This is found in Geoffrey of Monmouth's Historia Regum Brittaniae (History of the Kings of Britain. See Other Works). Geoffrey has...

"This lake contains sixty islands, and receives sixty rivers into it, which empty themselves into the sea by no more than one mouth. There is also an equal number of rocks in these islands, as also of eagles' nests in those rocks, which flocked together there every year, and, by the loud and general noise which they now made, foreboded some remarkable event that should happen to the kingdom."*

This seems to come from Nennius, but it is unclear where he gets the information about the prophetic gathering of the Eagles.