Project founder John Darby, a retired zoologist, said the grebes had "finally come out of lockdown" with the first recorded egg of the season on October 28.
Two other nests on the lake-anchored man-made wooden breeding platforms had a single egg each that had been laid in the last day or two, and a pair that arrived in August had finally started to build a their nest.
"This is the eighth year of the project and the first time other than the first year of my study, which started in October 2013, that birds have not bred in September and only just made it into October.
"Our records to date show that 12% of all eggs are laid in August-September and a further 19% in October, so everything is well behind the norm."
Mr Darby said he could only guess as to why the birds were breeding so late.
"Weather-wise it has not been that stable, especially with the amount of rain we have received, and the lake levels have been going up and down in a matter of days," he said.
There had been a lot of noise from pile driving on a nearby development recently, but the appearance of the eggs suggested the birds had got used to the noise or were not bothered by it.
