Hundreds of razorbills and guillemots have washed up on shorelines between Humberside and Orkney over the past few weeks, including around 15 at Warkworth.
And birdwatchers have reported many more sightings of the birds swimming unusually close to shore and expressing concern about their ‘listless’ behaviour.
“It’s a real mystery,” said Tom Cadwallender, a freelance ornithologist who formerly worked for the Northumberland Coast Area of Outstanding Natural Beauty (AONB).
“On our part of the coast you would normally expect the razorbills and guillemots from the Farne Islands to be getting ready to go out to sea for the winter.
"For some unknown reason they are hanging around inshore and the ones which are still alive are not as active as they should be. They should be feeding quite frequently but they many are just sitting on the water.”
There have been many other reports on social media of their unusual behaviour.
‘We're finding lots of moribund guillemots and razorbills - birds taking no evasive action as the boat approaches,’ said one.
Another said: ‘Even I, no birding expert noticed this unusual behaviour of a guillemot not 20ft from us and 4ft off the shore at Druridge Bay. Looked very listless and not at all bothered by our presence.’
There were also reports flocks of guillemot and razorbill on the River Tyne, at least as far upriver as Royal Quays marina.
"There are several schools of thought,” said Tom.
“One is that we had an run of northerly winds over the last few weeks which may have created turbulence at sea that may prevent them from feeding – but that has settled down now.”
"It appears there are plenty of sand eels on our bit of the coast so it’s difficult to explain. I’ve been birdwatching on the Northumberland coast for 50 years and this is unprecedented.
"Hopefully we will find out soon what is happening but it is quite alarming.”
The seabird unit at the UK Centre for Ecology and Hydrology (UKCEH) is investigating.
Dr Francis Daunt, seabird ecologist at UKCEH said: “We are seeing unprecedented numbers of guillemots dying for the time of year, over a huge area from northern England to Orkney.
"We are also seeing very unusual behaviour. Birds have been seen feeding very close to beaches in amongst swimmers, when normally they steer clear of people, and have been observed up to 20 miles up rivers, which is unheard of for this marine bird.
"These are signs that the birds are getting desperate in their search for food.
"The dead birds are little more than skin and bone with many half their usual weight, which is catastrophically low. So it appears that the birds are dying from starvation.
"This suggests that there is a lack of their fish prey in the sea, but people are seeing a lot of feeding flocks by our coasts at the moment, so perhaps starvation is being caused by something else.
"We have tested 10 birds for avian flu, all of which came back negative, and we are now looking at the possibility of toxic poisoning from algal blooms.”