The reference to a "kosher" bird being one that does not דרס is from the Mishnah Hullin 3: אֲבָל אָמְרוּ חֲכָמִים, כָּל עוֹף הַדּוֹרֵס, טָמֵא. כֹּל שֶׁיֶּשׁ לוֹ אֶצְבַּע יְתֵרָה, וְזֶפֶק, וְקֻרְקְבָנוֹ נִקְלָף, טָהוֹר, but the precise meaning of הַדּוֹרֵס here was variously interpreted but later authorities.