Predation of the shelduck Tadorna tadorna on the mud snail Hydrobia ulvae

Many waders and ducks rely exclusively on invertebrates for their food and can deplete populations of these invertebrates. The mudsnail Hydrobia ulvae and the shelduck Tadorna tadorna are both widespread and abundant and in previous studies H. ulvae has been identified as the main food source for shelduck. The objective of the study was to quantify the predation of shelduck of H. ulvae on an intertidal mud flat in north Wales in order to estimate the contribution of shelduck predation to H. ulvae mortality and to estimate the importance of H. ulvae in the diet of shelduck. The contribution of shelduck towards the observed mortality was estimated from the number of H. ulvae per dropping, the number of droppings produced per hour and the number of shelduck found at the study site. Shelduck predation hardly made a contribution to the observed mortality of H. ulvae. Examination of the energetic requirements of shelduck shows that H. ulvae cannot be a significant part of the diet. In conclusion, the predation of shelduck on H. ulvae was irrelevant for both the population dynamics of the mud snail and for the energy requirements of shelduck in the study area.