If we should come across the nest of a kosher bird and wish to take the eggs or the chicks for ourselves, we are not permitted to take the mother as well.
This mitzvah is what is called a “lav shenitak la’aseh,” a prohibition that, if violated, triggers an obligation. Specifically, if a person did take the mother with the young, he becomes obligated to send her away, as we shall see in the next mitzvah. (See Talmud Makkos 15a-b for a full discussion of a “lav shenitak la’aseh.”) When he sends the mother bird away, it’s a wash. However, if the mother bird dies or if another person sends it away after the first person has taken it, then the first person cannot rectify his violation of this mitzvah.
The reason for this mitzvah? Oh, there’s a loaded question! Short answer, let us compare it to the prohibition against slaughtering a cow and its calf in the same day, which we said in Mitzvah #294 teaches us not to destroy God’s species with reckless abandon. We will, however, discuss this matter in much greater detail in the very next mitzvah.
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractate of Chullin starting on page 138b. It is codified in the Mishneh Torah in the thirteenth chapter of Hilchos Shechitah. This mitzvah is #306 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #189 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.