Antoine Prevel is a volunteer with Sea Shepherd, an international, non-profit marine conservation organization.

"We patrol these beaches every day of the week and we find Guillemots nearly every day. It depends on the weather conditions."

According to the French League for Birds, over 500 birds have been found dead along the French Atlantic coast since the start of the year, with no exact explanation.

In La Rochelle, scientists are trying to understand the reasons behind the phenomenon.

Jerome Fort is the Head of Research at CNRS National Research Institute:

"Guillemot birds are a species that will not survive if they don't feed for two or three days, they have no energy reserves, so they need to feed almost constantly. So when there's a storm of strong winds such as recently, these birds will find it hard to feed properly, while having to use more energy and will die of exhaustion."

The recent beachings happened after storms or strong winds.

Fort says climate change, which brings more frequent and intense storms that exhaust the birds, is one explanation.

But he said there were several other possible explanations, including micro plastics and chemical contamination, which could contribute to weakening the seabirds.

Another factor could be that the fish they prey on have become rare and have relocated closer to the shores due to climate change and overfishing.