Britain’s sea birds are heading for their worst breeding season on record due to a decline in the small fish they eat, according to a BBC investigation.

BBC One programme Countryfile has found the fish are becoming smaller – forcing birds to work harder for a catch with less protein. Scientists believe warmer waters have depleted plankton stocks, which form the basis of food for many such fish. Environmentalists argue that tackling climate change is the only solution. Scientists examining bird populations on islands like the Shetlands say the number of kittiwakes has halved in the past five years. They fear that with vital catches of fish such as sand eels getting smaller, breeds like puffins and guillemots may also decline in the future.

Marine scientists have been monitoring plankton in Plymouth for 70 years. These microscopic creatures are at the heart of the ocean food chain, and all sea life depends on them. Rising water temperatures have led to sudden and dramatic changes – with the plankton breeding at the wrong time, and in the wrong place, threatening species like cod. Fewer plankton make it harder for the fish, and therefore the birds, to survive.