A total of 21,341 dead birds were found in western France by 500 volunteers working on three weekends in January and February, the French League for the Protection of Birds (LPO) said on Wednesday.
A further 2,784 were taken to sanctuaries.
A series of storms hit the French coast along the Bay of Biscay and Brittany in January and February.
They caused extensive damage on land but also had a devastating effect on wildlife, the LPO reports, pointing out that the figure is probably an underestimate, given that sailors have reported seeing many birds' bodies floating at sea.
The worst-affected species were the Atlantic puffin and the guillemot with the razor-billed auk also suffering badly.
Most of the casualties were due to a lack of food because storms blew the birds to areas where less was available.
Others may have died of exhaustion because giant waves prevented them settling on the water and forced them to use more energy than usual, while the LPO suspects that some boats may have emptied oil under cover of the storm, further endangering seabirds.