The National Zoo's panda, Mei Xiang, gave birth to a cub late Sunday night, zoo officials confirmed Monday morning after monitoring their 24-hour "panda cam."

Mei Xiang has kept her cub pretty well hidden in a nest of hay in her den, but the little tyke can be heard on the camera's audio, according to a press release. Panda cubs are born extremely small -- about the size of a stick of butter.

"I'm cautiously optimistic as we haven't seen the cub yet, but we know that Mei is a good mother," said Zoo Director Dennis Kelly in a statement. "Like everyone else, I'm glued to the panda cam for my first glimpse of the cub!"

(View a photo gallery of mother Mei Xiang and Tai Shan, the last giant panda cub born at the zoo)

The cub was born at 10:46 p.m. Sunday, according to the zoo. It marks Mei's second cub from artificial insemination and up until the birth, officials weren't even sure if she was actually pregnant as an ultrasound in late August was inconclusive. But staff prepared her den anyway as the mom-to-be was showing other signs of pregnancy, including "extended periods of time body licking and cradling toys," the zoo said.

Video footage of the birth from the zoo's Panda Cam:

Giving birth in captivity is rare -- Mei had a less than 10 percent chance of conceiving via artificial insemination and she has had five pseudopregnancies since 2007. Her only other birth was to Tai Shan in 2005.

According to Chinese tradition, the cub won't get a name until it is 100 days old. In 2005, the pandemonium -- or panda-monium? -- over the naming of Mei's last cub created its own sideshow to the country's excitement over having a cub. An internet poll was created giving voters a choice between four names, all approved by the China Wildlife Conservation Association. But there was still a grass roots movement of sorts to enter "Butterstick," the nickname given by a zoo staff member, as a contender.

Tai Shan won out with more than 202,000 votes. It means "peaceful mountain."

The zoo's giant pandas and its cubs are all property of China. In 2009, Tai Shan was returned to China.