Gerald Lenton,62, of Peterborough was convicted by a jury at Carlisle Crown Court of threatening the Frank Bird poultry factory in Carlisle.
He had denied the charge.
Evidence had been given that Lenton had telephoned the plant and told a receptionist in a fake Northern Irish accent:
"We are animal rights. We have an incendiary device on your premises."
Police then spent three hours searching the plant but found nothing. The warning had been taken seriously because 12 years before a device had been found at the plant after a similar warning.
The prosecution accepted that Lenton was a hoaxer who was not in fact an animal rights activist but a truck driver picking up a consignment of frozen chickens from the plant.
In his defence Lenton admitted that he had phoned the company but only to say that the gate into the plant was stuck and that he could not get his lorry inside.
Lenton, who has previous convictions for dishonesty, was bailed for a sentencing hearing on March 27.