I met an individual who told me that he refuses to do Kaparos on chickens because he feels that the way it is done is cruel to the chickens, being that amongst other factors the slaughtering takes place in view of all the other chickens, and this causes unnecessary fear in the chickens. I answered him by asking him as to how he knows the chickens know what is going on and that they are scared by this. He said to me that he once heard that this is even written in Jewish law. Can you please clarify.
In general, it is correct that it often occurs with the chaos of Kaparos that unnecessary pain is caused to the chickens, and hence it is the absolute moral, ethical, and halachic obligation upon both the organizers and the people performing the ceremony of Kaparos to be careful to their utmost to diminish the amount of suffering and pain which is caused to the chickens, so it not be a mitzvah that comes as a result of an Aveira. Thus for example, the chickens should be kept in a shaded area away from the sun, be provided ample water and food, and not be thrown around [which can enter into questions of Treifos]. Likewise, the handlers of the chicken by the people performing the Kaparos should be with gentleness, without exerting too much force that can cause unnecessary pain to the chicken. And now to your main question:
Yes, it is indeed correct the Jewish law discourages the slaughtering of chickens in view of other chickens both due to reasons of Tzar Baalei Chaim and due to reasons of Treifos, as we will explain below. Accordingly, it is proper for the organizers to slaughter the chickens a slight distance away from the other chickens, and it is likewise proper for the people holding the chicken in line for slaughter to not hold the chicken in a way that it can view the slaughtering of the other chickens.
Nonetheless, all this is only Lechatchila, and Bedieved if this was not adhered to and the chickens witnessed the slaughter of the other chickens, it remains kosher, especially if it was slaughtered that day, and is having its lungs checked as is commonly done amongst Mihadrin Hashgachas.
To note, that for whatever reason, the widespread custom amongst all slaughterhouses that I have witnessed is even initially to slaughter the chickens in front of each other, having boxes of chickens lined up right next to the slaughter, who then slaughters the chicken in full view of all the other chickens. Seemingly, this is done due to that being stringent in this [and distancing the slaughtering of the chicken from the other chickens] would cause a delay between the slaughtering of each subsequent chicken, which in the end of the day would tremendously raise the price of chickens for the kosher consumer. Hence, they rely on the fact that the chickens are anyways being slaughtered within the next hour or so, and as well as due to the argument that today’s chickens are no longer frightened by this occurrence.
Explanation: The Talmud records that a shriveled [raisin like] lung of an animal or bird deems the animal a Treifa. One of the ways that the lung of an animal can become shriveled and raisin like is if it is frightened. It is brought in the Rishonim and ruled in the Shulchan Aruch that slaughtering another animal or bird in front of it causes this to happen, and hence if one slaughters another animal in front of the animal or bird and its entire lung, or it’s majority, cringed as a result, then it is deemed a Treifa, and is forbidden to be eaten.
Now, the Rashba rules that this worry is only applicable if there will be a delay between this witnessing of another animal being slaughtered and its own slaughtering. If, however, the animal will also be slaughtered right away, then there is not much of a worry and one is not required to check the lungs to verify that this shriveling did not take place.
Nonetheless, the Achronim conclude that it is improper even in this case to slaughter the animal in front of other animals who will be slaughtered right away, and hence they explicitly call out against the custom by Kaparos on erev Yom Kippur to slaughter the chicken in front of the other chickens, and state that doing so is a worry of Treifos due to the potential shriveling of the lung, and is also improper due to Tzaar Baalei Chaim. Nonetheless, Bedieved they agree that we do not have to require the lungs to be checked if this took place, and one may rely on the ruling of the Rashba. They explain that there is no difference between chickens or cows in this regard.
Based on all this, it is clear that by Kaparos one must try to shield the slaughtering of the other chickens from one’s chicken, and that one should try to diminish in their suffering as much as possible. And just as we have showed mercy to them may G-d show mercy to us.
Sources: See regarding a cringed raisin like lung which is caused by the animal experiencing fear: Michaber Y.D. 36:14; Chulin 54a-55b; Taz 36:20; Shach 36:29; Rashba Chulin 55b See regarding initially being careful not to slaughter the animal in front of other animals due to the above reason and due to Tzaar Baalei Chaim: Beis Efraim Y.D. 26; Yad Efraim on Shach; Pischei Teshuva Y.D. 36:16; Zivcheiy Tzedek 217; Ben Ish Chaiy Tazria 15; Kaf Hachaim 36:244; See regarding the custom today in the slaughterhouses to ignore this rule and slaughter the chickens in full view of the other chickens: Sichas Chulin 55b [Rav Yitzchak Rubin