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If a bird's nest chance to be before you on the way, in any tree or on the ground, chicks or eggs, and the mother sits on the chicks or on the eggs, you shall not take the mother upon the children.

Rashi

If [a bird's nest] chance to be

Except for one that’s prepared.

You shall not take the mother

(Chullin 140): While she is upon the children.

Ramban

If a bird's nest chance to be before you

This, too, is an explained mitzvah; from ...