Hullin, Chapter Twelve, Mishnah Three
1) If the mother was hovering [over the nest]:
a) If her wings touch the nest, one is obligated to let her go;
b) If her wings do not touch the nest, one is not obligated to let her go.
2) 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.
3) 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:
a) Just as the young are living beings so the eggs must be such as [would produce] living beings; this excludes spoiled eggs.
b) 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.
4) 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.
5) 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.
6) 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.
Section one: The mother is considered to be sitting upon the nest only so long as at least her wings are touching the nest. If she is just hovering over the nest, and her wings are not touching, one is not obligated to send her away.
Section two: Although Deuteronomy 22:6 uses the plural form of eggs and fledglings, the obligation is still in place even if there is only one egg or one fledgling. This is because the Torah also uses the word nest which implies any nest, so long as there is at least one egg or one fledgling.
Section three: The prohibition applies only to fledglings that cannot fly or to viable eggs. It does not apply to a case where the young birds can already fly or to a case where the eggs are spoiled. This is derived through a midrash which compares the fledglings with the eggs. Just as the fledglings have proven themselves to be viable birds, so too the eggs must show signs of being viable. Spoiled eggs are not covered by the prohibition. And just as the eggs still require the attention of their mother, so too the fledglings must require the attention of their mother. A fledgling which can feed itself and fly, is no longer covered by the prohibition.
Section four: Even if the mother bird keeps returning to the nest, the person who finds her there must send her away before taking the young or the eggs. This is derived from the double appearance of the word shalah in the Torah, which I have translated as surely let the mother go. Although this is a common grammatical construct, the rabbis frequently use it as an opportunity for midrash.
Section five: One cannot fulfill the obligation by letting the young go, and taking the mother. Rather, the obligation is to send the mother away and then take the young.
Section six: If one let the mother go and took the young, he has now acquired the young birds and eggs. If he then puts them back in the nest and the mother comes and takes them, he is exempt from sending her away again. This is because the eggs are already his.