My TL;DR:

According to the RSPB, its annual compilation of confirmed cases of illegal killings of birds of prey has always been the tip of the iceberg of raptor persecution, which is mostly driven by a desire to protect game shoots involving species including red grouse, pheasants and partridges.

Since the bird flu outbreak began in Britain in 2021, hundreds of dead raptors have been sent directly to Defra for testing. In most cases, if a bird tests positive for bird flu, no further postmortem tests are undertaken.

Moreover rules prevent carcasses that test negative from being moved to other labs to be examined for signs of persecution such as shooting or poisoning.

Convictions are vanishingly rare because evidence is difficult to gather on remote estates with little public access.

Of 191 individuals convicted of bird of prey persecution-related offences from 1990 to 2022, 67.5% have been gamekeepers. Of two successful convictions in 2022, both were gamekeepers.