Gulls. Professor John C. Coulson

is a colonial breeder, nesting on a wide range of sites, including lakes, reservoirs, small moorland pools and tarns, marshes, sewage farms, clay pits, dunes, saltmarshes and industrial ponds. While numbers of nests in some colonies are easily counted, others are difficult to measure as the nests can be concealed by vegetation or lie on boggy ground, making access difficult or risking damage to the vegetation. Good vantage points do not exist near many colonies, and different methods are therefore needed here to estimate their size.

      The basic unit used to measure the size of a colony is the number of nests, applying the realistic assumption that two adult birds are associated with each nest. In very large colonies, sampling of nest density has been used and then extrapolated across the total area of the colony as delineated from aerial photographs or detailed large-scale maps. At some sites, pair and hence nest numbers have been estimated through the less reliable method of counting numbers of birds in the air when disturbed and then using a conversion factor, such as dividing by 1.55. Most Black-headed Gull census counts have errors, which should be considered when making interpretations of the national status of the species and possible changes in abundance over time.

      FIG 18. Part of the colony of Black-headed Gulls nesting with Sandwich Terns (Thalasseus sandvicensis) on Coquet Island, Northumberland. (John Coulson)

      Data for years since 1980 on Black-headed Gull colony size recorded in the UK Seabird Colony Register allows for an in-depth assessment. While the species is essentially a colonial breeder, solitary breeding pairs on the day of counting comprised 10 per cent of the sites in Scotland and 8 per cent in England. Whether many of these pairs had been solitary for the entire breeding season is uncertain, because most of the nesting sites were visited by counters only once. It is possible that some single pairs were the sole remainders of a group present earlier in the breeding season that had suffered predation or disturbance, and had deserted the site prior to the observer’s visit. Single pairs of Black-headed Gulls can probably breed without the need of stimulus of other pairs, although when they do so, they nest late and their breeding success is usually low.

      In Britain and Ireland, the numbers of pairs of Black-headed Gulls estimated at breeding sites (subsequently referred to as colonies for convenience, and including sites with single pairs) between 1980 and 2015 ranged from only one pair to more than 10,000 pairs. Overall, a third of colonies in Britain contained between 10 and 100 nesting pairs (Fig. 19).

      At the same time, the average size of colonies differed between Scotland and England, with 84 per cent in Scotland having fewer than 100 pairs (Table 8), while the comparable figure for England was a third lower, at 54 per cent. Colonies with 100 to 1,000 pairs were twice as frequent in England than in Scotland, while colonies of more than 1,000 pairs formed 13 per cent of all sites recorded in England, but only 2 per cent of those in Scotland.

      These differences reflect the type of nesting sites used in the two regions, with the majority in Scotland being inland on upland sites and usually associated with relatively small waterbodies, while in England large coastal colonies were more frequent and were sometimes extremely large. The distributions of colony sizes in Wales and Ireland were intermediate between those in England and Scotland, with 72 per cent of colonies in Wales and 62 per cent in Ireland having fewer than 100 pairs (Table 8).

      FIG 19. The size distribution of Black-headed Gull colonies (including sites with only one pair) in England and Scotland between 1980 and 2015, based on Seabird Colony Register data for 364 colonies surveyed in Scotland and 260 in England.

TABLE 8. The proportion of Black-headed Gull colonies in England, Scotland, Wales and Ireland with fewer than 100 breeding pairs, based on data from 1980–2015. Sites with a single pair present have been included.

TABLE 9. The percentage of Black-headed Gull colonies of different sizes in England and Wales in 1938, 1958 and 1980–2012. Sites with only one pair are included.

      A comparison between the size of colonies reported in the 1938 and 1958 censuses made in England and Wales, and more recent data collated in the Seabird Colony Register, show progressive changes in colony sizes over time (Table 9). There has been an overall decrease in the proportion of colonies with fewer than 100 pairs since 1958, while the proportion and total numbers of large colonies with more than 1,000 pairs has more than doubled over the same period. Many colonies – particularly small ones in England and Wales – have disappeared for various reasons, including disturbance, predation and drainage, while others – particularly large ones – are now protected within nature reserves, although even a few of these have disappeared.

      Persistence of colonies

      While some Black-headed Gull colonies persist for many years, in general they are not as permanent as those of the Kittiwake or Herring Gull (Larus argentatus). Over the past hundred years, many Black-headed Gull colonies have disappeared while new ones have become established. Small colonies are particularly susceptible to decline, and this is especially true of those on upland moors, where the birds nest around small bog pools, reservoirs and lakes. A consequence of this is that some of these mobile groups can be missed during census work.

      Various reasons have been suggested for the desertion of colonies. In some cases these are speculative, and include drainage, military activity, competition from increasing numbers of large gulls, aggression by Mute Swans (Cygnus olor) and Canada Geese (Branta canadensis), egg collecting, keepering, and predation by Foxes and American Mink. Rats and Foxes have been a persistent problem for Black-headed Gulls nesting at Blakeney Point in Norfolk, and no young were reared at the colony in 2000. In 2014, Fox predation at the same site resulted in very few chicks being reared by the 2,200 pairs present, while in 2016 an appreciable increase of rats caused the gulls to abandon the colony and, presumably, move elsewhere.

Ravenglass colony

      The site of the very large Black-headed Gull colony on the coast of Cumbria at Ravenglass is owned by the Muncaster Estate. There has been continuous collection of eggs there since the seventeenth century, a practice that was recorded as being extensive in 1886. Del Hoyt has suggested the colony had an annual yield of 30,000 eggs over many years. Neither Michael Shrubb’s searches (2013) nor my own have uncovered documentation to confirm or contradict this statement, but the colony was certainly used consistently as a source of freshly laid eggs for much of the twentieth century by the estate, and workers also took some young from time to time. The collection of eggs was well organised and was stopped each year on an agreed date, which left time for successful breeding by birds laying late or with repeated clutches. David Bannerman recorded that the estate collected 72,398 eggs at Ravenglass in 1941 and 24,568 in 1951, when a further 6,000 eggs were taken (presumably illegally) by others.

      In 1954, the site was leased to Cumberland County Council as a local nature reserve and egg collecting ceased. The gulls were protected and studied in the ensuing decade by Niko Tinbergen’s research group from Oxford University. During the period 1954–68, the colony size remained between 8,000 and 12,000 pairs, but during this time predation on eggs, chicks and adults occurred from time to time and breeding success was low. In 1968, a full-time warden was appointed, and he reported that 8,100 pairs nested. From then onwards, a marked decline continued each year; Neil Anderson’s study in 1984 found only 1,300 pairs nesting, and the colony was totally deserted in 1985 (Anderson, 1990). Five pairs nested there in 1988 and three pairs in 1989, but none since (Fig. 20). Predation by mammals over several years, particularly Foxes, was thought to be the cause of decline