Shelduck © Steve Round
WeBS counts show that our two estuaries are the top sites in Britain for Shelducks, with five-year peak mean figures far exceeding the threshold for international importance of 3,000 birds, averaging 14,500 (Mersey) and over 11,000 (Dee). This status is based on the moulting flock on the Mersey, peaking in July or August, and the maximum on the Dee, usually in October. Midwinter totals are much lower, typically 4,000 on the Dee. The numbers actually staying to winter on the Mersey have dropped considerably since the moulting flock became established, previously reaching 4,000 to 5,000 birds, but since the late-1990s seldom above 3,000: could it be that the pressure of the huge moulting flock actually depletes the food available until the hydrobia reproduce again in the spring?
Most of the Shelducks wintering here are thought to be British breeders, perhaps with some immigrants from Scandinavia and continental Europe (Migration Atlas). The origins of our birds were nicely illustrated by two ringed females found within three weeks of each other in 1990/ 91: one, caught during an overnight ringing session on the drained New Brighton Marine Lake had been hatched the previous summer on the edge of the Solway in Cumbria, and the other, found dead at Frodsham, was a bird-of-the-year from 1989 ringed at Slimbridge, Gloucestershire so those two made movements of around 160 km in opposite directions to converge on the Mersey.
Although the map shows that they are quite widely distributed, there are few birds wintering inland and the vast majority of the county’s birds are in the estuaries. The only non-estuarine tetrads with flocks counted in three figures were Frodsham Marsh no.6 bed (SJ47Z), with a maximum of 220 birds and Fiddlers Ferry (SJ58M) with up to 100 birds. Both of these are feeding and loafing sites adjacent to the tidal Mersey, however, and only two truly inland sites registered a flock of more than ten birds: Sandbach Flashes (SJ75E/ J) with up to 45 birds, and flooded fields alongside the river Weaver at Aston (SJ57N).
A century ago, Coward (1910) wrote of large flocks frequenting the banks of the Dee and Mersey estuaries, but his ‘large flocks’ were of hundreds, rather than the thousands of the present day: the largest counted was more than 800 strong, near Denhall on the Dee in February 1906. Bell’s assessment was much the same in 1962: ‘the numbers wintering in the estuaries are variable, and are difficult to assess over these areas, but may number several hundred’. More quantitative information came from the wildfowl counts, forerunners of WeBS, with, for instance, more than 2,000 at Gayton and 660 on the Mersey at the same time on 16 February 1969. Numbers wintering on the Mersey rose spectacularly to reach 12,000 in 1980/ 81, then gradually levelled out over the next decade to the figures quoted above, around 4,000 to 5,000 birds on each estuary.
Their diet is much the same as in the summer, predominantly mud-snails and tubifex worms, although they are also attracted to grain, but in winter Shelducks have to spend more than half of their time feeding. As the winter passes, adult birds become more restless, and during February aggression amongst the flock members increases as they anticipate the coming season.
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