The Royal Swan Upping is the annual census taking and a health check of swans on the Thames. It occurs ever year in the third week of July. It began in the 12th century.
The swans on portions of the river that go through Oxfordshire, Buckinghamshire, Berkshire Surrey, and Middlesex are owned by the Queen.
Queen’s website http://www.royalswan.co.uk
Marlow Town Council “Royal Swan Upping 2022”
The Guardian “A cygneture event: what is swan upping?”