Q: My local farmer’s market sells duck eggs. Can I assume that they are kosher?
A: Although the Torah lists twenty-four non-kosher species of birds, we no longer know precisely to which species the Torah is referring. The Gemara provides alternative ways to ascertain whether a bird is kosher, but notes an instance in which a mistake in identification transpired. For this reason, Rashi states that we do not rely on the Gemara’s criteria, and we eat only the few species of birds for which we have a mesorah (oral tradition) that attests to their kosher status.
The Baal Ha’maor, one of the Rishonim, maintains that he has a tradition that all birds with wide feet and a wide beak – those more or less resembling a goose – are deemed kosher if they meet the Gemara’s criteria. The Shulchan Aruch quotes this statement, indicating that all varieties of duck are kosher. However, the Rema states that we do not rely on this view, and concludes that one should eat only those varieties for which we have a mesorah.
Although no two birds look exactly the same, a mesorah will cover all birds that look similar and have no significant differences among them.
Practically speaking, we have a mesorah on the more predominant varieties of chickens. The turkey is also considered to be a kosher bird, yet poskim have raised the question that it would seem impossible for there to be a mesorah on turkey, as turkeys are native to North America and were unknown before the continent was discovered. Due to this concern, there are some individuals who do not eat turkey. Various answers to this question have been offered, and common practice is to consider turkey to be kosher.
As far as ducks are concerned, we have a mesorah regarding Pekin ducks – also referred to as American Pekins or White Pekins. Pekin ducks are therefore deemed to be a kosher species. Regarding other varieties, such as black-billed ducks, poskim debate whether there is a mesorah for them. Common practice is to be stringent. There is further discussion regarding Mallard ducks which sport bills that are not completely black but have black spots or bands. STAR-K does not certify Mallard ducks.
There is also a century-old dispute regarding Muskovy ducks. In 1860, Rabbi Yissochar Dov (Bernard) Illowy took a position as a rabbi in New Orleans. Upon his arrival, he found Jews eating Muskovy ducks. He felt there was no mesorah allowing their consumption and forbade them. He corresponded with Rabbi Nathan Adler, Chief Rabbi of England, and Rabbi Shimshon Raphael Hirsch of Germany, both of whom agreed with his decision. Although other poskim disagreed and felt that the ducks were kosher, common practice in America is not to eat them. Before buying duck eggs, one would need to ascertain (in a halachically valid way) which species of ducks produced them. Only Pekin duck eggs are kosher; other varieties of duck eggs should be avoided.
 ויקרא יא,יג-יט ודברים יד,יב-יח
 חולין דף נט ע”א ואילך
 רש”י שם דף סב ע”ב ד”ה חזיוה
 רז”ה הובא ברא”ש שם פ”ג סי’ ס
 שו”ע יו”ד סי’ פב סעי’ ג
 רמ”א שם
 עי’ בדרכ”ת שם ס”ק כו בשם שו”ת משיב דבר וספר ערוגת הבושם
 עי’ בפת”ש שם ס”ק א ודרכ”ת שם ס”ק ו
 ויש מקילין, עי’ באתר OU Position on Certifying Specific Animals and Birds – OU Kosher
 עי’ באתר https://www.kashrut.com/articles/ThreeBirds/
 עי’ מש”כ הפמ”ג שם שפ”ד סי’ פו ס”ק ז בענין ביצים מאווזים שחוטמיהם שחורים, ולפי הצד הראשון שם יש מקום להקל כאן, אבל לפי הצד השני שם יש להחמיר.