Our Gemara on Amud Beis teaches that unfertilized eggs do not incur the obligation of sending the mother bird away. Ohr Sameach (Laws of Shechitah 13:7) remarks that the Rambam in the Moreh Nevuchim (III:48) explains the reason for the mitzvah of shiluach haken is that the mother bird experiences the same anguish as a human mother at the loss of her children; in the emotional realm, humans and animals are the same (see the section in the Moreh on shiluach haken and the preceding section on mother and child animals and prohibition to slaughter them on the same day.) Therefore, the Ohr Sameach explains that it makes sense that there is no mitzvah by unfertilized eggs, since the mother’s instinctive attachment is not activated over dead eggs.

It is noteworthy that the Rambam equates the subjective experience of the human mother to that of the animal mother. If that is so, one may wonder: how does it help to send away the mother bird? After all, I don’t think it would help a human mother to send her away while taking her offspring, God forbid. A simple answer is that just as we try to minimize animal pain when we slaughter them, but ultimately this is their purpose and human need trumps their need, here too the idea might be to minimize the suffering, or at least symbolically minimize it. I am not satisfied with this answer, because given the strong statement that the Rambam made equating animal suffering to human suffering, it just does not make sense. Slaughtering is different because the death indeed may be instantaneous and painless. Here, the mother bird is going to be without her chicks after all. I believe a better answer is to keep in mind that though there is no difference between animal emotion and human emotion according to the Rambam, he is not equating intellect. It may very well be that the short-term episodic memory and capacity of a bird may be so limited that once the chicks are not in front of her, she doesn’t remember them or know about them.