The number of cygnets on the Thames is "slightly lower" than last year as the annual census of the swan population on the river is under way.
In February, more than 30 swans from the Queen's Windsor flock died from an outbreak of bird flu.
The Queen's Swan Marker David Barber says after one day it looks to have only had a small effect on cygnet numbers.
Swan Upping dates to the 12th Century, when the ownership of all unmarked mute swans in open water in Britain was claimed by the Crown in order to ensure a ready supply for feasts.
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