VAYIKRA
Toras Kohanim is quite lofty, speaking to the echelon of Jewish society about the holy laws of the mikdash, purity and korbanos. It is interesting to note that the first lesson in this sefer is about the small Aleph, and the humble Derech Eretz of Moshe, not entering without invitation.
The Medrash relates that a Talmid Chacham who does not have derech eretz, נבלה טובה ממנו - the carcass of a dead animal is better than him. What an astounding comparison!
One of striking halachos in this parsha of korbanos is מליקת העוף - the unique manner in which a bird was sacrificed in the mikdash. The Kohen would use his sharpened, lengthy thumb nail to cut into the back of the bird's neck severing its artery and trachea. In a similar vein and related to Pesach, in the parsha of והיה כי יביאך it describes the mitzvah of kedushas b'chorim and the firstborn donkey is redeemed with a lamb or its head gets axed off from the back of its neck. On this statement the 3rd son, the Tam asks like we are asking; מה זאת???
Perhaps we can answer that generally, an animal is regarded as a base physical entity that is blended with its higher purpose as it functions as a tool for human productivity. Properly slaughtering a kosher animal elevates it as its blended function now serves an even higher purpose.
But there are some cases of extremities - a bird is a bit detached from our world. It belongs somewhat to the world of Ruach, to the heavens. A chamor, as is well known is considered the most physical, material of all domestic animals and it represents Chomer - base physical matter. Both of these creatures do not get a normal "balanced" slaughtering but a "detachment" of their two extremes - their head from their body.
A Talmid Chacham that lacks derech eretz is more detached from this blended harmonious existence than a dead carcass that still functions with some purpose for its hide.
The preparations for Pesach are a true opportunity to test this blend as we get involved with such base activities as cleaning and looking in the חורין וסידקין - dark dirty places for a higher purpose l'shem mitzvah.