Sylvia the swan was snatched from her nest on the moat of Markenfield Hall, in Ripon, as she sat protecting her unborn cygnet.
The mother-to-be, was one of the two Black Swans on the moat, along with male swan Sebastian.
The pair had spent most of April making a huge nest on the bank at the front of the Hall in preparation for the hatching.
Lady Deirdre Curteis, who owns the hall with her husband Ian Curteis, began to grow concerned when she found Sebastian sat alone by the moat on Saturday, April 20.
Closer inspection found that Sebastian was sitting on the nest and Sylvia was nowhere to be seen.
Lady Deirdre said: "I thought it was peculiar as Sebastian - the male - always swims across when he sees anyone coming, but there was just one swan and it was sitting on the nest.
"We searched for Sylvia for days but found no trace anywhere - not even a feather”.
After a frantic few days of searching, the owners consulted their Swan expert and came to the conclusion that Sylvia had been stolen.
Lady Deirdre said: "Our Swan man told us that it is not uncommon for thieves to take advantage of nesting birds, snatching them at their most vulnerable, but to think of it happening here - and when her egg was so close to hatching - is heart-breaking."
Markenfield Hall’s moat has been patrolled by a pair of Black Swans since Lady Deirdre and the late Lord Grantley completed their phase of the Hall’s restoration in the early 1980s.
The couple were given the first pair by a friend who was the Curator of Birds at London Zoo.
The birds and their descendants have lived at the North Yorkshire hall ever since.
Now Lady Deirdre is searching for a new female mate for Sebastian before the hall opens to the public on Sunday, May 5.
She hopes the young swan will accept a new female and "live here in peace once more."
Markenfield Hall will be open to the public from Sunday, May 5 to Sunday, May 19 and again from Sunday, June 16 to Sunday, June 30.