Positive commandment · Mitzvat Aseh #148 of 613
To send away [the mother bird] from the nest [when taking the young]
לשלח הקן
The verse · Devarim 22:7
שַׁלֵּחַ תְּשַׁלַּח אֶת־הָאֵם וְאֶת־הַבָּנִים תִּקַּח־לָךְ לְמַעַן יִיטַב לָךְ וְהַאֲרַכְתָּ יָמִים
“but thou shalt surely let the mother go, and take the young to thee; that it may be well with thee, and that thou mayst prolong thy days.”
The Rambam’s words
To send away [the mother bird] from the nest [when taking the young], as [Deuteronomy 22:7] states: "You shall surely send away the mother."
לשלח הקן, שנאמר: "שלח תשלח את האם ואת הבנים תיקח לך" (דברים כב,ז).
Why · Sefer HaChinuch #545
To send away the mother bird if he takes it upon the young
“It is from the roots of this commandment [that it is] to put into our hearts that the providence of G-d, may He be blessed, is upon all of His creatures — with the human species individually, as it is written (Job 34:21), “For His eyes are upon a man’s ways, etc.”; and upon the other species of animals generally, meaning to say that His desire, may He be blessed, is towards the existence of the [particular] species.”
Who & when
“And this commandment is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Ritual Slaughter הלכות שחיטה