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Black-headed Gulls breed from Iceland, the Faeroes, Ireland and Iberia and across the middle latitudes of Eurasia. In winter many of those that nest in Eurasia move south, some reaching sub-Saharan Africa, southern Arabia, the Indian sub-continent and southeast Asia. Those that nest in UK however mostly remain within the British Isles.
    At least 1.7 million are estimated to winter in Great Britain. Over two-thirds of those in England and Wales are thought to be migrants from continental Europe or Iceland. The numbers breeding in Britain fell during the late 18th and early 19th centuries as a result of marshland drainage and the collection of eggs for food, then, following the passing of the Sea Birds Preservation Act of 1869, later increased to an estimated total of 38,000 pairs in 1938 and 147,000 pairs in the 1988-91 Breeding Atlas. This latter figure seems to have been something of a peak: a survey in 2000 produced an estimate of 128,000 pairs while Bird Atlas 2007-2011 recorded a 22% range contraction since the 1968-72 Breeding Atlas.
    In Wiltshire Black-headed Gulls have long been common winter visitors with flocks of 1000 or more feeding during the day and roosts estimated at as many as 17,500 at the Cotswold Water Park(CWP) in the 1980s. The only summer records were of birds foraging in the county from breeding colonies in surrounding counties, until 1993 when two pairs attempted to breed at the CWP, but their nests were flooded out before they could hatch their young. The following year ten pairs nested though only one succeeded in raising young (the other nests were thought to have been predated by foxes). In 1996 eleven pairs nested successfully on a floating raft provided initially for Common Terns. After the provision of a second raft, numbers increased to 40 pairs which successfully raised young in 2000, rising to a maximum of 70 pairs on the rafts and 18 pairs elsewhere in the CWP in 2009. Thereafter the numbers diminished to 47 pairs in all in 2010, 15 pairs in 2012 and only 5 pairs in 2013.

