This year’s Swan Upping, dating back 800 years, started at Sunbury, Surrey, and went up the Thames to Abingdon, Oxfordshire, between Monday and Friday last week.
Experts in scarlet uniforms counted and weighed cygnets along the way, to check the health and numbers of swans.
David Barber, in his 21st year as the Queen’s Swan Marker, said: “Monday and Tuesday were very good, but the rest of the week was a disaster.
“On Friday, when we rowed from Moulsford to Abingdon Bridge, we only found one family with four cygnets.”
The Uppers passed by Windsor and Maidenhead on the Tuesday, where the cygnets were a reasonable size, but the few found further up the river were very small.
The cold start to the year has pushed back the breeding cycle, causing eggs to hatch later and leaving cygnets vulnerable.
Mr Barber said: “Hopefully next year improves.”
He will now put together a report on the findings, to be delivered to Buckingham Palace next week.
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