Giant panda Bei Bei rests at the Smithsonian's National Zoo in Washington.

He was born in Washington and took his first steps in the US capital, but at age four, it's time to head "home" to China.

Bei Bei, the last young panda living at the National Zoo, will leave Washington on Tuesday aboard a specially outfitted FedEx plane, as planned even before he took his first breath.

Such are the strict rules of "panda diplomacy."

In front of the zoo's panda enclosure on a chilly day, schoolchildren in winter coats and hats line up to say farewell, many of them with signs reading "Bye bye Bei Bei."

The National Zoo a branch of the venerable Smithsonian, the world's largest museum and research complex -- has organized a week of events to celebrate the departure of Bei Bei, which means "precious, treasure" in Mandarin.

Kids can write postcards for the panda, which will go with him to China, watch zookeepers offer him special treats, attend Q&A sessions with zoo personnel or just watch him play.

Through it all, Bei Bei.... munches on bamboo.

Giant panda Bei Bei eats bamboo at the Smithsonian's National Zoo in Washington.

Marty Dearie is one of Bei Bei's keepers. He saw the male cub born in August 2015 on the "panda cam" which is trained on the zoo's pandas around the clock. Bei Bei, who now weighs a healthy 250 pounds (114 kilos), lives with his parents, dad Tian Tian and mom Mei Xiang.

"Our job has been to get him prepared for the trip to China from the moment he was born," Dearie told AFP. "We knew that was coming it's part of the loan agreement."

China's "panda diplomacy" has evolved over the years.

At one point in time, Beijing gave pandas to friendly nations, but now that the animals are considered "vulnerable" to extinction by the International Union for Conservation of Nature, China lends pandas to zoos around the world.

Once a panda reaches the age of four, he or she is repatriated to breed with other animals at sanctuaries in China.

Bei Bei's siblings, brother Tai Shan and sister Bao Bao, have already been returned to China.

Tian Tian and Mei Xiang will remain at the National Zoo until at least December 2020, when a new deal would need to be negotiated.

Honey and sugarcane

Moving a giant panda to another continent is no easy task.

For several weeks, Bei Bei's keepers have been training him to sit in the crate in which he will travel to China on the nonstop 16-hour flight from Washington to Chengdu.

"At this point, we were able to close both doors and he sits in there comfortably," says Dearie.

Giant panda Bei Bei sits in a tub eating bamboo at the Smithsonian's National Zoo in Washington.

A keeper and a veterinarian will make the trip with Bei Bei. The National Zoo staff will prepare a special stash of treats for him -- like most giant pandas, he eats constantly.

"We'll probably be taking 15 or 20 kilos of bamboo with him at least. He'll also get apples, sweet potatoes, carrots. We'll probably take some sugarcane. There might be honey, biscuits," Dearie explains.

FedEx is paying for the flight and the special outfitting of the Boeing 777 -- called the "Panda Express" for the occasion.

"Bei Bei and his 'luggage' will be the only shipment on the aircraft," said FedEx spokeswoman Rae Lyn Rushing.

Both the public and his keepers have been enthralled with Bei Bei from the get-go: from his first steps, to an operation in 2016 to remove a lemon-sized chewed-up mass of bamboo from his intestines, to his upcoming departure.

Bei Bei's name was revealed at a ceremony in 2015 presided over by then US first lady Michelle Obama and Peng Liyuan, the wife of current Chinese President Xi Jinping.

Agence France-Presse