Bava Kama 55a: Shmuel said: It is forbidden to mate a domestic goose with a wild goose. Rava bar Rav Chanan asked: Why not? Just because one has a long beak and one has a short beak? If so, it should be forbidden to mate a Persian camel with an Arabian camel, since one has a thick neck and the other has a thin neck.
בבא קמא נה ע”א: אמר שמואל: אווז ואווז הבר ־ כלאים זה בזה. מתקיף לה רבא בר רב חנן: מאי טעמא? אילימא משום דהאי אריך קועיה והאי זוטר קועיה, אלא מעתה, גמלא פרסא וגמלא טייעא, דהאי אלים קועיה והאי קטין קועיה, הכי נמי דהוו כלאים זה בזהִ אלא אמר אביי: זה ביציו מבחוץ, וזה ביציו מבפנים.
The Avnei Nezer was asked about a species of goose with a long neck that was brought to Warsaw from a faraway part of Russia. The Jews in that part of Russia had been eating it, but the Avnei Nezer did not consider that a “tradition” (as per the Rema Yoreh Deah 82:3, who says that no bird may be eaten unless we have a tradition that it is kosher). His reason is that a tradition is only reliable if it comes from a place where the people are Bnei Torah and many talmidei chachomim live among them. But this place originally had no Jewish community; Jews had settled there only recently, and it was not a place of talmidei chachomim.
However, quoting our Gemara, he shows that having necks of different length or thickness is not enough to define two birds as being of different species. If this is true for the prohibition of kilayim (interbreeding), it is true as well for the rule of eating only birds for which we have a tradition. Therefore, our tradition to eat geese is enough to permit these other geese, despite their slightly different appearance.
The Avnei Nezer mentions that the Chasam Sofer (YD 74) also sets down this rule: that anything that is not enough of a difference to be considered kilayim can be considered the same species for the purpose of relying on tradition to eat it.
[See Bechoros 7a in this sefer where we quoted part of this Chasam Sofer. The Chasam Sofer actually advocates a bigger leniency than the Avnei Nezer. While the Avnei Nezer is just saying that another goose species that looks slightly different is also kosher, the Chasam Sofer says that even if we know that the father is not kosher, there is reason to permit the offspring based on the opinion that אין חוששין לזרע האב – we don’t go after the father. This, combined with reliance on the Tzemach Tzedek, who permits the mallard duck, forms the basis for his ruling.
The implication is that if kosher ducks or chickens were bred with unknown species that might not be kosher according to any opinion, even if the offspring look similar enough to known species, they might not be kosher even according to the Chasam Sofer, because we have to take into account the opinion that we do follow the father. Additionally, there is no way to know if the other species was introduced as a father or a mother.]
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