http://revach.net/article.php?id=4726

Section: Taryag Mitzvos   Category: Taamei Hamitzvos
Parshas Ki Seitzei: Avnei Nezer - When The Bird Uses His Sechel

The Avnei Nezer of Sochatchov gives a unique reason for the mitzva of Shiluach HaKein. He says that all the living creatures are subservient to man because man has Sechel or intelligence while other creatures are purely physical. Man's purpose in this world is for his intelligence to conquer materialism internally within himself, and therefore gives him power of the material world around him.

The essential difference between intelligence and materialism is that intelligence allows you to think about things other than your own physical needs. When the bird sits on her egg, she does it out of pity on her child. This pity is akin to Sechel as it is an act of giving to another. While the bird is acting out in a manner akin to Sechel, man may not display superiority over it. Only when the bird takes its Sechel and flies away may you take the egg. (Iturei Torah)