(CNN) -- A day after a jubilant first delivery, giant panda Mei Xiang gave birth to a second cub Saturday night, but it was stillborn. Washington's celebrity panda gave birth to the first cub Friday, to the excitement of hordes of fans watching it on public panda cams. The cub, about the size of a stick of butter, appeared to be doing well. So was its mom, who was cradling it in her den at the Smithsonian's National Zoo. "It is robust, healthy and fully formed," the zoo said in a Twitter post Sunday after the cub's first neonatal exam.
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NEW: First cub "is very active, very vibrant and very pink," the zoo says
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The giant panda gave birth to a second cub Saturday, but it was stillborn
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Giant pandas are one of the world's most endangered species
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The first neonatal exam reveals Mei Xiang's panda cub is healthy
@placeholder experts began watching Mei Xiang a couple of weeks ago.