1)    The custom is to perform the ritual of “Kapparos” on the eve of Yom Kippur. One who is worried that the local slaughterer will not be alert enough on that day, may perform it any rime during the ten days of atonement. A person must not think that the chicken is his atonement. Instead, he must think that everything that is being done to this chicken should really have been done to him for his sins, and by thinking about this he will come to repent. One should not insist on the color of the chicken or to pay extra for a white one, as this is close to walking in the ways of the idol-worshippers, but if he comes across a white one, he should take it and say nothing. (Mishne Berurah 605)

2)    One may not perform Kapparos with something which is fitting to be offered up on the altar like pigeons and doves so that it does not appear as if he is offering up offerings outside the holy temple. Because of this, people take chickens or fish. One may also take geese or other animals which are unfit for offerings. One can also fulfil this custom with coins in which case the custom is to use the amount which one would otherwise pay for the chicken. (ibid and Kaf Hachaim ibid)