Hundreds of birds were found dead on a North Wales road on Tuesday 10th December. They were sent to be tested to discover how they died.
About 225 starlings were discovered with blood on their bodies in a lane on Anglesey, North Wales Police said.
Dafydd Edwards, whose partner found the birds, said it was as if "they had dropped down dead from the sky".
The Animal and Plant Health Agency has collected them for testing and will examine whether they could have been poisoned.
North Wales Police said it was investigating the "very strange" discovery and has appealed for information.