The basic sound of the marsh at this time of year - it's keynote - is the hiss of last year's reeds swaying in the wind together with the low bass of sea. They are both a constant, and powerful, presence. The songs, calls and cries of birds and animals appear through, above and out of this background. Bitterns boom intermittently day and night but mostly at dawn. Their low notes, repeated 3 or 4 times, are one of nature's strangest sounds and carry long distances. Cettis warblers utter their brief explosive phrase loud and clear, geese fly past honking, reed warblers churr and twitter, ducks squabble and splash, water rails squeal, little grebes trill and pipe beautifully. Most are short and unpredictable – sonic events that emerge from the background. Sometimes sounds are very close, intimate murmurings or sharp cries, unidentifiable. A large expanse of reeds hides it's inhabitants extremely well. Listening reveals their presence in brief clues and unexpected surprises.