Shulchan Aruch Chapter 604:
The order of Erev Yom Kippur (ערב יום כיפור) – 2 Paragraphs
Note: The REMA is in brackets 
1. It is a Mitzvah to eat on Evev Yom Kippur and to have a lot of meals. [It is forbidden to fast on that day even a fast for a dream. If one made a vow to fast on it, see 570:2]
2. We don’t fall on our faces on Evev Yom Kippur. [Also, we do not say למנצח and מזמור לתודה. Similarly, we don’t say before daybreak a lot of סליחות (penitential prayers). But there are places where the custom is to say a lot of סליחות (penitential prayers). Everyone should go according to his custom. Regarding the saying of אבינו מלכנו on Erev Yom Kippur, there is an argument among אחרונים (later authorities). The custom of this province is to not say it unless Yom Kippur falls out on Shabbat where this is not said so you say it on Ever Yom Kippur, in the morning prayer.]
Chapter 605: The custom of כפרות on Erev Yom Kippur (1 Paragraph)
1. There are those who have the custom to do an act of expiation on Erev Yom Kippur by slaughtering a chicken for every male son and to say verses on it. One should not partake in this custom. [However, there are some Ge’onim who write about this custom and most of the אחרונים (later authorities) write about it and it is the custom in all these areas. One should not change it since it is the custom and is well established. The custom is for a man to use a rooster and for a female to use a chicken. A woman who is pregnant should use a rooster and a chicken since maybee she will give birth to a son. We select poultry for the children like it says “if your sins will be like crimson, like snow I will whiten them”. The custom is to give the Kaparot to the poor or to exchange them with money and give that to the poor. There are places where the custom is to go by a cementary and to give a lot of charity. These are all beautiful customs. Some do the slaughtering immediately after they are finished. In addition, they rest their hands on it like a sacrifice and they throw the guts of the animal on roofs or in the courtyard of a place where you are able to take the birds from.]
Translated by Jay Dinovitser www.shulchanarach.com
 Mishna Brura: The Torah writes “and you shall afflict your souls on the ninth of the month in the evening”. To the novice, it seems that one should fast on the ninth of the month, which is Evev Yom Kipppur. However, the Rabbis through the tradition maintain that quite the opposite is true, it is a commandment to eat on that day. If the Torah wanted you to fast, it would have wrote “one the ninth of the month in the evening you shall afflict yourselves until the (next) evening”. But the Torah did not say this and it never explicitly says to fast on that day but instead said the fast is the next day, on The Day of Atonement.. The funny language of affliction is to show that G-d gives us credit for eating as though we fasted and afflicted ourselves.
 To say the prayer of תחנון (Takh-an-un).
 Some authorities claim this resembles witchcraft or pagan practices.
 M.B. If she has a son then the rooster will cover him. If she has a daughter then the chicken will cover both of them.
 I sort of miss the connection but I think the Hebrew word for “whiten” used in the verse, ילבינו is similar to the Hebrew word meaning “son”, בן.
 M.B: The chickens steal and the insides are the first place where the stolen food goes. This is symbolic of distancing ourselves from stealing.