“By now swallows should be everywhere and should have built their nests. Some could even have their first eggs.

“If in the next two weeks numbers of migrants are not up to their usual levels we would begin to wonder whether they are going to arrive.”

Mr Stancliffe believes some of the birds may have been grounded by bad weather on the Continent during their migration.

This has forced them to use the energy they built up over the winter for migration, making them stick around on mainland Europe to fatten up again before they continue their journeys.

He added: “There is also a suggestion that over the winter swallows were not able to build up the weight they needed to migrate. “There was a cold snap south of the Sahara and swallows were found in poor condition, so some may not have made it here.”

Summer visitors are at risk at every stage of their migration, flying the gauntlet of birds of prey and human hunters.

Climate change could also be worsening their problems with extreme weather such as droughts and storms.

Andre Farrar, of the Royal Society for the Protection of Birds, said: “Swifts and house martins seem very scarce. 

“I have heard several people say their house martins are not back yet. We will watch with anxiety as the season unfolds.”

The concerns come with bird experts recording dramatic reductions in the populations of migrants over the past 40 years.

Willow warblers and cuckoos have declined by about 60 per cent and spotted flycatchers, tree pipits and turtle doves by more than 80 per cent. But the relatively small number of visitors so far this spring does not mean that the breeding season will be a disaster.

Mr Stancliffe said: “This might not be such a bad thing as there will be less competition for the birds that are here. 

“If we have a phenomenal summer, birds like swallows could raise two broods.”

To send sightings of summer visitors – and any other birds – log on to www.bto.org/birdtrack