Ki Teitzei

IF A BIRD'S NEST HAPPENS BEFORE YOU ON THE WAY, ON ANY TREE OR ON THE GROUND, YOUNG BIRDS OR EGGS, AND THE MOTHER BIRD IS CROUCHING ON THE YOUNG BIRDS OR EGGS, DO NOT TAKE THE MOTHER ON THE YOUNG (Devarim 22:6)

Is there a reason for this mitzva or is this simply a Divine decree?

Ramban says there is a reason.  However he addressed two sources that seem to contradict this.

One source is a Mishna (Brachos 33a) that someone who prays for mercy "as Your mercy extends to a bird's nest" should be silenced ...[for implying the mitzva of sending away the mother bird is because of G-d's mercy on the bird].

This Mishna seems to suggest the mitzva has no ethical purpose, but this is not so.  Ramban explains the mitzva of sending away the mother bird is not because of G-d's mercy.  But we are commanded this precept so that   we develop the trait of mercy.  The mitzva is for us, not for the birds.

A second source is Bereshis Rabbah 44:1 which reads "What difference does it make to the Holy One whether an animal is slaughtered from the front of the neck or the back?  Surely you must say the commandments have been given only to refine men."

This may sound like G-d's decrees are arbitrary with no other purpose than to compel our loyalty to Him.  That is not what Bereshis Rabbah means.   The details of the commandments do not affect G-d.  They do affect human beings.  The commandments are for our sake, to improve us.

None of this means that G-d is unconcerned with birds!   But G-d's own mercy is not the reason for the mitzva.  After all, G-d can take care of the bird Himself.  The reason for the mitzva is to develop mercy in us.