The Mishna teaches that the simanim of all kosher animals are stated in the Torah; they have split hooves and chew their cud. However, the simanim of kosher birds are not stated. Rather the Chachamim say there are 24 birds that are prohibited to eat because they are dories. Rashi explains this means that they pick up their food from the floor with their fingers and eat it, and they don’t peck from the ground. The Rabeinu Tam and the Ramban explain that they trap and pounce on their prey and eat it while it’s alive, not waiting until it dies.
The Ramban explains that the siman of drisah by birds is the greatest siman; any bird who is dores is always assur. The Torah assurs them since their blood is heated up because of its cruelness thereby bringing forth bitterness and poison. Therefore when a person eats a non-kosher animal, cruelness will be transferred to the person.
So too, explains the Ramban, is with animals. None of the kosher animals trap and pounce on their prey, to eat it while it is alive. This is only done by the non-kosher animals which the Torah prohibits since they too bring forth a cruel nature. The Ramban adds that one who suckles the milk of a chazir will become a leper as it brings out a bad nature.
Just as non-kosher food brings out bad character traits, kosher food brings out good character traits. The Midrash says: milk contaminates and milk purifies. We find this with Ontininus. When Rebbi was born, the king decreed not to circumcise any boys. His parents did not obey and circumcised Rebbi. The king found out and summoned them to come. On the way, Rebbi’s mother switched him with a non-Jewish baby, Ontininus, and nursed him. The king checked the baby and found he was uncircumcised and sent them back home. The general said, “I saw that they circumcised him! Hashem performs miracles for them all the time.” Then and there the king annulled the decree. The Yerushalmi says that Ontininus ended up learning Torah, converting and circumcising himself! The pure milk of Rebbi’s mother helped influence Ontininus to come close to Hashem and His mitzvos.
May Hashem help us to attain good character traits by eating kosher food and become kadosh as the parsha ends “והייתם קדושים, and you shall be holy.”