A four-year-old North American black vulture on the loose since the weekend has so far ignored all efforts to entice her back into captivity.
Guinevere is being harried by gulls and crows
Guinevere flew off during a training session at the Tropical Wings World of Wildlife in South Woodham Ferrers, Essex, on Saturday.
Crows and seagulls harried and chased her as she tried to rest on rooftops and trees on her fifth day of freedom.
But handlers failed to woo her back with dead chicks and rabbits.
Guinevere has a 5ft wing-span and stands 2ft 6in tall.
"They can travel a long way. I'm afraid this could go on for some time," said Lucy Murray, conservation manager at the wildlife centre.
"We are trying to keep track of her movements and hopefully we'll be able to coax her back."
Mrs Murray said Guinevere was bred in captivity, only ate carrion and would not attack pets or people.
"She won't be interested in anything living," added Mrs Murray.
"She probably wouldn't know how to go about killing something. She's a gentle giant and wouldn't hurt a fly."
Mrs Murray went on: "She won't attack other birds even though she's much bigger. She'll probably be scared to death. They won't like it because they'll see her as a threat."
Guinevere's mate, another black vulture named Lancelot, was "missing her terribly", Mrs Murray said.