Hundreds of birds which were found dead on a road died due to trauma from impact with the ground, preliminary findings have indicated.
About 225 starlings were found dead on a lane on Anglesey on 11 December.
The Department for the Environment, Food and Rural Affairs is continuing its investigation, but it is unclear why the birds plunged to the ground.
Dafydd Edwards, whose partner found the birds, said it was as if "they had dropped down dead from the sky".
Defra added it had investigated several incidents of mass mortality in groups of starlings previously. It said testing for bird flu was negative.
North Wales Police said it believed it had an explanation into the deaths on Wednesday.
Starling expert Prof Anne Goodenough from the University of Gloucestershire said one theory was the birds had been taking part in a mass murmuration and had been disorientated by sun reflected from a wet road.
"You could have had quite a lot of glare from that, that could have potentially confused the birds," she said.
"We know that can happen to birds, so we get swans and geese, for example, crash landing onto solar panels because they look like lakes.
"So that kind of almost visual hallucination can occur - whether that is the case here, we don't know."
What are starlings?
- Smaller than blackbirds, with a short tail, pointed head, triangular wings, starlings look black at a distance but when seen closer they are very glossy with a sheen of purples and greens
- Noisy birds, starlings spend a lot of the year in flocks
- They usually lay 4-6 eggs in mid-April
- Starlings are very tenacious and adaptable birds
- Over the centuries they have expanded their numbers and range in the wake of farmers, wherever suitable conditions became available. They used to be uncommon birds in the UK
- Starling numbers have declined markedly across much of northern Europe and the UK. The decline in the UK started during the early 1980s and has continued ever since
- Long-term monitoring shows starling numbers have fallen by 66% in Britain since the mid-1970s