Chullin 65a: Any bird that catches its prey and eats it is not kosher. Any bird (among those that do not catch their prey – Rashi) that has a fourth toe, a crop and a peelable gizzard is kosher.
חולין סה ע”א תניא, רבן גמליאל אומר: דורס ואוכל ־ בידוע שהוא טמא, יש לו אצבע יתירה וזפק וקרקבנו נקלף ־ בידוע שהוא טהור. ופירש”י ואם אינו דורס ויש לו עוד שלשה סימנין דהוו להו ארבעה בידוע שהוא טהור.
In 1998, Rabbi Shmuel Wosner was asked about the practice of chicken breeders to mix various chicken breeds into the chickens in order to increase their profits. (Farmers have bred chickens that grow faster so that they reach slaughter weight at only 5-7 weeks – twice as fast as in the 1950’s. Commercial chickens today also require less food.)
It is known beyond a doubt, he says, that some of the male chickens inserted into the bloodline were breeds for which we have no mesorah and, following the Rema (YD 82:3), do not eat. And even if we were to rely on the Mechaber, who holds that we can eat any bird that has the four kosher signs: a crop, a gizzard that can be peeled by hand, a fourth toe, and is not a bird of prey – here the breeders are non-Jews or ignorant Jews, who are not checking for these signs. (In fact, it is known that the commercial chicken is descended from the Cornish, which was bred from the Malay for the purpose of cockfighting.)
The Chasam Sofer’s heter on a duck whose father may have been a mallard (see story on Bechoros 7a) would not apply here, because there, there was a doubt as to whether the father was a mallard, the problem with a mallard is only the lack of mesorah (but it has the kosher signs), and there is an opinion (the Tzemach Tzedek) who permits the mallard. (But in the case of chickens, the breeding was definitely done, and the chickens are descended from breeds that were developed for fighting, which may be considered “dores”.)
Also, the Chasam Sofer is talking about a case when only the father was questionable, so we have the additional benefit of relying on the opinions that the product of two factors (zeh v’zev gorem) is permitted, and that the father’s side doesn’t matter. (In the chicken breeding industry, hybridization took place on both father’s and mother’s side.)
Furthermore, the Chasam Sofer gave a heter for an isolated case, but here we would be permitting all chickens eaten by Jews everywhere, for all time to come. We can’t base the general practice of the Jewish people on a “bedieved.”
Therefore, Rav Wosner concluded, we must wake up and watch over our slaughterhouses to make sure they buy only chickens that are free of crossbreeding, a breed that has an unquestionable mesorah on both father’s and mother’s side.
Subsequently, some argued that the commercial chicken, although new, should be permitted because Jews have been eating it for the last 50 years, and thus a “mesorah” has taken hold. They compared this to the turkey, which originally had no mesorah but became accepted by most Jews. Rabbi Moshe Zev Zorger, Dayan of Satmar Yerushalayim, responded that this comparison is flawed. The chief argument to permit the turkey is that, after its discovery in the new world, it was raised in Jewish backyards for centuries and Jews had ample opportunity to observe its natural habits; they could see that it was not a bird of prey. This constitutes the necessary “mesorah.” But the commercial chicken is raised exclusively on factory farms and was almost never observed by religious Jews.
Others argued, based on the Chasam Sofer (see Bava Kama 55a), that the commercial chicken is similar enough to the original species to be permitted without a mesorah. But its astounding rate of growth, its high ratio of breast meat, short legs, minimal feathers and inability to reproduce naturally may be considered significant changes. Furthermore, the eggs of these broilers have been observed to be the same on both ends, unlike kosher eggs that have one end narrower than the other (Avodah Zarah 40a).
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