Blair said they have had eight nests occupied before, but not with eggs and chicks.
“That is absolutely a record,” she said.
Blair said she was “cautiously optimistic” about the chicks’ chances.
She said: “There are predators around, the weather – I’d love to say that we’re going to stay with a lovely and warm summer now until September, but I think we all know don’t we?”
Ospreys were once found throughout the UK during the warmer months until they were driven to extinction as a breeding bird in England and Wales in 1847, and in Scotland in 1916.
They returned to the UK in 1954 and there are now close to 300 breeding pairs, according to the Wildlife Trusts.