km from the colony a few days after marking.

      FIG 22. Numbers of Black-headed Gulls nesting on the Sands of Forvie, 1986–2017. Data mainly from the Seabird Colony Register.

      Ythan Estuary (Sands of Forvie) colony

      Black-headed Gulls have nested at the Sands of Forvie at the mouth of the river Ythan, near Aberdeen in north-east Scotland, for many years. In the late 1980s, about 500 pairs nested there annually, along with Sandwich Terns (Thalasseus sandvicensis), Common Terns (Sterna hirundo) and Eiders (Somateria mollissima). Fox predation became intense in about 1990, when breeding Sandwich Terns and Black-headed Gulls abandoned the area and the number of young Eiders declined. In 1994, steps were taken to reduce and then exclude Foxes from the breeding areas, and in 1998 the first Black-headed Gulls returned to breed. Numbers increased rapidly in the following years and reached more than 1,500 pairs in 2002. This level was not maintained, but numbers exceeded 500 pairs each year up to 2012, when the colony size increased annually until 1,921 pairs were recorded 2016 (Fig. 22).

      Foulney and other colonies

      The colony at Foulney in north-west England (Fig. 23) showed a pattern of decline spread over a few years in the late 1980s and early 1990s, before breeding there ceased altogether sometime in or before 1996. As elsewhere, predation of eggs and nestlings was considered the main cause.

      FIG 23. The decline and subsequent total desertion of the Black-headed Gull colony at Foulney, a coastal site in Cumbria. Data mainly from the Seabird Colony Register.

      FIG 24. The rapid build-up of a Black-headed Gull colony at the RSPB reserve at Saltholme, near Stockton-on-Tees, between 2005 and 2017. Data mainly from the Seabird Colony Register.

There are many other examples of Black-headed Gull colonies that showed appreciably smaller numbers each year before breeding stopped entirely. The national census in 2000 listed 12 large colonies that had disappeared since 1985, plus a further five that had lost more than 89 per cent of the maximum numbers of breeding pairs over recent years. In contrast, nine colonies showed major increases within a short time period, including the colony at Langstone Harbour in Hampshire, which in the 15 years between 1985 and 2000 increased more than 60-fold. Many new colonies had become established and subsequently grew rapidly, such as those at Hamford Water in Essex, Coquet Island in Northumberland, Larne Lough in Northern Ireland, Loch Leven in Scotland, the Ribble Estuary in Lancashire and Saltholme (Fig. 24), a nature reserve near Stockton-on-Tees, County Durham, that has been managed by the RSPB since 2007.

      Reasons for colony declines

      Egg collecting, particularly if stopped early in the season, has carried on year after year at several Black-headed Gull colonies without resulting in abandonment. And at Ravenglass, Neil Anderson’s investigations (see above) ruled out a possible link between the decline there and contamination associated with the nearby Sellafield nuclear fuel reprocessing and decommissioning plant. In many areas – particularly the fenland of eastern England – colonies have been lost due to drainage and expanding agriculture, which have removed otherwise suitable nesting sites. However, it seems that the main cause of decline and disappearance of Black-headed Gull colonies is predation of eggs, young and sometimes adults. The presence and increase in numbers of mammalian predators, particularly Foxes and American Mink, have been associated with the decline of the colonies at Sunbiggin Tarn, Ravenglass and the Sands of Forvie. During his studies at Ravenglass, Hans Kruuk found that four Foxes killed 230 Black-headed Gulls in one night, and a total of 1,449 adult gulls were killed by Foxes