Common in damp woodland, marshes, river-banks and other damp areas throughout the British Isles. Widespread and common in Hampshire, but scarce on the Isle of Wight, with only two records so far this century. Wingspan 30-34 mm. The main confusion species is
Ruddy Highflyer H. ruberata, which is often reddish-brown and has a more westerly and northerly distribution, but grey forms of Ruddy Highflyer are difficult to distinguish from May Highflier. Larva feeds on Alder, over-wintering as a pupa.