In Parshas כי תצא, one of the commandments given to the Jewish People was if you want to take a baby bird or and bird’s egg, you have to send away the mother bird. As the Pasuk says:
ז שַׁלֵּ֤חַ תְּשַׁלַּח֙ אֶת־הָאֵ֔ם וְאֶת־הַבָּנִ֖ים תִּֽקַּֽח־לָ֑ךְ לְמַ֙עַן֙ יִ֣יטַב לָ֔ךְ וְהַאֲרַכְתָּ֖ יָמִֽים
7 Send away the mother, and take the young for yourself, in order that it will be good for you and prolong (your) days.
Why would we let specifically the mother bird go? The Sforno says that by sending away the mother bird from the nest, the family can keep going and not be extinct. The Torah says that one that does even a small act of kindness, will get reward in this world by eating and enjoying fruits, and in עולם הבא, the reward still remains.
This is like a Daf in Masecess Shabbos. Daf קכז, amid א. It says:
אָמַר רַב יְהוּדָה בַּר שִׁילָא א״ר אַסִי א״ר יוֹחָנָן שִׁשָּׁה דְּבָרִים אָדָם אוֹכֵל פֵּירוֹתֵיהֶן בָּעוֹלָם הַזֶּה וְהַקֶּרֶן קַיֶּימֶת לוֹ לָעוֹלָם הַבָּא וְאֵלּוּ הֵן הַכְנָסַת אוֹרְחִין וביקור חוֹלִים וְעִיּוּן תְּפִלָּה וְהַשְׁכָּמַת בֵּית הַמִּדְרָשׁ וְהַמְגַדֵּל בָּנָיו לְתַלְמוּד תּוֹרָה וְהַדָּן אֶת חֲבֵרוֹ לְכַף זְכוּת
Rav Yehudah bar Shila said in the name of Rav Assi, in the name of Rabbi Yochanan: There are six things which a person eats the fruit of in this world, and the principal remains for him for the world to come. And they are: Bringing in guests, and visiting the sick, and concentration in prayer, [Iyun means literally to investigate something deeply. “Iyun tefillah” has several different meanings, not all of which are praiseworthy.] and getting up early to attend the Beis Hamidrash, and one who raises his sons to learn Torah, and one who judges his friend with the cup of merit. [This refers to an old-fashioned scale, where objects were placed in cups on each side. This refers to one who judges others favorably.]
Let’s get back to the Pasuk. Doesn’t this seem like a big reward for a small Mitzva? Rashi says the following:
למען ייטב לך וגו'. אִם מִצְוָה קַלָּה שֶׁאֵין בָּהּ חֶסְרוֹן כִּיס אָמְרָה תוֹרָה "לְמַעַן יִיטַב לָךְ וְהַאֲרַכְתָּ יָמִים", קַל וָחֹמֶר לְמַתַּן שְׂכָרָן שֶׁל מִצְווֹת חֲמוּרוֹת:
IN ORDER IT WILL BE GOOD FOR YOU. If an easy commandment which does not entail monetary loss, the Torah said, “so that it would be good for you and prolong your days”, how much more so for the granting reward for difficult Mitzvos.
So Rashi is saying that this was a big reward and you didn’t even lose anything in the process, so imagine how much bigger the reward would be for a big Mitzva. Also if you look at maseces Chullin, it says that the reward is actually in עולם הבא.
Lastly, the Chizkuni says that the reason the Pasuk says the reward is prolong days because the reward is the same as the fulfillment of the Mitzva. Because you didn’t kill the mother or its baby birds, the family can still be alive, and you will have a longer life.