Kaporos is a ritual related to Yom Kippur, the holiest day of the Jewish calendar. During Kaporos, we pray for atonement and that punishment for our sins be diverted to a dumb animal, in this case a chicken.

Several reasons have been suggested for the choice of a chicken to perform the Kaporos rite: 1) In Aramaic, a rooster is known as a gever. In Hebrew, a gever is a man. Thus we take a gever to atone for a gever. 2) A chicken is a commonly found fowl and relatively inexpensive. 3) It is not a species that was eligible for offering as a sacrifice in the Holy Temple. This precludes the possibility that someone should erroneously conclude that the Kaporos is a sacrifice.

It is customary to use a white chicken, to recall the verse (Isaiah 1:18), "If your sins prove to be like crimson, they will become white as snow." In any event, one should not use a black chicken, as black is the color that represents divine severity and discipline. Nor should one use an obviously blemished chicken.

A male takes a rooster; a female uses a hen. Ideally every individual should use their own chicken. If, however, this is cost prohibitive, one fowl can be used for several individuals. So an entire family can do Kaporos with two chickens—one rooster for all the males and one hen for all the females.

In the event that more than one person share a Kaporos chicken, they should do the Kaporos together, not one after the other. For one cannot do Kaporos with a "used" chicken.

A pregnant woman should perform Kaporos with three chickens—two hens and a rooster. One hen for herself, and the other hen and rooster for the unborn child (of undetermined gender). Or, if this is too expensive, one hen and one rooster will suffice (and if the fetus is female, she shares the hen with her mother).

If a chicken is unavailable, one may substitute another kosher fowl (besides for doves and pigeons, as they were offered as sacrifices in the Holy Temple). Some use a kosher live fish; others perform the entire rite with money, and then giving the money – at least the value of a chicken – to charity.
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It is of utmost importance to treat the chickens humanely, and not to, G-d forbid, cause them any pain or discomfort. Jewish law very clearly forbids causing any unnecessary pain to any of G-d's creations. The repugnance of such an unkind act would certainly be amplified on this day, the eve of the day when we beseech G-d for – perhaps undeserved – kindness and mercy. In fact, the Code of Jewish Law suggest that we take the innards and liver of the Kaporos chickens and place them in an area where birds can feed off them. "It is proper to show mercy to the creatures on this day, so that in Heaven they should have mercy upon us [too]."

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