Effects of Grazing on Grasslands by Wintering Whooper Swans

The effects of grazing by Whooper Swans (Cygnus cygnus) on agricultural grasslands in winter were investigated in northwest Ireland. Exclosure cages were used to compare aboveground biomass in ungrazed plots with those grazed by swans. Grazing intensity was measured by recording cumulative dropping densities in fixed plots. Measurements of aboveground biomass were made on three occasions: midwinter (January), early spring (late March) and late spring (late May) at two different study sites in different years. There were significant yield losses between grazed and ungrazed plots for each sampling period. These were highest in spring when losses of up to 65% were recorded. Swan grazing intensity was positively correlated with yield losses and aboveground biomass and the correlation was especially strong in late spring. Geese were also present in fields at one of the study sites and, while they also had a significant effect on biomass, swan grazing had a proportionately stronger effect. While the results illustrate the potential severity of grazing by swans, they should be interpreted with caution as previous studies have shown the interactive effects of a range of variables, including weather conditions, which make it difficult to predict the response of a sward to grazing.