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      FIG 25. Movements of Black-headed Gulls ringed as nestlings in south-east England and recovered outside the breeding season, based on 976 recoveries. Right: Similar mapping, but based on 447 recoveries of Black-headed Gulls ringed in north-east Britain. Note that fewer birds moved to Iberia from the north-east and that birds from the two regions have different winter distributions in Ireland. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

      The BTO Winter Gull Roost Survey in 2004 estimated that more than 1.6 million Black-headed Gulls were using winter roosts in the United Kingdom and Northern Ireland., and since the total did not include all winter roosts, there are probably more than 2 million individuals wintering here. This number can be compared with 280,000 breeding birds in the United Kingdom and Northern Ireland, which with immatures added comes to about 360,000 individuals. As most of these remain here in winter, it suggests that some 77–82 per cent of wintering Black-headed Gulls recorded in 2004 had moved here from the Continent. Gabriella Mackinnon and I obtained a comparable figure of 71 per cent, derived from ringing recoveries up to 1980 (Fig. 26). Both percentages are rough estimates, but it is clear that a major proportion of the wintering Black-headed Gulls in Britain and Ireland originate from mainland Europe. Without new estimates, it is not known whether these numbers from the Continent have declined in more recent years as a result of the relatively mild winters.

      Some readers may find it strange that, despite the satisfactory status and abundance of the Black-headed Gull as a breeding species in Britain, it has been Amber-listed here since 2015 as a species of conservation concern. This is based on the belief that numbers of wintering Black-headed Gulls in Britain have decreased by 33–50 per cent over the last 25 years, based on counts at winter night roosts. Since the British breeding numbers do not appear to have reduced (here), a possible explanation is that a major decline has occurred in northern Europe – the area from which many of the British wintering gulls originate. However, there is little evidence of this apart from Denmark. An alternative explanation for the decline in winter numbers in Britain is that more individuals are remaining on the European mainland during the milder winters encountered in recent years. A preliminary analysis of recent winter ringing recoveries of Black-headed Gulls marked on mainland Europe suggests that the change of wintering areas is more likely to be the explanation. If this is correct and the species is not declining appreciably in Europe as a whole, but is modifying its wintering areas, then is it really a matter of conservation concern? In contrast to the assessment in Britain, the International Union for Conservation of Nature (IUCN) still regards the Black-headed Gull worldwide as a species of Least Concern.

      FIG 26. The percentage of Black-headed Gull recoveries reported in Britain and Ireland in each month that were originally ringed in mainland Europe. Note the trend for first-year birds to return to the Continent later. The recoveries of a few Continental birds in May and June may represent rings reported some time after finding.

      Arrival of wintering birds

      The numbers of Black-headed Gulls arriving in Britain and Ireland for the winter boosts the breeding season population by a factor of about four. These visitors come from a wide range of countries in northern Europe, with most starting their journey at locations around the Baltic Sea (Figs 27 and 28). Their arrival is spread over several months, from late July to early November (see Fig. 26).

      The countries of origin of Continental Black-headed Gulls visiting Britain varies slightly with their final destination, with birds flying south-west or west to reach their wintering area (Figs 27 and 28). The exceptions are individuals reared in Iceland, some of which winter in Scotland and Ireland, but most of which avoid England and Wales. Other Black-headed Gulls reared in Iceland have been recovered from the east coast of North America, a route apparently taken by all of those reared in Greenland, although this wintering area is totally avoided by birds breeding in mainland Europe and in Britain.

      Fig. 29 shows the number of recoveries of foreign-ringed Black-headed Gulls in Britain and Ireland for each month of the year up to 1985 in relation to those reported monthly from December to February, a period during which it is assumed that all individuals have arrived at their wintering areas. An appreciable proportion of foreign adults, possibly mainly failed breeders, arrive in Britain during July, but only a few young of the year have crossed the North Sea by the end of the month. A small proportion of first-year individuals reared abroad remain in Britain in the following summer, having failed to return to the Continent in the spring. These birds, entering their second year of life and already in Britain in July, are joined by more similar-aged immature birds moving here from the Continent.

      FIG 27. Black-headed Gulls ringed in winter in north-east Britain (to the right of the thick black line) and recovered abroad in a subsequent breeding season. Note the five Iceland recoveries. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

      FIG 28. Black-headed Gulls ringed in winter in south-east Britain (to the right of the thick black line) and recovered abroad in a subsequent breeding season. Note the single Iceland recovery. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

      FIG 29. Recoveries in Britain and

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