A duckling worth £600 is believed to have been stolen from Castle Espie Wetland Centre in County Down.
The two-week-old Mandarin duckling was introduced to the public on Monday, but police said they believe it was stolen between 13:00 and 15:00 BST that day.
The East Asian species is very valuable and can be sold at a high price, according to the bird sanctuary.
Due to its young age, the duckling requires a special diet, so they say its safe return "is a high priority".
"It's unclear why someone has chosen to do this, but we would like to appeal to anyone who has information that could lead to this duckling's safe return to get in touch," said a spokesperson for Castle Espie.
The charity said that while Mandarin ducks aren't rare, they are "in high demand for private collections in western Europe due to their ornate plumage".
The charity's marketing manager, Karl Simmonds, added: "As a leading conservation organisation, we take the utmost care to look after all of the species kept on-site.
"As a team, we are extremely saddened that someone had decided to take one of our Mandarin ducklings."
A PSNI spokesperson said: "I would appeal to anyone who was at Castle Espie between these times yesterday and saw any suspicious activity, or anyone who knows anything about this to get in touch.
"I would also ask anyone who is offered a Mandarin duckling for sale in suspicious circumstances to contact us."