A motorcyclist has died in Sydney's north while trying to avoid a car that had slowed to let a group of ducks cross the road.

Police say the 44-year-old man was riding along Lady Game Drive, Lindfield, about 4.30pm (AEDT) on Friday, when he hit a vehicle heading in the opposite direction after he tried to avoid a car that had slowed for the ducks.

He died at Royal North Shore Hospital, where he had been taken for emergency surgery.

About three hours later another motorcyclist died when he crashed into a railing on the Old Pacific Highway at Mount White, north of Sydney.

Motorists stopped to assist at the scene and called emergency services but the man, in his early 20s, was pronounced dead on arrival at Gosford Hospital.

Police will prepare reports for the coroner about both incidents.