The fossils were found just outside the Seasalter Levels Nature ReserveFossils of parrot-like birds thought to be about 55 million years old have been found on marshes in north Kent.
Two bird skulls, a pelvis and several bones found on Seasalter Levels are thought to date from the Eocene period when the area had a tropical climate.
They were found by local archaeologist Mark Harrison in the shoreline's ancient clay beside the sea wall of the Seasalter Levels Nature Reserve.
The fossils date from a time before humans and most other mammals.
"Mark's discovery indicates that Seasalter Levels has been a wildlife haven for millions of years," said Alan Johnson, North Kent Marshes reserves manager for the Royal Society for the Protection of Birds (RSPB).
"This is a rare and surprising find - birds' skeletons are so fragile, the chances of them being preserved are almost inconceivable.
The shape of the skulls tends to suggest that they were perching birds, which may have fallen out of the mangrove forest that grew in the area and into the mud."
The fossils will now undergo an identification process to determine their origin.
The marshes which form the nature reserve, south-west of Whitstable, were bought in September 2007 by the RSPB, Canterbury City Council, Swale Borough Council and Natural England.
Mr Johnson said that a huge variety of wetland wildlife which thrived in the area at one time could make a comeback as a result of conservation work.
"These fossils show how wildlife has adapted to climate changes in the past but the situation we now face is unprecedented as it is the result of human behaviour," he said.
"As conditions become increasingly hostile, reserves like Seasalter Levels have never been so important.
"Species struggling elsewhere may look to these restored, protected habitats to survive."