Definition of "seamew"

Now dance the lights on lawn and lea,The flocks are whiter down the vale,And milkier every milky sailOn winding stream or distant sea;Where now the seamew pipes, or divesIn yonder greening gleam, and flyThe happy birds, that change their skyTo build and brood

1849, Alfred, Lord Tennyson, In Memoriam A.H.H., canto CXV