Mishnah Chullin 12

¹The law of letting [the mother bird] go from the nest is in force both within the holy land and outside it, both during the existence of the Temple and after it, in respect of unconsecrated birds but not consecrated birds. The law of covering up the blood is of broader application than the law of letting [the mother bird] go; for the law of covering up the blood applies to wild animals as well as to birds, whether they are at one's disposal or not, whereas the law of letting [the mother bird] go from the nest applies only to birds and only to those which are not at one's disposal. Which are they that are not at one's disposal? Such as geese and fowls that made their nests in the open field. But if they made their nests within a house or in the case of Herodian doves, one is not bound to let [the mother bird] go. ²An unclean bird one is not obligated to let it go. If an unclean bird was sitting on the eggs of a clean bird, or a clean bird on the eggs of an unclean bird, one is not obligated to let it go. As to a male partridge: Rabbi Eliezer obligates [one to let it go]. But the sages exempt. ³If the mother was hovering [over the nest]: If her wings touch the nest, one is obligated to let her go; if her wings do not touch the nest, one is not obligated to let her go. If there was but one young bird or one egg [in the nest], one is still obligated to let the mother go, for it is written: “A nest,” [implying], any nest whatsoever. If there were there young birds able to fly or spoiled eggs, one is not obligated to let [the mother] go, for it is written, “And the mother sitting up on the young or upon the eggs:” Just as the young are living beings so the eggs must be such as [would produce] living beings; this excludes spoiled eggs. And just as the eggs need the care of the mother so the young must be such as need the care of the mother; this excludes those that are able to fly. If one let [the mother] go and she returned, even four of five times, he is still obligated [to let her go again], for it is written, “You shall surely let the mother go.” If one said, “I will take the mother and let the young go,” he is still obligated [to let her go], for it is written, “You shall surely let the mother go.” If one took the young and brought them back again to the nest, and afterwards the mother returned to them, he is not obligated to let her go. ⁴If one took the mother with the young: Rabbi Judah says: he has incurred [forty] lashes and he need not now let her go. But the sages say: he must let her go, and he does not incur lashes. This is the general rule: [For the transgression of] any negative commandment which has of a remedy by the subsequent fulfillment of a positive commandment one does not incur lashes. ⁵One may not take the mother with the young even for the sake of purifying the metzora. If in respect of so light a commandment, which deals with that which is but worth an issar, the Torah said, “In order that you may fare well and have a long life”, how much more [must be the reward] for the observance of the more difficult commandments in the Torah!