The King also spent time at the Animal and Plant Health Agency’s York Biotech Campus earlier in the day.
He talked to scientists and volunteers about a project to protect red squirrels by limiting the numbers of non-native grey squirrels.
They explained that oral contraceptives would be delivered through a feeding programme to slow the rate of the species’ population growth and help endangered red squirrels to thrive.
Dr Julie Lane, head of the National Wildlife Management Centre at the campus, told King Charles: “What a wonderful day you’ve given us.
“You’ve been championing our red squirrels for decades now, which is why we’ve been so keen to show you the work we’ve been doing here at the Animal and Plant Health Agency.
“Without you, this work would not be happening at all.”
The King was presented with a framed photograph of a red squirrel by field ecologist Erin Thomas, who explained how she took the picture herself at the Yorkshire Arboretum, at nearby Castle Howard.
He thanked Thomas, saying “that’s fantastic”.
