It is feared there are fewer than 2,000 breeding pairs in the world
Volunteers are looking for one of Europe's rarest seabirds, which has been seen off the Cornish coast in increasing numbers in recent years.
For the third year running, the SeaWatch South West Survey is looking for the endangered Balearic shearwater off Gwennap Point at Porthgwarra.
The data is being collected to find out how important Cornwall is for the bird and what can be done to protect it.
The Balearic shearwater is a dove-sized relative of the albatross.
Breeding colonies are only found on five islands in the Mediterranean, but the birds there are being threatened by building developments, feral cats and rats.
After the breeding season, the birds head northwards, visiting the seas around the South West.
It is believed there are now fewer than 2,000 breeding pairs remaining in the world, compared with 3,300 in 1991.