Herd composition and dispersion in the Whooper Swan

Transcription

1 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 accompanied 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 prebreeders. 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 no 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

2 HERD COMPOSITION IN THE WHOOPER SWAN Table i. Aggregates of 14 counts of Whooper Swans Cygnus cygnus on Loch Park, Banffshire, At SW end Adults Young Broods SW middle Adults Young Broods Middle NE end Adults Young Broods also made it difficult to get an unbiased picture of the grouping of swans on the loch. The difference in geographical distribution between swans with young, occurring chiefly in the narrow part of the loch (middle to north-east end) and other swans which keep together at or near the wider south-west end is statistically significant at the 0.01% level. It should perhaps be mentioned that, of the swans in the south-west to middle section, groups of 21 and 47 adults without young in fact occurred at the fringe of the south-west end of the loch. NUMBERS OF FIRST WINTER SWANS IN THE HERDS Airey (1955) pointed out that larger herds of Whooper Swans contain relatively few young birds, but he gave too few data to substantiate this. Boyd and Eltringham (1962) have shown that Whooper Swan herds in Britain are mostly small, containing fewer than 10 birds, although a large proportion of the wintering population is gathered into comparatively few big herds. For the purpose of comparing the proportion of first-winter swans in herds of different sizes I have fixed the upper limit of a small herd arbitrarily at 20 swans. A total of 115 small herds of a mean size of 8.2 contained 26.4% of firstwinter birds, while 57 herds of a mean size of 40.6 contained only 15.1% (table 2). This pattern of dispersion in Whooper herds, which again is statistically significant at the 0.01% level, may be due to (1) disturbance causing adults without young to gather on more open water (as at Loch Park) or Table 2. Proportion of first-winter Whooper Swans Cygnus cygnus in small and large herds, south-west Scotland, north England and Northern Ireland, 1955/ /58 Small herds (up to 20 swans) Herds Total Adult Young J955/ / / Others Large herds (21 swans and over) Herds Total Adult Young (26.4%) ' i(i5-i%)

3 BRITISH BIRDS (2) non-breeders remaining in the large groups in which they spent the summer. Miss E. A. Garden (in litt.) saw herds of Whoopers in Iceland in July 1958, as well as many small groups. The data used to determine the proportion of first-winter birds in small and large herds are drawn largely from south-west Scotland, north England and Northern Ireland. In all, 1,377 swans were counted in 1955/56 and 799 in 1956/57, compared with the totals of 1,042 and 423 respectively used by Boyd and Eltringham (1962) in calculating the proportion of first-winter swans in the British wintering population in those years. Some duplication may occur in respect of English records, but as much of the data are drawn from different sources they provide an effective comparison. They show that among the swans counted the proportion of first-winter birds closely resembles that found by Boyd and Eltringham in the same years in their comprehensive survey (table 3). Table 3. Proportion of first-winter Whooper Swans Cygnus cygrtus in 1955/56 and 1956/57 The percentages in the last column are taken from Boyd and Eltringham (196Z) Total counted Adult Young Percentage Boyd and Eltringham 1955/ / % 19-8% 18.5% 17-4% COMPARISON OF THE HERDS IN BANFFSHIRE AND NORTHERN IRELAND Lough Beg, which lies along the borders of Co. Antrim and Co. Londonderry, has probably been colonised by Whooper Swans only during the last twenty or so years, although at peak numbers the herd is one of the largest in Britain. Similarly, Major R. F. Ruttledge (in litt.) has no records for Co. Mayo and Co. Galway before At Loch Park the swans arrive in October, reach peak numbers in early November and leave about the middle of the same month (fig. 1). At Lough Beg the peak is later, in January or February, although in 1956 a large number of swans was present in October (fig. 2). Loch Park is clearly no mote than a staging post, while Lough Beg is a wintering place. But peak numbers at Lough Beg are late compared with peak numbers in Scotland generally, which occur in November (Boyd and Eltringham 1962). Although there is no firm evidence, certain facts suggest that there may be a south-westerly movement through Scotland to Northern Ireland and a return to Iceland direct from there. These are, firstly, that in south-west Scotland peak numbers occur in January and February, and in the 28

4 HERD COMPOSITION IN THE WHOOPIR SWAN FIG. I. Whooper Swans Cygnus cygnus at Loch Park, Banffshire, FIG. 2. Whooper Swans Cygnus cygnus at Lough Beg, Northern Ireland, Hebrides in March (Boyd and Eltringham 1962) (although the man}' small lochs in the islands make swan counts doubtfully reliable), and, secondly, that there is no spring passage through such important autumn stations as Loch Park and the Loch of Strathbeg in north-east Scotland. Alternatively, the Whooper Swans which winter at Lough Beg may 29

5 BRITISH BIRDS be of different origin from the Scottish birds. The evidence that British wintering swans are of Icelandic stock rests on three ringing recoveries (one from the Hebrides, one from Stirling and one from Co. Down) and the absence of young birds in the population at Loch Spiggie, Shetland, after a summer which was unusually bad in Iceland but normal in Scandinavia (Venables and Venables 1950). Hilprecht (1956) referred to large concentrations of Whoopers on the west coast of Denmark but said that they are also generally distributed along the south west coasts of Norway and Sweden, as well as the German portion of both North and Baltic Seas. These were considered to be of the Scandinavian and not the Icelandic race (though the latter is not very certainly distinguishable) and the only two ringing recoveries relate to birds ringed on the south Swedish coast and recovered from north Russia. There is a marked difference in the proportion of first-winter swans at Loch Park and Lough Beg. In 1955/56 there were 7.7% of firstwinter birds (all counts during the season) and in 1956/57 6.0% at Lough Beg, compared with 15.2% and 24.2% at Loch Park. Mean brood size is similar at the two places (table 4) so that the difference is due to fewer, not smaller, broods. Table 4. Brood size in Whooper Swans Cygnus cygnus at Loch Park, Banffshire, and Lough Beg, Northern Ireland, 1955/56 and 1956/57 Loch Park Broods counted Mean size Lough Beg Broods counted Mean size 1 955/ / While the proportion of first-winter swans at Loch Park resembles that in the wintering population as a whole (Hewson unpublished), as might be expected from a migration staging point, the much smaller proportion at Lough Beg illustrates the tendency for large herds to contain relatively fewer first-winter swans. Small Whooper herds in Ireland have a larger proportion of first-winter birds, as elsewhere. ACKNOWLEDGEMENTS I am very grateful to all those who sent information, often detailed, particularly A. F. Airey, Professor M. F. M. Meiklejohn, Major R. F. Ruttledge and A. J. Tree. Dr. R. M. Cormack, Dr. G. M. Dunnet and Dr. R. Richter gave advice during the preparation of this paper. SUMMARY (1) Whooper Swans Cygnus cygnus on autumn passage at Loch Park, Banffshire, form larger groups and are more wary when not accompanied by first-winter birds. Family groups disperse more widely to feed. 3

6 HERD COMPOSITION IN THE WHOOPER SWAN (2) 57 large Whooper Swan herds of a mean size of 40.6 contained 15.1% of first-winter birds, while 115 small herds of a mean size of 8.2 contained 26.4 ( /o The difference is due to fewer, not smaller, broods. (3) The proportion of first-winter swans in the sample examined resembled in 1955/56 and 1956/57 that found by Boyd and Eltringham (1962). (4) At Loch Park, where Whoopers pass through in autumn but do not winter, the proportion of first-winter swans resembles that in the wintering population as a whole. At Lough Beg in Northern Ireland, a wintering place, the proportion of first-winter birds was only 7.7% in 1955/56 and 6.0% in 1956/57, compared with 15.2% and 24.2% respectively at Loch Park, illustrating the tendency for large herds to contain relatively few first-winter birds. (5) The late peak at Lough Beg may be due to continuing south-westerly movement of swans during the winter, with a direct return northward to Iceland. REFERENCES AIREY, A. F. (195 5)-.'Whooper Swans in southern Lakeland'. BirdStudy, 2: BOYD, H., and ELTRINGHAM, S. K. (1962): 'The Whooper Swan in Great Britain'. Bird Study. 9: HILPRECHT, A. (1956): Hockerschwan, Singschwan, Zwergschwan. Wittenberg- Lutherstadt. VENABLES, L. S. V., and VENABLES, U. M. (1950): 'The Whooper Swans of Loch Spiggie, Shetland'. Scot. Nat., 62: