National Mute Swan Survey 1990

Abstract

INTRODUCTION Following the ban in England and Wales on most sizes of lead weight for fishing in early 1987, the Mute Swan (Cygnus olor) population appeared to respond rapidly and the National Waterfowl Count (NWC) index for the species increased markedly in the winters of 1987/88, 1988/89 and 1989/90 (Salmon, Prys-Jones & Kirby 1988, 1989; Kirby, Waters & Prys-Jones 1990). The British population was relatively stable from the mid 1950s to 1986/87 (Owen, Atkinson-Willes & Salmon 1986, Kirby, Delany & Quinn in press), but this apparent overall stability masked considerable population declines in some regions and increases in others. Changes at a regional level have been revealed by successive national breeding season surveys in 1955/56, 1961, 1978 and 1983 (Rawc1iffe 1958, Campbell 1960, Eltringham 1963, Ogilvie 1981, Brown & Brown 1984b, Ogilvie 1986) and by analysis of long-term trends in regional abundance using winter count data (Kirby, et al. in press). There have also been detailed regional studies of declines in the valleys of the Trent (Coleman, Minton & Coleman 1991), the Warwickshire Avon (Hardman & Cooper 1980) and the Thames (eg Birkhead & Perrins 1985) and of a steady increase between 1977 and 1982 in the Lothians (Brown & Brown 1984a). Boyd & Ogilvie (1964) and Ogilvie (1967) demonstrated that hard winters can result in considerable Mute Swan mortality and, in the late 1970s and early 1980s, the impact on swan numbers of lead poisoning caused by the ingestion of anglers' fishing weights came to be realised (Simpson, Hunt & French 1979, Goode 1981, Birkhead 1982, Birkhead 1983, Birkhead & Perrins 1985). Comparison of the annual population indices obtained from NWC data for the Mute Swan with indices for three species with broadly similar habitat requirements (Great Crested Grebe, Canada Goose and Tufted Duck) showed that the Mute Swan was exceptional in not having experienced a considerable increase in population since 1955 (Ogilvie 1986). Following the apparent success of the ban on lead weights, as reflected by the marked increase in the Mute Swan population in 1988 and 1989, the then Nature Conservancy Council (NCC) asked The Wildfowl & Wetlands Trust (WWT), the British Trust for Ornithology (BTO) and the Scottish Ornithologists' Club (SOC) to undertake a national Mute Swan survey during the 1990 breeding season with the following objectives: a) to assess population changes since the 1983 census and describe their geographical pattern, particularly in relation to the ban on the use of lead weights by anglers; b) to estimate the current sizes of local and national populations; and c) to provide a baseline for future monitoring of the species. This report addresses objectives b) and c) only, since the data from the 1978 and 1983 surveys are not yet ready for analysis. Even in respect of objectives b) and c), the findings must be regarded as provisional since full statistical tests have yet to be made. This report provides a description of the distribution and abundance of breeding and non-breeding Mute Swans during 1990, with the results being presented in both tabular and map form. Preliminary examination of the habitat data collected and a discussion of factors affecting Mute Swan populations are also included. Further, more detailed, analyses including comparison with earlier surveys will be undertaken in due course.