Another interpretation, “you shall surely send away” – Rabbi Berekhya said: There is a certain demon that shoots forth like an arrow and flies like a bird. From where is this derived? It is as it is stated: “You will not fear the terror of night, nor the arrow that flies by day” (Psalms 91:5). The Holy One blessed be He was saying [here]: ‘If you fulfilled the mitzva of dispatching the mother bird, I will deliver you from those [kinds of demon].’
Another interpretation, there are mitzvot for which the reward granted is wealth, and there are mitzvot for which the reward granted is honor. What is the reward granted for this mitzva? It is that if you do not have children, I will give you children. From where is this derived?
It is as it is stated: “You shall surely send away the mother.” What is the reward that you receive? “And take the offspring for yourself” (Deuteronomy 22:7).