In Yorkshire, dead birds have been found at locations including Filey and Runswick Bay, and there have also been mass deaths off the Northumberland and Scottish coastlines.
It is not known if the phenomenon - which seems to particularly affect guillemots - is caused by disease, a marine pollution event or long-term environmental pressure.
Species such as puffins, razorbills and kittiwakes have also been affected.
Bird flu has been ruled out but poisoining from algae blooms is among the possibilities being assessed.
The furthest north deaths have been recorded is Orkney.
The bodies show signs of malnutrition, suggesting low fish stocks could be to blame, and the guillemots have been seen up to 20 miles inland feeding in rivers and along beaches, despite them normally avoiding people.
Mass death events are known as 'wrecks' but usually occur in winter following storms.
Maria Prchlik from RSPB Bempton Cliffs in East Yorkshire said: “Extreme weather, pollution and disease can kill seabirds. If prey fish are scarce, seabirds can be weakened through starvation. We don’t know the exact cause here, but we know climate change is driving prey fish numbers down in our seas and creating more extreme weather events. The world is in a nature and climate crisis with humans and wildlife already experiencing the impacts. We need urgent action from governments to help revive our world.”