שלח תשלח את האם ואת הבנים תקח לך, "you must be sure to send away the mother and take the young for yourself.⁠" According to the plain meaning of the text the Torah's directive here is designed to protect the species from extermination. Even though the Torah permitted slaughtering of living creatures for the enhancement of man's lifestyle, the Torah did not mean to give a carte blanche for exterminating any species thereby. If one were to take both mother bird together with her young (and eat them) it would be symbolically similar to exterminating that particular species (Nachmanides). The Torah adds the words: "in order that that it will be good for you,⁠" i.e. if you will display concern for the survival of the species of these birds, God in turn will display His pity for you and will increase your days on earth as compensation for not having wiped out that family (species) of birds (Ibn Ezra).
When the Torah chose the expression והאם רובצת על האפרוחים או על הביצים, "and the mother is roosting (reclining) on the chicks or the eggs,⁠" (verse 6) the Torah revealed to us by that that the "eggs" mentioned here are equivalent to the "young ones" mentioned in verse 7 seeing that eventually the young chicks will emerge from these eggs. This also teaches that the command to dispatch the mother bird is applicable only when the eggs have advanced to the stage where it is practically certain that healthy young chicks will emerge from them (Chulin 109). If the eggs are broken or otherwise show signs that they will not be hatched, there is no need to send away the mother bird in order to take the eggs. The reason the Torah mentions both the chicks and the eggs in the plural is because this is the normal scenario. Even if the mother bird is brooding over only a single egg or chick the same legislation applies (Chulin 109).

Maimonides, in his More Nevuchim 3,48, offers the following rationale for this commandment: The Torah does not want the mother bird to experience the anguish of watching to see herself being deprived of her chicks. The situation is analogous to the prohibition of not slaughtering the calf and its mother on the same day (Leviticus 22,25), so that the cow does not watch the death of its calf. The love of a mother animal for its young is not determined by intellectually rationalised feelings but by something more profound in the psyche of the animal. It reflects its power of imagination. If the Torah is so concerned with the feelings of birds and four-legged domesticated animals, how much more is it concerned with the feelings of human beings! The legislation therefore is designed to teach us not to practise cruelty even when such cruelty is only of an emotional nature.

A Midrashic approach based on Tanchuma Ki Teytze 2: There is no commandment which is easier to fulfill than sending away the mother bird. Compare the reward offered by the Torah for a commandment which requires so little effort to fulfill: "in order that it will be good for you and will prolong your days.⁠" We may appreciate this better by reference to a parable. A king employed a number of labourers to work in his field to plant there and he did not tell them exactly what their wages would be. In the evening, every labourer who had planted a tree was given one gold coin. When the labourers observed this (extraordinarily large payment) they were amazed and concluded that if planting a single tree, an undemanding task, was worth a gold coin to the king, how much greater would be the payment for labourers who planted a number of trees per day. The commandment of שלוח הקן, the sending off of the mother bird, is similar. It does not require financial cost in order for us to fulfill it and yet the reward is great. We may extrapolate from that what kind of reward God will grant us for the fulfillment of commandments which require the spending of a lot of money, physical exertion, and possibly involve saving someone's life. The reason that God spelled out the reward for only a few of the commandments is to encourage us to fulfill all of them. We may compare the commandments to trees producing fruit. Some tree may produce a fruit which is superior to the fruit of another tree. Similarly, there are commandments the reward for which is greater than the reward for fulfilling some other commandment.
We know, for instance that the reward for being hospitable and providing food and shelter for strangers is having children. We have proof of this from our matriarch Sarah (Genesis 18, 6-10) and we also know this from the woman in Shunam who was blessed with a child by a promise from the prophet Elisha whom she hosted on a regular basis (Kings II, 4-16). Other commandments such as honoring father and mother are rewarded by long life (Exodus 20,12), as is the commandment to send away the mother bird even though the principal meaning of this long life is understood to refer to life in a world beyond this terrestrial life on earth,
[In the following list of verses from scripture the author illustrates allegorically the eventual fate of the Gentile nations as retribution for their not having observed this commandment in their relationship to the Jewish people. Ed.]. Isaiah 16,2 writes: "like fugitive birds, like nestlings driven away, Moav's villagers linger by the fords of Arnon.⁠" The prophet refers to Jeremiah 17,11: "like a partridge hatching what she did not lay, so is one who amasses wealth by unjust means, in the middle of his life it will leave him, and in the end he will be proved a fool.⁠" Immediately after this Jeremiah continues: "O throne of Glory exalted from of old. Our sacred shrine! O Hope of Israel, O Lord!⁠" How do these verses form a continuation from what the prophet wrote before? The partridge brings eggs belonging to other species of birds and broods on them until they hatch from their shells. When these chicks hatch, the chicks feed on the partridge plucking its feathers and making it defenseless until they eventually devour it. Who is the cause of the partridge's misfortune? None other than the partridge itself. If it had not stolen other birds' eggs this would never have happened to it. This is what is going to happen to the nations of the world who dared stretch out their talons against the throne of God, i.e. destroy our Temple. They destroyed and burned God's Sanctuary, exiled the Jewish people and, as a result, in the words of Tzefaniah 2,9, "Assuredly, as I live, declares the Lord of Hosts, the God of Israel: 'Moav shall become like Sodom, and the Ammonites like Gomorrah; clumps of weeds and patches of salt and desolation evermore. The remnants of My people shall plunder them, the remainder of My nation shall possess them.'" Thus far the Midrash.

We find that Solomon drew a comparison between two commandments, treating them as one. This is what he meant when he said in Proverbs 30,17: "the eye that mocks a father and disdains homage due a mother, the ravens of the brook will gouge it out, young eagles will devour it.⁠" Concerning the above verse the Jerusalem Talmud Peah, chapter 1 declares that the eye mocking the father is a reference to the commandment to honour father and mother. The words "and disdains the homage due a mother" are a reference to the commandment of dispatching the mother bird prior to taking her young. Whereas in the Torah we read about the reward for observing these commandments, Solomon informs us about the punishment in store for violating it.

An investigative/scientific approach: the reason behind the commandment to dispatch the mother bird is to teach us empathy and to encourage us to turn our backs on all forms of cruelty. The latter is a negative virtue. The commandment is parallel to that of not slaughtering the calf on the same day as its mother. It is in line with what we learned in the oral Torah that the reason ritual slaughter of animals has to be performed by cutting the animal's throat is because this is the least painful method of killing the beast. Killing the animal by hitting it on the neck (such as the עגלה ערופה) is much more painful. The foregoing is the opinion of Maimonides in his Moreh Nevuchim, and I have already mentioned part of it earlier.

A Kabbalistic approach: the words שלח תשלח את האם are a reference to the emanation בינה, the source from which the lower seven emanations derive their spiritual input in every seven year (שמטה) cycle to renew the face of the earth. The earth is the קן העולם, the nest of the world. It is known as אם העולם, "mother of the nest (world),⁠" like the "mother bird,⁠" seeing it hatches all creatures.
You will find the following comment in the Sefer Habahir item 104: Rabbi Rechumai said the reason that the Torah commands שלח תשלח את האם, "be sure to dispatch the mother,⁠" whereas it does not say that one must dispatch the father bird, is to render recognition and give honor to the "mother" of the world seeing that the emanation בינה is perceived as the emanation everyone calls out for as we know from Proverbs 2,3 כי אם לבינה תקרא, "for you call out for insight.⁠" [This may also be an allusion to Proverbs 1,8: ואל תטוש תורת אמך "do not abandon the teachings of your mother.⁠" Ed.]
The emanation בינה as the emanation immediately "above" the lower seven emanations each of which symbolizes one of the seven days of creation, is perceived as the mother, i.e. the one giving birth to these other seven emanations.
Concerning the words ואת הבנים תקח לך the same Rabbi Rechumai writes that it refers to the בנים she has raised. This is a reference to the seven days of the week and the seven days of Sukkot. Thus far Sefer Habahir.
The dispatch of the mother bird legislated here applies only to pure birds, i.e. the ones permitted for consumption by Jews. This is a compliment and a form of religious service for these birds as they are considered above and beyond the domain of the seven days of creation, a domain which has not been completed (limited) by a number being given to it. [The reference appears to be to the emanation בינה, which is "higher" than the seven emanations below it. Ed.] A parallel to this is found in Psalms 147,8 ולתבונתו אין מספרת, "its insight has not been assigned a number such as have the days (which would limit it). Not only the emanation בינה but also the tenth emanation מלכות, (from כתר down) is known as אם as we know from the fifth commandment כבד את אביך ואת אמך, a commandment addressed to people in that sphere. [I assume that the word ואת in that commandment provides the linkage between בינה and מלכות. Ed.]
Our sages in Berachot 35 citing Proverbs 28,24 where Solomon lambasts those who rob father and mother, speak about man robbing God (in connection with enjoying food etc., without first reciting a benediction). The reason the simile is extended to both father and mother [who cannot both represent God. Ed.] is that "father" refers to Hashem, whereas "mother" refers to כנסת ישראל, the spiritual concept of the people of Israel.
The Torah promises long life as the reward for fulfilling both the commandment of honouring father and mother and the commandment of dispatching the mother bird. The only difference in the wording of the reward is that in connection with honouring father and mother the "long life" is promised על האדמה, an expression repeated in connection with having true weights (Deut. 25,15). In connection with our commandment the words על האדמה "on the earth" are missing, suggesting that the Torah speaks of extra terrestrial life in the hereafter, or as the sages put it "in a world which is all long life.⁠"
The meaning of the words והארכת ימים is a reference to the תאריך אף מן הימים, "you will also enjoy other days,⁠" i.e. after the world limited to ששת ימים, the six days of creation has come to an end.

There is also a different approach concerning this commandment, i.e. that the reward promised extends to the entire world seeing that the very chasing away of the mother bird awakens feelings of concern, worry and pity in her seeing she suspects something will happen to her chicks. It is these feelings and the appeal to God which they engender that triggers God's largesse for His creatures on earth. Seeing that the mother bird is trying to take her own life due to her anguish, the angel in charge of all the birds is aroused asking God's mercy for her. At that point, Hashem, Who is known as רחמיו על כל מעשיו "extending His mercy to all His works,⁠" (Psalms 145,9) is activated to extend this mercy in increasing amounts to all creatures experiencing anguish. Looked at from this vantage point, you may conclude that the effects of fulfilling this commandment are indeed very far ranging, so much so that it triggers mercy for all the inhabitants of this world. This is the extended meaning of the words למען ייטב לך והארכת ימים, i.e. the dispatch of the mother bird will be good for you in that it results in your life span being extended. An extension of one's life span is always due to the attribute of Mercy having been invoked.

I have found the following in Devarim Rabbah 6,3: "the reward for fulfilling the commandment of dispatching the mother bird are children (being granted them).⁠" God formulated this reward as follows: 'if you presently do not have children, I will grant you children.' How is all this contained in the wording of the text? The Torah writes שלח תשלח את האם ואת הבנים תקח לך, "if you will surely dispatch the mother you will be able to acquire the children for yourself.⁠" This Midrash alludes to the mystical aspect of this commandment I already discussed.
This also clarifies that the emanation (attribute) בינה is the one responsible for the granting or denying of children. This is the meaning of the peculiar expression ותתפלל על ה' reported in Samuel I 1,10 where Channah is described as praying for children. We would have expected the text to say ותתפלל חנה אל ה', that Channah prayed "to the Lord,⁠" not על ה'. The expression על ה' suggests a domain above that of the seven lower emanations, a domain of a world which is totally good, totally enduring, everlasting. This is implied in the words והתענג על ה', (Psalms 37,4) where the psalmist says: "seek favour of the Lord and He will grant you the desires of your heart.⁠" If one penetrates to that emanation, to that attribute of God, one will have one's wish for children granted.
Seeing that this is so the legislation before us concludes with the words למען ייטב לך והארכת ימים, "so that it will be good for you and you will enjoy long life.⁠" We have learned in Niddah 70 "what is a person to do in order to be endowed with wisdom?⁠" Let him ask for mercy from the One Who owns wisdom, for it is written וה' יתן חכמה, "the Lord will grant wisdom" (Proverbs 2,6). The Talmud there continues: "what is someone to do who yearns to have children?⁠" "Let him ask for mercy from the One to whom the children belong.⁠" as we know from Psalms 127,3: "sons are the provisions of the Lord, the fruit of the womb, His reward.⁠"
It is worthwhile to analyse in this saying how the sages arranged the requirements (objects of prayer) in the descending order of the emanations. [Wisdom is stored in the higher emanation חכמה, followed by the next lower emanation בינה, which is the source of children. Ed].
This commandment belongs to the category we described earlier as מקובלות, [the author had divided the commandments into three groups, מושכלות, מקובלות,חוקים, compare beginning of commentary on Parshat Chukat Ed.] Normally, the sages instituted a benediction to be recited prior to the performance of any of the commandments which the author described as מקובלות. We do not know of any such benediction before the performance of this commandment. Why is this so? The reason is that it is not one of the commandments man should strive to fulfill, seek out opportunities to observe. If the occasion arises, the commandment needs to be fulfilled. This is why the whole paragraph is introduced with the words כי יקרה, "if it so happens.⁠" Even if the opportunity arises, this commandment applies only if the finder wants to take the eggs or chicks for himself.