Dr Ian Evans of Natural England went out to Spain in the 1990s to collect wild red kites for release in the Chiltern Hills. He said the ones returning this week may be of Spanish descent.
"Those birds we took from Spain in the '90s have done really well in Britain - we're talking 4,000-plus pairs in the UK now, which is an incredible success story," he told BBC News.
In the 1990s, red kites in Spain were doing well in comparison to the UK, where years of human persecution, including egg collecting, poisoning and shooting, had pushed the bird to near extinction.
While red kites in the UK have since boomed, populations in some parts of southern Spain have gone the other way due to a number of factors.
Dr Evans said the reason to take the birds back to Spain and release them in the wild was to "secure the future of kites globally".
The juveniles are only a few weeks old, but already have most of their feathers and can feed independently.
A first batch of 15 birds has already been flown out, with the project kept under wraps until now.
The birds will perfect their flight in aviaries before being released fitted with the latest technology for monitoring how they adapt to their new habitat.
The RSPB's Duncan Orr-Ewing said 30 birds will be released this year, with plans to release 30 more birds in each of the next two years.
"You need to find 90 to 100 birds to create a sustainable population in a given area," he said. "That should be sufficient to create a new breeding nucleus of the birds."
Red kites are found across Europe and numbers have increased overall in recent years, although there are still drastic declines in southern Spain, Portugal, and locally in Germany. The UK is now home to more than 10% of the world population of red kites.