Male bustards do not become fertile until they turn five
Nineteen Russian Great Bustards are being freed on Salisbury Plain in Wiltshire as part of a project to bring them back to the wilds of Britain.
The scheme hopes to repopulate the UK with the birds, once common but hunted to extinction.
The Great Bustard Group has released 70 birds over the last five years and hope to create a viable UK population.
The females have been nesting for the past two years although no chicks have been seen as yet.
The young birds are vulnerable to predators and their survival in the wild in the first 10 weeks is critical.
David Waters, of the Great Bustard Group, which first released 22 birds in 2003, said: "Today marks the half-way point and we're very pleased with the results so far.
"We did not expect to have nesting females so early on. The indications are very positive indeed."
He added: "We've managed to recover clutches of eggs which did not hatch and the likely explanation for this is that they were not fertilised.
"Male bustards do not become fertile until after they turn five. The chaps are currently a bit young so whilst they're willing they're not yet able."
The survival rate of the released bustards is just over 20%.
Mr Waters said: "We are looking at nearly 80% dying, which is a remarkably good survival rate for birds in the wild."
The group estimate that at least two of the first release of bustards, originally from Russia's Saratov region, are still alive among a wild UK population.
Female great bustards can live for up to 12 years, while males can live for 20 growing as bulky as 20kg (44lb) making them the heaviest flying birds in the world.