Some of Norfolk's rarest birds have been found dead following an extended winter and late spring which have left the region's wildlife struggling for survival.

The bodies of four stone curlews – one of the UK's most threatened birds, which has a stronghold in the Breckland area – have been found in fields in Norfolk in recent days. Four others have been found in Suffolk and Wiltshire.

The species is a crow-sized bird with long legs and large yellow eyes which enable it to locate food in the dark, as it feeds at night.

It is estimated there are fewer than 350 breeding pairs of stone curlews in the UK – and about 70pc of them are in Norfolk and Suffolk.

RSPB officials believe the dead birds had returned from their wintering grounds in Africa and Spain and struggled to find enough food to survive. The bodies weighed around 300g, compared to a healthy weight of 450g.

Simon Tonkin, the RSPB's senior farmland conservation officer in the east of England, said the simple solution of putting out seed for the starving birds was not a viable option, due to the complexity of their diet of invertebrates and insects.

He said: 'The stone curlew relies on all manner of things for food, like ground beetles, spiders, earwigs, woodlice and earthworms. If you have got frozen ground conditions things like that are going to be hard to find.

'If you were to put food out you would need such a range of invertebrate species that it would be virtually impossible. They would need such an abundance of what would normally be available from spring onwards, so supplementary feeding with seed would simply not work.

'What is absolutely vital is making sure the habitat exists for the things which the stone curlews eat, and ensuring there is an abundance of prey to mitigate for the cold conditions, even in extremes of weather.

'We know that many farmers are working very hard for the stone curlew, but we need to provide more of those habitats. If you get the habitat right for the species which the birds feed on, then you get it right for the stone curlew.'

Elsewhere there have also been reports of short-eared owls and barn owls found dead after cold weather hampered their ability to hunt.

RSPB conservation director Martin Harper said: 'I can't remember a spring like this – nature has really been tested by a prolonged period of very cold weather.

'The discovery of eight stone curlews is a stark reminder of how fragile this species is. This amounts to around 1pc of the total UK population of these birds but the total number of deaths is likely to be higher. Many of these birds are only here because of the dedication of farmers who have been creating safe habitats for them in key areas.

'We are still getting calls from members of the public about strange sights in their gardens as birds like yellowhammers and reed buntings struggle to find enough food in the countryside. But we are also hearing about a steady trickle of swallows making their way up through the country and with temperatures on the rise the situation could start to look different in the coming days.

'As the global temperature continues to rise to this is another reminder that we must ensure our landscapes are in the best state possible to help wildlife cope with the increasingly unpredictable weather it will bring.'

Although springtime usually brings an influx of migrant birds returning to UK shores to build nests, the RSPB said very little activity had been recorded by nature enthusiasts.

Meanwhile, scarce reports of birds like chiffchaffs, willow warblers and blackcaps have sparked added fears from conservationists after last year's poor breeding season.