Bechoros 7a: Rabbi Yehoshua ben Levi said: a female non-kosher animal cannot become pregnant from a kosher male, nor can a kosher female animal become pregnant from a non-kosher male.
בכורות ז ע”א: האמר ר׳ יהושע בן לוי: לעולם אין מתעברת לא טמאה מן הטהור, ולא טהורה מן הטמא.
The Chasam Sofer was asked about a farm that raised female ducks, and a male wild duck (mallard) was seen frequenting the area. There was one great authority who permitted the mallard duck: the Tzemach Tzedek (Rabbi Menachem Mendel Krochmal of Nikolsburg, 17th century). However, the common practice is not to eat it, since we have no tradition to do so. Was it allowed to eat the offspring of these female domestic ducks?
The Chasam Sofer permitted them with three arguments. Although, as he notes, each of the arguments have possible objections, when taken together, these arguments are sufficient grounds for leniency.
First of all, Rabbi Yehoshua ben Levi says (Bechoros 7a) that a kosher animal cannot reproduce with a non-kosher animal. So either these offspring are from a male domestic, or else the wild duck is kosher. The possible objection to this argument is that perhaps Rabbi Yehoshua ben Levi’s rule is true only of mammals, not birds.
Secondly, the offspring look like domestic ducks, so there is no evidence that they were from this male mallard. We can assume that they are like the majority of ducks in the world, which are fathered by their own kind. The objection is that there would still be a Rabbinic prohibition: living animals are not nullified in a majority.
Third, even if the father is indeed the mallard, we can rely on the Tzemach Tzedek who permits the mallard. Although usually we don’t follow the Tzemach Tzedek, we can rely on him in this situation where it’s a hybrid, and the halacha is inconclusive on the question of whether the father matters – חוששין לזרע האב. (See Yoreh Deah 16:2 and Shach 17.)
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