Swans used to be considered a delicacy to eat, but they're a protected species in the UK now, and the Queen owns all of the swans on open water in the United Kingdom.
Swan upping is the traditional means by which the swans on the Thames are apportioned among the three proprietors.
Its main practical purpose today is to conduct a census of swans and check their health.
It occurs annually during the third week of July and is under way.
TRT World 's Sara Firth explains.