In the book of Deuteronomy Chapter 22 verse 6-7 it’s written:
“If a bird’s nest chance to be before thee in the way in any tree, or on the ground, whether they be young ones, or eggs, and the mother bird sitting upon the young, or upon the eggs, thou shalt not take the mother bird together with the young:
But thou shalt surely let the mother go, and take the young to thee; that it may be well with thee, and that thou mayest prolong thy days.” (King James)
This is the place to ask why should we do such a cruel deed, because its double cruel:
1) The little chicks will have no mother.
2) The mother bird will have no rest since it lost its nest, and it will fly and cry till infinity asking to kill itself to find some rest.
But the Name who has mercy for all Her creatures when She hears their singing She mercies the bird, and all wandering creatures who have to roam all over the world, because they have no home, and their suffering is included in the suffering of the mothering bird, and She feels compassion for her, and since man caused the exile of the bird, the man’s deed is the reason for Her compassion for all of creation.
Source: The book of Zohar, The ladder explanation Vol 21, Zohar chadash, Midrash Ruth, Article Nest To The Bird, Pages 26 -28.