Was chicken always Kosher? If not, when did it become so? I know that a Talmudic rabbi (I think Yossi Haglili) ate fowl and milk together, so it must have been considered "parveh" at some point in time, later to be moved to "Basari.(meat)" Do you have a source that attributes to Haglili the combining of meat and milk? Is there a source that explains the process that brought the definition of chicken from "Parveh to Basari"? Is there a possibility that chicken was originally not kosher at all? I was told that that the kashrut of chicken was a matter of controversy between the Karaites and the Rabbanites, with the Karaites contending that the Chicken was "a teaming bird" prohibited in the Torah itself, and therefore not suitable to be offered on the altar (although all of this is a bit unclear to me).
Thank you for your time,
Regarding "parve fowl":
The Mishnah in Hullin (8:4) already discusses this issue (See also Mechilta de Rabbi Yishmael Mishpatim, Parasha 20; Mechilta de Rabbi Shimon bar Yochai, chapter 23 and Sifrei Devarim, piska 104). R. Yosi Haglili allowed the eating of fowl and milk even rabbinically, and as a matter of fact, the Babylonian Talmud describes how in Tannaitic times there were certain areas where that was the accepted law (see Hullin 116a and also Shabbat 130a).
Rabbi Yosef Caro's decision in the Shulkan Arukh (Yoreh De'a 87:3) is that it's prohibited Miderabbanan (rabbinically), though it is not a biblical prohibition. It would seem that after the period of the Mishnah it became an accepted practice to consider fowl "meat."
My conclusion is chicken was always considered kosher. The issue was just a question of whether chicken could be eaten with dairy products. It has the characteristics the rabbis required for kosher fowl. I don't know anything about the Karaites' position on this matter.