Workers at the National Zoo were surprised when Mei Xiang, a giant panda, unexpectedly delivered a second baby about four hours after the first.

This birth is only the third time that giant panda twins have been born in the United States, and zoo staff have been working hard to take care of the babies.

It can be difficult for pandas to care for twins. To increase the likelihood of their survival, keepers have been swapping out the cubs. They take one cub and care for it, leaving the other with Mei Xiang. The cubs can be swapped every few hours, though it depends on the cooperation of the mother. As zoo director, Dennis Kelly said Sunday "these are her cubs and she's not too keen on us taking them from her."

Both cubs seem to be in good health. Both squealed loudly, which is a good sign. Copper Aitken-Palmer, chief veterinarian at the Smithsonian's Conservation Biology Institute, examined the baby pandas. She contacted experts in China for help. There are a greater number of pandas in captivity in China, and experts there have more experience with cubs. The cubs' eyes have not opened yet, and veterinarians could not determine their sex.

Mei Xiang has previously delivered two surviving cubs since 2005. One, Bao Bao, still lives at the National Zoo and recently celebrated her second birthday. The other, Tai Shan, lives in China. He was born in 2005. Mei Xiang also gave birth to a stillborn cub about 26 hours after the birth of Bao Bao. In 2012, she gave birth to a cub that survived less than a week due to liver difficulties.

Twin births are not especially rare among pandas, but this is only the third set of twins born in the United States. In one of the other cases, the twins did not survive, so zookeepers are working closely with experts to care for these newborn pandas.

Though the cubs will likely not be introduced to the public for months, the zoo does have Panda Cams available online.