Maurice Bullen likes to give his students a hearing test. He plays two recordings and asks if they can tell them apart.
They are both Second World War fighter planes: one's a Spitfire, the other a Hurricane. Only one student manages to distinguish between the two droning engines.
"It's like that with birdsong," he goes on to explain. "When you tune in your ears, you can really sort out the difference between one bird and another."
This class is not so much birdwatching as birdlistening. Maurice and his wife, Kathy, set great store by the power of the human ear.
He plays another recording, taken in his garden the day before. It contains a medley of songs from a blackbird, greenfinches, a blue tit, jackdaws, and the toot of a moor-hen from a lake a quarter of a mile away.
Using the sound as a base, the Bullens go on to describe Brit-ain's birdlife, as vividly as any natural history film.
Later in the course there will be weekend birdwatching trips. But here students get an outline of our resident birds, migrants and winter and summer visitors. There is talk about what you can do in your own garden to encourage birds, and the dos and don'ts of nesting boxes.
Birdwatching, explains Maurice Bullen, is more than just a pleasant pastime. Keeping track of who goes where helps to monitor endangered bird species, such as the grey partridge, the song thrush and the skylark.
The statistics are sobering. We now have only 5 per cent of our original grazing marshland and only 3 per cent of our ancient meadows left. More than 100,000 miles of British hedgerows have been lost.
"The whole thing is interlinked," says Maurice. "If the habitat goes, the birds go." It's as simple, and dreadful, as that.
This class took place at The Civic Centre, 2 Gloucester Street, Wotton-under-Edge, Gloucestershire GL12 7DN. Tel: Stroud College, 01453 763424.