The Shelduck Population of the Ythan Estuary, Aberdeenshire
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The Shelduck population of the Ythan estuary, Aberdeenshire I. J. PATTERSON, C. M. YOUNG a n d F. S. TOMPA Introduction ing season on a muddy estuary and nesting mainly in rabbit burrows among sand dunes. The Shelduck T adorna tadorna is a large and The present study was carried out in conspicuous species whose spectacular separate periods by the three authors; in moult migration has attracted much atten 1962-1964 by C.M.Y., in 1966 by F.S.T. and tion (Hoogerheide&Kraak, 1942; Coombes, since 1968 by I.J.P. The aim of this paper is 1950; Goethe, 1961a, 1961b). As one of the to describe changes in population size over territorial ducks, the social structure and this period and discuss the various popula regulation of its populations pose par tion processes which might have contributed ticularly interesting problems. There have, to these changes. however, been few detailed studies of local populations, an exception being Hori’s (1964a, 1964b, 1965,1969) study on Sheppey Study area in the Thames estuary. Here the majority of breeding birds hold territories on freshwater The Ythan estuary, 57°20'N, 2°00W, 21 km fleets in grazing marshes and nest in hollow north of Aberdeen is well separated from trees, haystacks and farm buildings in close other estuaries suitable for shelduck popula proximity to man. The population on the tions, the nearest being at Findhorn on the Ythan estuary, Aberdeenshire, contrasts with Moray Firth 100 km northwest and at Mon the Sheppey one in being much further north, trose 75 km south. The intervening coast is with the birds staying throughout the breed chiefly rocky with some sandy beaches and Figure 1. The Ythan estuary and Sands of Forvie National Nature Reserve, showing the areas of the former used by Shelduck broods. 16 Shelduck population, Ythan estuary 17 only scattered pairs of Shelduck occurred The annual cycle there during the study. The estuary is 7 km long and up to 0-6 km Most of the Shelducks were absent from the wide (Figure 1) with a mixture of muddy Ythan from early July until March. Re shores and bays, sand and gravel beaches coveries of twenty-one ringed birds during and mussel Mytilus edulis beds. Roads and this period were all from moulting areas on tracks run close to the shore along the entire the northwest coast of Germany and from length, so that all parts of the relatively the southern North Sea coasts. The first Shel narrow area can be observed easily. On the ducks returned to the Ythan during Novem east (seaward) side is the Sands of Forvie ber but numbers remained low over mid National Nature Reserve, a complex area of winter, in contrast with the Thames area dunes. Seven dune ridges are oriented at right where there were large wintering flocks angles to the coast stretch from the southern which dispersed to other areas in February tip of the peninsula to the village of Collies- leaving only the breeding population (Hori, ton (Landsberg, 1955; Burnett, 1964). The 1964a, pp. 333, 335). Marked Ythan Shel dunes at the southern end are mobile and ducks have been seen in wintering flocks on sparsely covered with marram grass Ammo- the Eden estuary near St Andrews where the phila arenaria; tow ards the n orth there is number in late winter greatly exceeded the increasing vegetation cover with extensive local breeding populations (Boase, 1959 and heather Calluna vulgaris. R abbits Orycto- personal observations). It is likely that re lagus cuniculus were abundant in the dunes turning moult migrants first assemble in large throughout the study, their burrows provid estuaries like the Eden and later disperse to ing most of the Shelduck nesting sites. Tem breeding areas. porary and permanent pools occurred in Shelducks arriving on the Ythan during many of the dune valleys. winter and early spring formed a loose flock North and west of the estuary is mixed usually centred on the largest mudflat, the farmland where a few scattered pairs of Shel Sleek (Figure l)though sometimes, especially duck occurred on lochs and ponds. in severe weather, near the mouth of the estuary when the higher salinity nearer the sea prevented the mud from freezing. The Methods flock was usually widely scattered while feed ing but the birds roosted at high tide in a dense A large part of the study was based on the group on an island in the Sleek. Two Shel case histories of individually marked Shel ducks caught in this winter flock have been duck. These were caught as ducklings just recovered in subsequent weeks in wintering before fledging by chasing them or by driving areas further south suggesting that some them into nets, and as adults in the winter birds may return to the major winter flocks, flock by baited funnel traps (Young, 1964). although records of colour-marked in A few were caught on territories and a few dividuals showed that most birds stay con females were netted at the nest burrow. Each tinuously once they arrived. In the Sheppey bird was given a unique ring combination of population (Hori, 1964a, p. 336) adults coloured celluloid (1962-1966) or ‘Darvic’ similarly left the wintering area on the rigid P.V.C. (from 1968) rings. Only black, Swale channel and dispersed to freshwater white, red, yellow, green and blue were used pools nearer the nesting area in early to avoid confusion in the field. Since the spring. Shelducks spent much of their time on open During March, pairs began to leave the mudflats, leg rings were easily identifiable up Sleek flock and dispersed over the whole of to 300 m. From 1969, combinations of dye the muddy parts of the Ythan estuary. The spots on the white parts of the plumage, establishment of territories has already been using ‘Durafur Black R’ a fur dye kindly described by Young (1970a) who showed supplied by I.C.I. Ltd, allowed identification that in 1962-1964 a constant number (seventy of birds at greater distances, on water or in to seventy-two) of pairs were territorial while long grass. the remainder of the population remained in Other techniques used in particular parts a flock either on the Sleek, or, later, on the of the study will be described in the relevant upper parts of the estuary. Most of the flock sections. birds were 1- or 2-year-old pre-breeders but some were paired adults which quickly occu pied territories when the owners were re moved. This contrasts with Hori’s study where, after a similar dispersal of adults to territories on freshwater fleets, the remaining 18 I. J. Patterson, C. M. Young and F. S. Tompa flock was made up entirely of immature birds estuary from the former territory site al and unpaired adult males. though the parts of the estuary used by Concurrently with the establishment of broods included many also used for terri territories, pairs from the Ythan began to tories (though most of these were abandoned visit the Forvie sand dunes (Figure 1). Soon by the brood stage). The parents, particularly after dawn, single pairs left the estuary and the males, vigorously defended an area round flew or walked around the dunes visiting the brood against other Shelduck and were burrows for some time before usually assem also seen to attack Wigeon Anas penelope, bling in groups, as described by Young Redshank Tringa totanus. Common Tern (1970b). He called these groups ‘parliaments’ Sterna hirundo and Eider Somateria mollis after Coombes ( 1949) although the latter may sima as well as potential predators such as have been referring to groupings on feeding the Herring Gull Larus argentatus. grounds. Young (1970b) found that the same Crèches of young Shelduck have been individuals were seen at the same place on widely reported (Kirkman, 1913; Boase, several occasions over periods of some weeks. 1938, 1959, 1963; Coombes, 1950; Gillham Hori (1964a) found similar groups with con & Homes, 1950; Bannerman, 1957; Isakov, sistent membership, which he called ‘com 1952; Hori, 1964a, 1964b, 1969). These were munes’, in the nesting area and showed that also seen on the Ythan, though the group the association between the pairs persisted size was smaller (maximum twenty-seven) throughout incubation ; birds which lost their than at other places. Creches were detected eggs continued to visit the same nesting area by the mixing of young at different stages of until the last pair in the group hatched their plumage development within one brood brood. Young (1970b) also found that pairs (criteria described by Gollop & Marshall, which failed to hatch ducklings were more 1954), and by dye-marking ducklings. Water- likely to be seen in gatherings in the nesting soluble food dyes were injected into each egg area than were successful birds. of selected clutches about 4 days before the During the incubation period the male expected hatching date, using the method remained alone on the estuarine territory developed by Evans (1951). Ducklings with where the female joined him to feed when she their down dyed red or blue could be dis left the nest. Females which lost or deserted tinguished in the field for at least 4 weeks. their clutches returned to their territories Crêching was seen to occur when broods which were maintained for some time before came together while feeding and as the the pairs abandoned them and joined the parents interacted. Afterwards, in many non-territorial flock. This flock gradually cases, some or all of the young from one increased until the general departure on brood joined the other parents. Most crèches moult migration in early July.