Zebra Finches in UK chimneys in the 30’s

Hi, Did you see period drama “Upstairs, Downstairs” on the ABC last night. A bulky cup nest fell down the chimney of an English manor house in the 1930’s. It contained one egg, which hatched to produce a baby Zebra Finch, which fledged and flew away singing like a canary. I suppose it was an in-joke by the producer? Harmless fun I suppose! Richard