Herd composition and dispersion in the Whooper Swan By Raymond Hewson INTRODUCTION FROM A LOCAL STUDY of the Whooper Swan Cygnus cygnus at Loch Park, Banffshire, it became apparent that, within the herd that gathered during the autumn passage, swans in family parties (i.e. adults accom- panied by young birds) were less wary and fed more widely spread out along the narrow part of the loch than swans without young—a category which probably includes failed breeders as well as pre- breeders. These birds tended to form large compact flocks at the widest part of the loch and when the herd was disturbed readily flew there from other parts of the loch, the family parties remaining behind. To discover whether this behaviour affected the pattern of dispersion in winter herds, a request for information on herd size and composition was published in Bird Study, 3 : 226. At the same time detailed records of herd composition at Lough Beg, in Northern Ireland, were kept by A. J. Tree during the winters of 1956/57 and 1957/58. These data have now been examined. WHOOPER SWANS AT LOCH PARK Lech Park is a long narrow loch in a steep valley at an altitude of 600 feet in upper Banffshire. It is about 1,630 yards long but only 70 to n o yards wide over more than four-fifths of its length. At the south-west end, beyond a small island, the loch widens to about 15 o yards and is somewhat deeper, with an area relatively free from aquatic vegetation in the middle. Up to 200 Whooper Swans gather in October and depart in November. When undisturbed they feed over the whole area of the loch, except the deep part just mentioned, grazing on broad-leaved pondweed Potamogeton natans, Canadian pondweed Elodea canadensis and stonewort Chara spp. These swans have been regularly counted since October 1955 as part of a local population study, an account of which is to appear in a forthcoming issue of Bird Study. On 14 occasions between October 1957 and October 1962 when the birds were not disturbed during counting, a record was kept of the location of the various groups (table 1). Because the swans fly down to the south-west end, where the whole herd gathers if thoroughly alarmed, it was not easy to make this sort of count regularly, and in many cases time was too short to do so. Other causes of disturbance 26
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