What does it mean that the day is high and the night is cold, and the birds are idle and warm?

The day is high and the night is cool, and the birds are leisurely and warm. These two sentences mean to write the autumnal equinox season, and to divide day and night at the same time. The time before the autumnal equinox is long and the night is short. After the autumnal equinox, the weather in the north is getting colder and colder, and geese will fly south to warm places.