Fifty years ago, Mediterranean gulls were rarely seen in the UK
Hundreds of pairs of breeding Mediterranean gulls in the Medway estuary have become a "highly significant colony", the RSPB has said.
RSPB wardens identified more than 200 pairs of the protected bird off the north Kent coast - more than a third of the UK's population.
The bird protection charity said that as recently as 50 years ago, the species was rarely seen in the UK.
The population across the UK is thought to consist of fewer than 600 pairs.
RSPB warden Gordon Allison said the greatest concentrations of Mediterranean gulls were found on the charity's island reserve at Nor Marsh.
Mr Allison, who went out in a boat to record the birds, said: "It is very exciting to have such a significant breeding population of this special bird right on our doorstep. A real success story."
Other colonies of seabirds on the Medway include Sandwich terns and little terns.
The Nor Marsh island site is not open to the public, to protect it from disturbance, but there are views over the Medway from the Motney Hill RSPB reserve and Gillingham's Riverside Country Park.
Mr Allison said the gulls could be seen from both points, along with the spectacle of thousands of other birds currently using the estuary as a winter migration route from the Arctic to Africa.