Is the mitzva of shiluach haken only if you need the eggs?
There is a responsa of the Ben Ish Hai brought in the fourth volume of the set published by Ahavat Shalom, that says it must be for the sake of taking the eggs(or young) on account of Tzarat Ba'alei Haim, as the mother may not return and you would thus be the cause of the starvation of the young.
This is a big dispute between halachists who are more rationalist vs. those who are more mystically oriented.
According to 'rationalists', who view it as an act of mercy, if you don't need the eggs / chicks, then it is an act of cruelty, for no reason, so of course you should not chase away the mother bird. Ones who hold like this include Rokeach, Meiri, Rabbenu Bachya, Ran, Rambam, and Tosafot. There are also Acharonim who maintain this.
According to 'mystics', who view it as a deliberate act of cruelty to instigate Divine mercy, then of course one should chase away the mother bird, even if one does not need the eggs. Ones who hold like this follow the rationale for the mitzvah stated in the Zohar, and include Chavot Yair, R' David Luria, R' Chaim Vital, and the Aruch Hashulchan.
These are not complete lists.
But it would seem to indicate a shift, post the promulgation of the Zohar, in how to view the nature of the mitzvah and consequently how one should conduct oneself in the most typical situation, where one does not need the eggs.)
For more information on this, read the essay by Rabbi Natan Slifkin on Shiluach HaKen, the transformation of a mitzvah.