All lines are blocked between Worthing and Chichester, according to Southern Rail.
Southern said train services running through these stations may be delayed by up to ten minutes. Disruption is expected until 5pm.
"There is a report of a swan on the line in the Ford area. Our colleagues at Network Rail are having to take a short block of the line to remove the swan.
"This means that all trains are at a stand.
"We're sorry for any disruption caused to your day."