Up to 3,000 dark-bellied brent geese are making their way from their breeding grounds in Arctic Russia for their winter stopover in Pagham Harbour.
They'll be flying more than 135,000 miles to get here.
The RSBP says it promises to be one of the region's most awe inspiring natural wonders.
The first geese were spotted by the wardens arriving in mid-September, and their numbers are expected to steadily grow to their peak in January.
Rob Carver, the nature reserve’s manager said: “Tidal inlets like Pagham Harbour offer the geese everything they need for their winter stop-over, as they feed up on plants growing on the mudflats, and grasses on surrounding farmland.”
“The geese will be with us until March offering a fantastic wildlife spectacle not to be missed. I would encourage people to visit Pagham Harbour and enjoy this sight before the geese leave us again next year.”