David: More or less right. Here is a letter from "British Birds", 76/8 (Aug 1983), p.353:
"'Twitcher' is actually a John Izzard-Bob Emmett word which was coined in the middle 1950s to describe our good friend Howard Medhurst, alias 'The Kid'.
"Birdwatching transport was very much a two-wheeled affair in those days. John Izzard and his girlfriend, Sheila, rode a Lambretta, whilst Howard rode pillion on my Matchless. The Lambretta had a unique luxury built into it: a back-warming, lap-warming dog, 'Jan', which used to travel jammed between John and Sheila. There was no such creature comfort on the Matchless; on arrival at some distant destination, Howard would totter off the back of my machine and shiveringly light up a cigarette. This performance was repeated so regularly up and down the country that it became synonymous with good birds, and, as we all felt a slight nervous excitement at the uncertainty involved in trying to see a particular bird, it became a standing joke, and John and I would act out a nervous twitch to match Howard's shiverings. This led us to describe a trip to see a rare bird as 'Being on a twitch'. Inevitably, this led to the term 'twitcher'. It was our association with the Portsmouth Group in the New Forest that extended the term into more general use. In the late 1960s, it became a derogatory term to describe unscrupulous tick-hunters (and as far as I am concerned it still is). It is pretty safe to say, however, that Howard Medhurst was - in the nicest possible way - the original twitcher.
"R. E. Emmett"