Trevor Weekes, founder of East Sussex Wildlife Rescue, said: “Catching any bird is exceptionally difficult. Magpies are extremely intelligent and are some of the most problematic birds we deal with.
“It’s not always possible to catch them. Roughly 60% of all the buildings we go to we are successful, but that means there are about 40% where we can’t reach them.
“It all depends on the layout of the building.”
A spokesperson for Waverley Borough Council also said it was working with the supermarket to help with the capture of the bird.
“We are satisfied that the store has implemented appropriate measures to protect food safety, including enhanced cleaning procedures and additional controls around areas where food is displayed,” it said.
Sainsbury’s also said it had “regular checks in place and appreciate our customers’ patience while we work to resolve this as quickly as possible.”
As magpies are legally protected from being harmed, anyone wishing to remove the bird must first apply for a licence to do so.
The licence allows for the use of cages or net traps to catch the birds, before safely releasing them into the wild.
Weekes said although every effort is made to catch the bird humanely, in some cases the bird will die before it can be rescued, while in others it may be “destroyed” if all other avenues have been exhausted.
Follow BBC Surrey on Facebook, external, on X, external, and on Instagram, external and listen to BBC Radio Surrey on Sounds. Send your story ideas to southeasttoday@bbc.co.uk, external or WhatsApp us on 08081 002250.