People always fly low to catch insects before it rains. What kind of birds are called "weathermen"?

Swallows fly very low and it will rain in the sky.

Swallows feed on insects. When it's going to rain, there will be a lot of water vapor in the air and some bugs.

Zi Fei is not high, so he can only fly around near the ground. Insects are active near the ground and swallows fly low.

Fly to hunt. Due to the chaotic airflow before the rain, swallows fly high and low, erratic.

So "swallows flying low" is a sign of "it's going to rain". There are also "dragonflies in droves" among the people outside.

The same is true of the proverb "Fly low around the sky and rain for three days".