A bird in the hand / Ann Cleaves.

First published in 1986 by Century. Reprinted in 2018 by Pan Macmillan.

In England's birdwatching paradise, a new breed has been sighted a murderer. Young Tom French was found dead, lying in a marsh on the Norfolk coast, with his head bashed in and his binoculars still around his neck. One of the best birders in England, Tom had put the village of Rushy on the birdwatching map. Everyone liked him. Or did they? George Palmer-Jones, an elderly birdwatcher who decided quietly to look into the brutal crime, discovered mixed feelings aplenty. Still, he remained baffled by a deed that could have been motivated by thwarted love, pure envy, or something else altogether. But as he and his fellow twitchers flocked from Norfolk to Scotland to the Scilly Isles, in response to rumours of rare sightings, George with help from his lovely wife, Molly gradually discerned the true markings of a killer. All he had to do was prove it before the murderer strikes again.