Numbers of puffins on the UK's remote Farne Islands may be down 12% in the last five years, the National Trust has warned as it takes its latest census of the seabirds.
Initial numbers from the five-yearly count on the islands off the Northumberland coast, which are managed by the Trust, suggest the population is down 42% on one of the islands.
The figures could spell an alarming decline for the birds, known as the clowns of the sea due to their colourful beaks and clown-like faces, compared to the last count in 2013 when nearly 40,000 breeding pairs were recorded.
The National Trust, which has been looking after the islands for 93 years, said it would be stepping up monitoring of the seabirds to better understand what is going on.
Having surveyed four of the eight islands where the census takes place, there appears to be an overall 12% reduction in breeding pairs across the sites.
Puffins also returned four weeks later than normal to their breeding grounds on the windswept islands, where they rear chicks in burrows, due to the prolonged, harsh winter, the National Trust said.
Atlantic puffins were listed as "vulnerable" to extinction on the International Union for Conservation of Nature's (IUCN) Red List of Threatened Species in 2015, amid falling global numbers.
Climate change could be contributing to food shortages and extreme weather hitting the birds, while they could also be threatened by overfishing, invasive predators such as rats on some islands and marine pollution.
National Trust ranger Tom Hendry said: "Initial findings are concerning.
"Numbers could be down due to stormy or wetter weather as well as changes in the sandeel population, which is one of their staple foods."
Figures from the two largest islands surveyed so far are contradictory, with numbers on Brownsman down 42% while Staple shows a 18% increase.
Numbers are down by around a third on the two smaller islands surveyed.
If the final results reflect the drop seen so far, the Trust will need to assess the birds more frequently, although they are "notoriously difficult" to monitor.
Mr Hendry said:
Puffin records on the Farne Islands date back to 1939 when just 3,000 breeding pairs were recorded.
The figures continued to climb steadily until 2008, but that year saw numbers fall by a third from 55,674 five years before to 36,835 breeding pairs, a drop thought to be because of climate change.
In the last census in 2013, there were 39,962 breeding pairs on the island.
The count is conducted by assessing burrows on the islands, looking out for birds with fish in their beaks, a sign of a hungry baby puffin or "puffling", as well as external signs they are using the nests including fresh digging, footprints, hatched eggshells or other signs of activity in the entrance.
Ranger Harriet Reid said: