Excuse me, what is the "wild goose" in Rain Wild Goose River?

This is one of the two sentences in the solar term song of the 24th lunar month in China. The full text is as follows:

In early spring, the sun is turning, and it is raining beside the Yanjiang River.

Scared crows crow, the spring equinox dries up,

Qingming busy planting wheat, Grain Rain farming;

Long summer goose feathers live, the birds are coming,

Mang seeds were shoveled away, but cotton was not harvested from summer to Sunday.

It is not hot in the summer, but hot in the dog days;

Beginning of autumn is busy playing indigo, and busy sharpening knives and sickles in summer;

In the Millennium, there is no land in the autumn equinox.

The frost has changed before the cold dew is cold;

Beginning of winter closed the land, Xiaoxue River was closed,

The snowy river is closed, so ships can't sail on the solstice in winter.

A slight cold is not too cold, and a bad cold lasts for three or nine days.

The "wild goose" of "the sun turns in beginning of spring and it rains by the river" refers to "Hongyan", also known as "Dayan". The solar terms of "rain" in ancient China are divided into three periods: "The first period is for Rex to sacrifice fish; Second, wait for Hongyan to come; Third, wait for the grass to sprout. " In this solar term, otters start fishing and put the fish on the shore, as if they were sacrificed first and then eaten; Five days later, the geese began to fly back to the north from the south; In five days, in the "moist and silent" spring rain, the spirit of the sun rises everywhere in the vegetation and begins to sprout. Since then, the earth has gradually begun to show a thriving scene.