Usually there is one dramatic day each year when numbers of these birds fill the skies, and last Tuesday (September 23) was the day.

Between 273,000 and 535,000 pairs of house martins breed in the UK. In September, they start on their long journeys to Africa, with birds from all around the country gathering over Sussex's cliffs before daring to cross the open sea.

Hester Phillips, of RSPB South East, said, "Anyone walking or driving by the Sussex coast on the day was likely to have seen loose flocks of martins whizzing about the sky as they say goobye for another year.

"It's one of my favourite sights of September. To think these tiny birds will fly all the way to Africa to spend the winter somewhere over the rainforests never fails to amaze me."

The largest single movement through Sussex in a day was recorded in September 1987 when 45,000 flew past Beachy Head. House martins spend most of the time in the air, and are even thought to sleep while flying at high altitudes.

Despite the large numbers currently seen along the coast, the UK population has declined by 38 per cent since 1970, with the bird now amber-listed as a species of conservation concern.

House martins now nest almost exclusively under the eaves of houses, and their decline is probably a result of many factors, from a lack of mud with which to make their nests to fewer insects to feed on.

Ms Phillips added, "With the future for house martins uncertain, now is a great time to put up a special house martin nestbox under the eaves of your house, and ensure they have a grand welcome back next spring."

For more information on nestboxes and wildlife-friendly gardening, visit www.rspb.org.uk