Dipper These birds start establishing territory from late February to early March. Their presence in streams is affected by the pollution levels of the water source. Like the grey wagtail, their main diet is water insects - found in well oxygenated, pollutant free water, the presence of this species on the Vyrnwy rivers indicates an excellent water quality. There are approximately seven pairs of dipper to be found across the reserve. Characteristically dippers will use a prominent stone within their territory as a 'launch pad' in their hunt for insects. They will sit on the stone and bob up and down whilst singing to announce their presence within their territory. When watched carefully their white eyelid can be seen to move across the eye.