Extra security had to be drafted in to an historic royal palace after people said their food was being stolen.
Concert-goers who were at Hampton Court Palace, south-west London, to see the likes of Eric Clapton, complained of cakes and sandwiches going missing.
Eventually a duck was identified as the culprit after being caught on camera taking the food for her ducklings.
Organisers are now urging people to bring secure hampers and cool bags to "be on the safe side".
Hampton Court Palace east front manager Mark Howarth said: "Robin the Duck, as we've nicknamed her, lives in the East Front gardens.
"As soon as everyone has gone into the show and the music starts she appears by the picnics to see what she can raid."
One guest added: "I thought I was losing my marbles when I couldn't find the rest of our sandwiches. I was amazed that anyone here would steal food. It's such a relief to discover that it was only the palace duck. She obviously has very expensive tastes because she took all my smoked salmon sandwiches. I suppose it also explains why the duck pate was left!"