From: Rabbi Moshe Sofer
To: Unknown recipient
A male wild goose (bar ooza habar), lacking a mesorah of purity, fathered chicks with domestic geese that then became mixed with many kosher geese. The Chasam Sofer rules leniently, permitting the offspring on multiple grounds: pure and impure species cannot truly interbreed, the female may have been impregnated by a kosher male, the offspring bear no resemblance to the questionable father, the prohibition of beryah is only Rabbinic, and the Tzemach Tzedek had actually permitted the wild goose in any case.
Source: Shu"t Chasam Sofer YD §74