Chullin, Chapter 12, Mishnah 5. This chapter has been devoted to the mitzvah of shiluach hakein, sending away the mother bird before taking her young. The final mishnah of the chapter, and of the entire masechet, teaches how far this prohibition reaches, and then closes with a lesson that extends far beyond birds and nests.
The mishnah rules: "Lo yitol adam", a person may not take, "em al banim", a mother bird together with the young beneath her, and it then adds a striking application: "afilu letaher et hametzora", not even when those birds are wanted for the purification of a metzora.
The birds of the metzora
Someone who has contracted tzara'at must pass through a sequence of stages in order to become pure again. Among them is a ceremony that requires two kosher birds. One is slaughtered over a vessel containing spring water. The second is dipped into that mixture of water and blood, and with it the metzora is sprinkled.
This is no small matter. The metzora has been living apart, cut off from his family and from the community in a state of impurity. This ceremony is his gateway back: back to purity, back to his home, back to the people around him. Enabling that return is a tremendous mitzvah.
And yet, even for the sake of such a mitzvah, one may not seize a mother bird sitting upon her young. The greatness of the purpose does not create a license to transgress. The halachah of shiluach hakein stands.
The kal vachomer that closes the masechet
The masechet signs off with an argument from the lesser case to the greater. The mishnah's words: "Umah im mitzvah kalah", if with regard to an easy mitzvah, "shehi ke'issar", one whose cost amounts to no more than an issar, a coin of tiny value, "amrah Torah", the Torah nevertheless promised, "lema'an yitav lach", so that it will go well with you, "veha'arachta yamim", and so that your days will be many, then surely, "kal vachomer", all the more so, "al mitzvot chamurot shebaTorah", with respect to the weighty mitzvot of the Torah.
Consider what such a mitzvah asks of a person. Freeing the mother from her nest costs virtually nothing in money and takes no more than a moment of one's time. To this trivial act the Torah nonetheless attaches an explicit pledge: goodness in one's life and length of days. Almost no expense, almost no effort, and an enormous reward is guaranteed in return.
If that is so, reason from the smaller case to the greater. How much more must blessing and reward await a person for the weightier mitzvot of the Torah, the ones that call for real exertion and real expense. Upon such a person the blessing rests on a far higher level.
Hadran alach Masechet Chullin.