The poultry farmer said his flock of 7,000 chickens looked healthy on Saturday morning, but by the evening 4,000 had died.
The cause of death of the birds is still under investigation.
France is the biggest poultry producer in Western Europe and producers feared their Christmas sales could be badly affected if bird flu was confirmed as the cause.
One alternative theory is that the chickens may have died from overheating when the ventilation for the shed failed.
Samples taken from the Sarrey farm were analysed at France's main testing laboratory in Ploufrangon and the French authorities sealed off the farm as a precautionary measure.
