On 8 February this year, at the Wildlife and Wetlands Trust at Slimbridge, the two big male birds were fitted with radios which transmit to satellites, in the hope of teaching scientists more about the swans' 2,500-mile journey to and from their breeding grounds in the Russian Arctic.
The radios - rectangular boxes 4ins long, with short aerials - were mounted between the birds' shoulder blades and secured with stretchy harnesses. They were programmed to come on the air every 13 days, then to transmit for eight hours continuously.
Pedro took off from Slimbridge at midday on 26 February, with his mate, Weaver. Three days later he was over northern Germany; by 13 March he had reached Jutland; then he went off the air, presumably because his battery had failed.
Abelhard proved a more reliable station. He did not leave Slimbridge until 17 March, but then set out with his mate, Mid-Off, in a party of 27 Bewicks and made rapid progress to the north east. By 10 April he was off the coast at Stanga, in Sweden; 11 days later satellites picked him up in the valley of the Kasari River in Estonia.
On 16 May he was north of Archangel, off the White Sea coast of Russia; 11 June found him at the mouth of the River Oma, 66:47 degrees north. On 24 June he reached the west coast of Kolguyev Island, at 69:06 north, where a series of five good fixes suggested that he had settled down there on a breeding site. One last message came through on 19 July, but a null reading from the activity sensor suggested that the device had fallen off.
Now the staff at Slimbridge are eagerly waiting to find out whether their two airborne research assistants have bred, and whether they will bring families back with them.
The particular fascination of Bewick swans is that they can be identified by the unique black-and-yellow patterns on their bills. Much has been learnt about them- not least, that they pair for life.
Pedro was identified as a cygnet at Welney in 1983: now, at 13, he is half-way through a normal lifespan. He spent his first 10 winters at Welney, then in 1993 appeared at Slimbridge with Weaver and three young. (Weaver had been identified as a cygnet in 1977). Abelhard is also a regular visitor to Slimbridge: he is at least 12 years old, and has brought Mid-Off there for the past six winters.
Behind these bare facts lie several minor tragedies. Both males, it seems, must have had former mates. What happened to them?
As the BBC programmes will show, many hazards beset the migrants on their way to and from the tundra. They can be forced down by bad weather, fly into telegraph wires, die of lead-poisoning from shotgun pellets, or be killed by primitive hunters. The sad fact (revealed by x-rays) is that nearly a third of the birds that winter at Slimbridge are carrying lead shot.
Mild weather and persistent westerly winds have made this a late swan season. In a normal year there would be a couple of hundred Bewicks at Slimbridge by now, but there are scarcely half that, and many are still in Holland, detained by a superabundance of their favourite pond weed.
Imagine them on their way, flying at 50mph, in V-shaped skeins for maximum aerodynamic efficiency, five or six thousand feet above the surface of the earth. Heading South will doubtless capture some of the romance and magic of migration, but mysteries will remain.
How do Pedro and Abelhard retain an image of their far-off wintering grounds, all through the Arctic summer? How do they navigate during their two-month passage? And how do they time their arrival to come in at night, causing a tremendous clamour among travellers who have already reached their journey's end?