Inside the nest, Reimu, while waiting for Marisa to return, is busy to preparing for the "rain confinement".
She dried the food Marisa gathered to preserve it. To protect the nest from incoming water she also digs a moat around their nest and creates a stair to the entrance. For Reimu, this is also her first "rain confinement".
Having being taught by the neighbouring elder yukkuri, she does her best to dig the ground.
Her children, who have grown into koyukkuri, hels her. Actually they don't do much to help, but this is part of learning.