Caroline Winslow still can recall some details of the day 50 years ago when she first saw pandas Ling Ling and Hsing Hsing — sent as messengers of friendship from China after US President Richard Nixon's historic visit.

"I remember … one of the things about when Ling Ling and Hsing Hsing came was hearing vocalization for the first time because they have such an unusual … it doesn't sound like a bear. They have some wonderful vocalizations. I remember being really excited about that," Winslow said.

She told China Daily that it was a "wonderful, magical" feeling and had no idea that she would spend 50 years being involved with pandas.

When Ling Ling and Hsing Hsing arrived, Winslow was an educational volunteer at Smithsonian's National Zoo & Conservation Biology Institute. Before their arrival, the zoo called for volunteers to observe them, collect data about how they behaved, and study their needs. Winslow "volunteered to do that, sitting and writing down everything that they did".

"They (the pandas) were mysterious but adorable animals … people were really excited. Nobody had seen giant pandas except in the photograph," said Winslow.

For decades, Winslow continued the observation and data collection, first as a volunteer and then as a staff with the Friends of the National Zoo program. She retired one and a half years ago but still volunteers in the zoo.

"In those days, we had no cameras. Everything was handwritten," she recalled. "I remember sitting in the dark at night in the panda house with a red light in the panda enclosure, trying to figure out what they were doing," said Winslow.

"And then, over the years, the program grew bigger and bigger. People are more and more interested in pandas and what we were learning about, and we got the cam system installed, and you know everything has just multiplied and gotten better."

Winslow commended the panda cooperation program between the US and China. The China Wildlife Conservation Society and the National Zoo signed the giant panda research and breeding agreement in 2000. Under the agreement, the current panda couple, Mei Xiang and Tian Tian, arrived at the zoo the same year.

"When it first (the cooperation program on panda) happened, it was groundbreaking, and nothing like that had ever happened before. And what I see are the results: After 50 years, the amount of research that has been done, the amount of papers that have been written, and the amount of knowledge on both sides and sharing and cooperating to create that, I think it is fabulous," Winslow said.

"And not only that, as they said, the cubs born in the US are now having babies in China. It is the result of working together. It is just extraordinary to me," she said. "The collaboration has worked and everybody has benefited, particularly the giant pandas."

Among Mei Xiang and Tian Tian's cubs — Tai Shan born in 2005; Bao Bao born in 2013; and Bei Bei born in 2015 — the three of them have since returned to China and are participating in scientific research on giant panda breeding based on the US-China agreement.