Every now and then, the birds would stop, along with the BBC Natural History Unit film crew accompanying them. Then they would fly from one end of a 20-foot clear glass tunnel to the other many times, with a camerman running along beside them.
It was the best way the BBC could find to film the small birds flying past the scenery they would naturally have encountered during their 6,000- mile flight from northern Europe to southern Africa.
It is extremely difficult to film a swallow in flight because of its high speed and tiny size. ``We didn't want to use special-effects techniques and make it look like Superman flying,'' said Nigel Marvin, producer of a forthcoming BBC1 series on animal migration.
So they plumped for the purpose-built tunnel with curved sides so that the cameraman would not be reflected in the transparent plastic. The swallows were trained to fly along it into a headwind created by a fan, making for some food at the far end.
``Even so, it was very difficult to follow them down the tunnel,'' said Mr Marvin. ``For every 200 minutes filmed we could only use one, which we think is a record."
The filming of the swallows against various landscapes in France, Spain, Gibraltar and North Africa took several weeks and cost several hundred thousand pounds, but the BBC will recoup the money in foreign sales.
The baby birds became available when they were orphaned. At the end of the filming they were released in Morocco to continue their journey south under their own steam.
It takes a swallow three to four months to fly from Britain to sub-Saharan Africa. They usually fly about 100 miles a day when it is light, always roost at night, and sometimes stop for a few days en route.
The average lifespan is three years, with very high mortality among the young birds making their first journey south and back north again for the next summer.