The Mishna on today’s daf – the opening Mishna of the second perek in Massekhet Ḥullin – teaches one of the most basic rules of ritual slaughter. When performing sheḥita the slaughterer must cut two simanim – the esophagus and trachea – in an animal, and a single siman – either the esophagus or the trachea – in a bird. In both cases, it would be sufficient to cut the majority of the simanim (or one of the simanim in the case of a bird). Rabbi Yehuda requires that the veins should be cut, as well.
The Gemara discusses the source for these laws and for the difference between the requirements for kosher slaughter for an animal in contrast to those of a bird. Aside from Biblical passages that are brought, the Gemara also quotes a teaching from a certain Galilean traveler who suggested that animals were created by God from the dirt of the earth, so the requirement for their slaughter is two simanim. Fish were created from water, so there is no need for any sort of slaughter. Birds, which were created from mud, need a single siman for ritual slaughter.
Rashi explains this reasoning by suggesting that the more solid the raw material used in creation, the greater the life-force of the creation, and consequently the greater the need for a more significant act of slaughter. Other commentators took a spiritual direction in explaining this matter.
In his Torat Ḥayyim, Rabbi Avraham Hayyim Schor offers a kabbalistic approach, suggesting that an animal that dies without proper slaughter becomes ritually defiled because it has suffered death at the hands of the Angel of Death. When an animal is properly slaughtered, however, its life-force is released in a pure, holy manner, and it does not become defiled. The greater the life-force, the greater the potential is for ritual defilement and therefore the greater the need for a more significant act of ritual slaughter to guarantee purity and holiness.
The Maharsha explains that it is not based on the animal’s life-force, rather the animal whose creation is from the earth is a more physical being than birds or fish, and therefore needs a greater effort of tikkun – “repair” – to prepare it for use as kosher food.