Gulls - Professor John C. Coulson Collins New Naturalist Library

Скачать книгу

Ireland of Black-headed Gulls ringed in mainland Europe expressed as a percentage of the monthly recoveries between December and February. Note the high proportion of birds entering their second year of life (which moult early) reported in Britain in July, and the late departure of first-year birds in spring, with a few apparently remaining in Britain during the summer.

      Numbers of all ages cross to Britain in August. By the end of that month, about half the adults that winter here have already arrived, but only 15 per cent of first-year birds have arrived. The early movement from countries bordering the Baltic Sea perhaps reflects the less favourable feeding conditions there. For example, the small tidal changes in the Baltic markedly restrict the extent of inter-tidal areas available for foraging, while the warmer and drier continental weather makes earthworms less available there in late summer.

      There is evidence of a lag in new adult arrivals in Britain in September, probably because most adults are still completing their moult and growing their longest primaries. The lack of, or reduction of, these long feathers must impair their ability to make long flights. A second peak of arrival occurs in November, and by the end of that month most of the adults that winter here have arrived, apart from a few birds that possibly turn up in December. The maximum numbers of Black-headed Gulls are present in Britain for only three months, from December to late February.

      The timing of immigration of first-year and adult Black-headed Gulls in autumn to Britain depends on the country of origin. Dates after 1 July by which the first 25 per cent of recoveries of young birds and adults from abroad are recovered have been used as an index of the arrival times of first-year and adult Black-headed Gulls from different countries in Europe (Table 11). In general, adults and young birds arrive on different average dates, but this is not constant for each country of origin and varies by more than 60 days. Young birds arrive earlier than adults from countries closer to Britain, but the reverse applies for those from more distant countries. This suggests that first-year birds start their migration earlier than adults but move much slower, perhaps making a series of short journeys interspaced with stopovers at rest areas lasting a number of days. In contrast, adults appear to make much longer flights to their wintering area and arrive in Britain from European countries at less variable dates. The interpretation of the trend line in Fig. 30 suggests that first-year birds on migration take about 43 days longer than adults to travel 1,000 km.

      FIG 30. Difference in the dates of the first 25 per cent of recoveries in Britain of first-year and adult Black-head Gulls of Continental origin (data from Table 8) in each 12 months from 1 July, plotted against the distance from country of origin to central England. Note that the first-year birds from countries close to England arrived earlier than the adults, but that the converse was true for those travelling further.

      Faithfulness to wintering areas

      There is extensive and convincing evidence that some adult Black-headed Gulls from the Continent return to the same immediate wintering area year after year. The best example is a gull ringed in Poland, whose ring numbers were recorded in the same central London park over eight consecutive winters. However, such faithfulness to wintering areas is not absolute, as there are many instances of birds from the Continent wintering in Britain one year and then remaining in mainland Europe (usually in the Netherlands or Denmark) in the following winter (Fig. 31). It has been suggested, but not yet proven, that relatively mild conditions in western Europe in some recent winters have resulted in fewer Black-headed Gulls migrating here; this effect has also been reported for several species of waders and other waterbirds.

      Evidence of the extent to which Black-headed Gulls use the same wintering area in successive years comes from individuals wing-tagged in north-east England, as these had a high probability of being seen if they returned in the following winter. Most tagged birds were breeding on the Continent, and so the annual return would involve many individuals migrating considerable distances. In the winter after tagging, those that returned were seen on an average of five different days, so it is unlikely that many others were missed. The study detected that 48 per cent of adults and 32 per cent of second-year birds, but only 17 per cent of first-year birds, returned to the same area in successive winters (Table 12). Allowing for mortality in the intervening year (estimated at about 15 per cent for adults and 30 per cent for first-year birds), 56 per cent of the surviving adults returned to the same wintering locality, with 38 per cent of those in their second winter when marked returning for a second time, but only 24 per cent of those marked in their first year.

      FIG 31. Black-headed Gulls ringed in winter in Britain and recovered more than 20 km away in a subsequent winter. Many individuals returned to Britain, but note the appreciable number that failed to do so in a subsequent winter and were recovered on the Continent. Reproduced from The Migration Atlas (Wernham et al. 2002), with permission from the BTO.

      A total of 38 marked individuals that did not return to the study areas were seen by other observers elsewhere in the winter following marking. These were reported mainly on the east coast of Britain, south of the study areas. Two extreme cases were recorded; one bird 450 km away on the south coast of England, and the other 300 km north at Aberdeen in Scotland. In addition, one remained in Denmark. It is evident from this that only a proportion of individuals of all age classes return to the same wintering area, but that faithfulness is stronger (although not complete) in adults. The reason why some birds return while others fail to do so remains unknown, but is not primarily related to their sex.

      Return of birds to the Continent

      The return migration from Britain to the Continent is much more synchronised than the arrival and begins in late March, with most adults migrating in April (Fig. 32). First-year birds tend to depart later than adults and some do not leave until May, while others (perhaps 10 per cent of immature birds wintering here) remain throughout the summer. These birds join the feeding adults associated with colonies in Britain (since they leave their wintering feeding areas) and presumably do not return to the Continent until they have overwintered here for a second time.

      FIG 32. The month of arrival of Black-headed Gulls entering Britain from the Continent in autumn (white bars) and leaving in spring (blue bars), based on monthly change in numbers of ringing recoveries of foreign birds in Britain. Note that for all age classes, the spring departure is much more synchronised than the autumn arrival dates.

      By the end of July, numbers of second-years have arrived in Britain and joined the few that spent the summer here. These birds are not constrained by breeding and moult about four weeks earlier than adults, so this might explain their early crossing of the North Sea. More arrive in late October and early November, and many late arrivals originate from further away, east of the Baltic.

      Distribution of Continental wintering birds

      The Continental Black-headed Gulls that winter in Britain do not disperse randomly across the country, and are much more abundant in eastern and southern regions. The proportions of

Скачать книгу