The birds, the UK’s smallest type of swan, have flown from their summer breeding grounds in Siberia as part of the annual migration.
They spend the winter feeding on the flooded fields around the Pulborough nature reserve.
Pete Hughes, RSPB Pulborough Brooks warden, said: “Bewick’s usually turn up in late November or early December.
“It’s possible that an early cold snap has meant they’ve left their breeding grounds in search of suitable feeding areas.”
The RSPB said during this time of year there is an influx of huge numbers of other winter migrants including ducks, geese, swans and waders.
The reserve is located off the A283, between Pulborough and Storrington.