(The 'bird wave' at the estuary of Yalu River in the harbor city of Dandong, northeast China's Liaoning Province.)

It is one of the most wonderful views on Earth: Countless birds gather and fly in unison. From afar, they look like a wave in the sky.

The "bird wave" is a flock in which different species of birds join and move together while foraging. The stunning phenomenon is more common in the tropical region of the Americas, while in China, it can be spotted at intertidal zones and wetlands during migration season. 

Dandong City in northeast China's Liaoning Province is a stopping point on the "bird highway" –  the East Asian-Australasian Flyway – where many shorebirds traveling from Australia and New Zealand take a rest before heading to their breeding grounds in Siberia and Alaska.

From late March to April, bird lovers also flock to Dandong to film these migratory birds, and the estuary of Yalu River is the most-welcomed spot as they can observe the magnificent view of "bird wave."

Unlike some insectivorous birds who form groups in sky when foraging for food, these shorebirds, or waders, forage on the mudflat. When the tides surge, thousands of birds lift off and form a large "bird wave."

(Check the video and see the waders flying up from the mudflat.)

But the tide is not the only reason that creates the impressive sight, and the mechanism of the bird wave is complex, almost a mystery.

One common theory is that a mixed-species foraging flock can improve the feeding efficiency of birds and protect them from other predators. 

Scientific researches found that a "bird wave" has a flock-leader species that takes a vital role in the flock. The core species usually stimulate the formation and maintain the cohesion of the flock. The attendant species usually join the flock after its formation. They like to follow the crowd and have a higher fitness.

If you want to know more about the mixed-species flock, tune in to our livestream from 10:45 a.m. to 11:45 a.m. BJT on April 11, 2023. Let's explore the mystery of the "bird wave" at the estuary of Yalu River! 

(All images and scripts contributed by photographer and bird lover Sun Xiaohong)

(If you want to contribute and have specific expertise, please contact us at nature@cgtn.com.)