Tractate Chullin - Chapter 12 - Mishnah 3 חולין פרק י״ב משנה ג׳

If 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.