Yechiel Michel Epstein

“ Section 1 There is a custom, called 'Kapporos', which is practiced by slaughtering a chicken for every male, and a hen for every female - and many of the great authorities do not rest easy with its practice. ”

“ Therefore, one should not change from following it, for it is a custom of old. The current practice is to take a chicken for a male, and a hen for a female. One takes two chickens for a pregnant woman, for perhaps she will bear a male. White chickens should be chosen, as the verse says: 'If your sins are like an old stain, they will be whitened like snow.' These chickens are then distributed to the poor, or their value in money is distributed to the poor. ”

“ They objected to the preference for white chickens, which is reminiscent of the practice of pagan worship, whose adherents would also search after white chickens for their rituals, as is seen in the Talmud in chapter 'Lifnei Aydayhen' (14.1) . Therefore one should not specifically seek after white chickens, though if one happens upon one he can take it (Magen Avraham, comment 3, and the Bach) .
One should not take turtle doves or young doves that would be fitting for a sacrifice (in Temple times) , so that one does not give the appearance of offering a sacrifice (see there) . ”