Must one own the chicken that he performs Kaparos with, or is a borrowed chicken also valid?
Traditionally, the chickens used for the custom of Kaparos are to be owned by the individual and then slaughtered on his behalf. Seemingly, a person does not fulfill the traditional custom by using a chicken that is not his, such as by borrowing someone’s chicken unless that person intends on giving them at least partial ownership over the chicken. Nonetheless, there is room to learn that one can fulfill his obligation even with a borrowed chicken. Practically, it is best for one to take it from the owner with intent to [at least partially] acquire it [or acquire it on condition to return] rather than simply to borrow it.
May a Kaparos organizer arrange for the chickens to be lent to the customers rather than sold to them?
As stated above, the traditional custom of Kaparos entails the use of chickens that every individual owns and then has slaughtered on his behalf. Accordingly, it is not befitting for organizers to arrange for the chickens to be lent to the customers and rather it is to be given to them in a form of ownership. It goes without saying that if the arrangement is for the chickens to be lent, that this must be publicized to all the customers, who naturally assume that when they pay money they are buying the chicken. If the organizers did not inform the public about this, and initially arranged with the chicken farm that the chickens would only be lent to the customers and then returned to the chicken farm, then they are considered to have stolen from the public. Likewise, most seemingly no one has fulfilled their obligation of the custom of Kaparos. Unfortunately, there are many organizers who do not pay attention to this crucial detail, either deliberately or do to a lack of knowledge, and it is hence imperative on the customers to be vigilant and question what will occur with the chickens after they are returned and who is in charge of the slaughtering. As all matters that relate to Jewish tradition, it is most proper for Kaparos organizers to have Rabbinical supervision over the process, in order to ensure that it is being done honestly, with integrity, and according to the demands of Halacha.
 So is the implication of all the sources in Poskim who discuss donating the chicken to charity, and what to do if you can’t afford to purchase a chicken, and no mention is made that it’s possible to simply borrow one. so also seemingly applies according to the opinion that the Kaparos ceremony is to be identical to a Karban, and the person thus must own it [Vetzaruch Iyun, however, from Erechin 21a and Rambam Maaseh Karbanos 14:9 if actual ownership is required by a Karban, or mere consent suffices]
 Reply of Rav Eli Landa; Reply of Rav Avishad;
 See Erechin 21a and Rambam Maaseh Karbanos 14:9 that one may have another person bring a Karban on one’s behalf, thus implying that it is not necessary to own the animal; In Kfar Chabad, the custom for many years was for the slaughterhouse to allow the residents to borrow chickens for Kaparos for free, and then return them to the slaughterhouse after the ceremony. [Heard from Rav Meir Ashkenazi]
 The answer below is based on a real case scenario that happened in which the organizers borrowed the chickens from the slaughterhouse and did not inform the paying customers that they are in essence only borrowing the chicken. The matter was brought up with a number of Rabbanim, and the main consensus was as stated below. See also Nitei Gavriel 10 footnote 18; Hamaor p. 15; Koveitz Beis Aaron Viyisrael p. 78; Shut Rabbanei Europe p. 326
 Reply of Rav Eli Landa; Reply of Rav Avishad; As even by Karbanos, in which we find precedent to allow someone else to bring the animal on your behalf, it is only valid if the person had knowledge that he was not the owner of the animal and nonetheless agreed for it to be brought. However, in the case where it was brought on his behalf without his knowledge then it is invalid. Seemingly, the same applies here, that since the people were unaware that they’re only borrowing the chicken therefore the entire ceremony is invalid. Furthermore, in this situation the chicken is not being slaughtered on behalf of those who performed the ceremony, and thus loses the main aspect of the ceremony which is to slaughter a chicken on one’s behalf.