Outraged islanders have raised the reward to catch whoever is poisoning rare birds of prey.
The inducement to find the person responsible for poisoning four peregrine falcons has risen to £15,500.
The sum has risen following contributions from the ornithological section of La Societe Guernesiase, the RSPB and members of the public.
Post-mortem examinations and analysis of stomach contents have demonstrated that the four birds had consumed fatal doses of a mixture of professional-use pesticides.
The first deaths were notified to the States Veterinary Officer David Chamberlain last year, when two peregrine falcons were found dead next to each other.
A further death was reported in February at a different location and a fourth confirmed last month.
Mr Chamberlain confirmed that all of these birds had died from overdoses of a mixture of highly toxic chemicals not normally available to the general public.
An anonymous donor initially put up a £5,000 reward to find the culprit.
Man arrested over 'threats to kill' in Guernsey
Jersey and Guernsey both lift trophies in Siam and Muratti weekend
Men pretending to be police arrested for £10K Guernsey convent fraud
Channel Islands' 10 wealthiest residents revealed
85 home planned for Forest site in Guernsey
Sewage outfall failure on Guernsey's east coast
Guernsey's airline sets up Southampton base
Cuts to services in Guernsey considered under tax review
