Gemara

(Mishnah): One may not bring cheese on the same table with any kind of meat, except for meat of fish and grasshoppers.

104a (Rav Yosef): This shows that the Torah forbids fowl cooked with milk. If it were only a Rabbinic decree, we would not forbid putting them on the same table. We do not make a decree to guard a decree!

Rebuttal (Abaye): If we allow fowl and milk on the same table, one may come to put beef and milk on the same table, and eat them together!

113a (Mishnah): One who brings fowl on the same table with cheese does not transgress a Lav.

Inference: If he would eat them together, he transgresses a Lav. This shows that the Torah forbids fowl and milk!

Rejection: No. It means that one who brings fowl on the same table with cheese will not come to transgress a Lav.

(Mishnah): Basar v'Chalav of Tahor Behemos may not be cooked together. If they were cooked, one may not benefit from it. If the meat or milk is from a Tamei Behemah, they may be cooked together, and one may benefit from it;

R. Akiva says, the Torah does not forbid meat of a Chayah or bird (with milk);

R. Yosi ha'Gelili says, "in its mother's milk" excludes birds, which do not have milk.

Avodah Zarah 74a (Mishnah): The following forbid a mixture of any amount: Basar v'Chalav...

Pesachim 30a (Rav): Pots (used for Chametz) may not be Kashered for use on Pesach. They must be broken.

Question: He should allow leaving them to be used after Pesach, Lo b'Mino!

Rishonim

Rambam (Hilchos Ma'achalos Asuros 9: 4): The Torah does not forbid eating meat of a Chayah or bird cooked in milk of a Chayah or Behemah. Therefore, one may cook them and benefit from them. Mid'Rabanan, opmn eat it.

Rambam (27): One who eats fowl after cheese or milk need not clean the mouth or wash his hands.

Rambam (15:27): If milk fell into a pot with fowl and gave taste, one may add more fowl until he is Mevatel the taste. The same applies to all similar cases.

Tosfos (Pesachim 30a DH Lishhinhu): Why did we ask that Rav should allow leaving pots to be use them after Pesach, Lo b'Mino? Even if he considered the absorptions to be Mashehu (a tiny amount), l'Chatchilah one may not be Mevatel an Isur! If this were permitted, Chachamim's Shi'ur of 60 would be meaningless. Everyone will add to be Mevatel the Isur! This is unlike Muktzeh wood, for which we permit adding to be Mevatel (Beitzah 4b), for it has no source in the Torah. One may not be Mevatel an Isur with a source in the Torah. R. Yosef says that here there are two reasons to be lenient. The absorptions give a detrimental taste, and it is Mashehu b'Eino Mino. The Ri says that this is considered b'Di'eved, for if we do not permit, the pots must be broken.

Mordechai (752): We learn from the Rambam that milk and fowl is mid'Rabanan, and l'Chatchilah one may be Mevatel an Isur mid'Rabanan. However, if the Isur has a source in the Torah, one may not be Mevatel it, e.g. Chetzi Shi'ur according to the opinion that it is mid'Rabanan. This is like Tosfos. It seems that the Rambam agrees with Tosfos. This is why he taught this Heter regarding milk and fowl, and not regarding Chetzi Shi'ur, which has a source in the Torah.

Beis Yosef (YD 99 DH v'Chasav b'Hagahos Maimoniyos): The Rambam does not hold exactly like Tosfos. Tosfos consider milk and fowl to have a source in the Torah, since the Torah forbids some meat and milk. Also Terumah of Peros is considered to have a source in the Torah, since mid'Oraisa Terumah applies to grain, wine and oil. They are unlike Muktzeh.

Rosh (8:5): The Rif brought Agra's Beraisa, which permits eating fowl and cheese wantonly. This shows that he holds that milk and fowl is mid'Rabanan, like R. Akiva. Agra permits fowl and cheese without washing the hands and cleaning his mouth in between. The same applies to Chayos and milk. He mentioned fowl, for it is more common. R. Tam says that he discussed fowl, for it does not cling to the hands, teeth and gums.

Hagahos Ashri (8:3, citing Or Zaru'a): We hold that milk and fowl is mid'Rabanan. A Mishnah teaches that if a drop of milk fell on a piece of meat, if it gives taste, it forbids it. This refers to meat of Behemos. If it fell on fowl, which is mid'Rabanan, it is Batel in a majority. In Avodah Zarah, we learned that meat and milk forbids b'Mashehu. This refers to meat of a Behemah, but fowl is Batel in a majority. Also Bahag says so.

Tosfos (104b DH Of): The custom is not to eat cheese after meat at all, even after fowl. Even though Agra taught that one may eat fowl and cheese wantonly, which connotes that fowl is first, perhaps it is because he holds that milk and fowl is mid'Rabanan, and we hold unlike this.

Gra (YD 87:12): The Shulchan Aruch (Sa'if 5) permits eating milk after eggs found inside a slaughtered chicken, even if they are unfinished. This is like Tosfos understood Agra.

Poskim

Rema (YD 87:1): One may benefit from any Basar v'Chalav that is forbidden only mid'Rabanan.

Shach (2): Also the Mechaber (Sa'if 3) holds that fowl with milk, which is Asur to eat only mid'Rabanan, is Mutar b'Hana'ah even mid'Rabanan. Sefer ha'Chinuch says so, and the Rambam in Halachah 4 and Perush ha'Mishnayos. The Magid Mishneh, Maharshal and Acharonim explain explicitly say so, unlike the Bach, who says that the Rambam forbids Hana'ah.

Shulchan Aruch (3): One may cook meat of a Chayah or bird in milk of a Chayah or Behemah and benefit from it. It is forbidden to eat only mid'Rabanan.

Rema: The custom is to make 'milk' from almonds and put fowl in it, since it (milk and fowl) is only mid'Rabanan. Regarding meat of a Behemah, one should put almonds next to the milk due to Mar'is Ayin (lest people think that he transgresses Basar v'Chalav).

Bach (66:9 DH u'Maharshal): The Maharshal (Yam Shel Shlomo 8:52) requires putting almonds near the milk even regarding fowl. The Rema is lenient, since milk and fowl is only mid'Rabanan. I agree with the Maharshal. Mar'is Ayin applies even to Isurim mid'Rabanan if it is in public. Regarding private matters, Chachamim decreed only about Isurei Torah. We hold like Tosfos, that the Halachah follows Chachamim, who forbid milk and fowl mid'Oraisa.

Shulchan Aruch (YD 88:1): One may not put even fowl or meat of a Chayah on a table on which he eats milk, lest he come to eat them together.

Shulchan Aruch (89:2): If one ate cheese, he may eat beef immediately afterwards, but he must clean his mouth and check or wash his hands. If he will eat fowl after cheese, he need not clean or wash.

Rema: Some are stringent to wait even for meat after cheese. This is the custom. If the cheese is hard, after it we do not eat even fowl, just like we do not eat cheese after meat. Some are lenient. One should not protest, just they should clean and wash the hands. However, it is good to be stringent.

Shach (87:4): Tosfos says that eating cheese after fowl depends on whether milk and fowl is mid'Oraisa, but one need not say so. It seems that even Tosfos said so only to justify the custom. Even if milk and fowl is mid'Rabanan, perhaps one may not eat cheese right after fowl. We can say that Agra discusses fowl after cheese, like the Rambam explains.

Shulchan Aruch (90:1): If one cooked udder with other meat, everything is forbidden unless there are 60 parts for Bitul. If it fell into another pot, we require 60 against the udder, because the udder itself became like Isur.

Taz (4): This is like the Rashba, who applies Chatichah Atzmah Na'asis Nevelah (a piece that became forbidden due to absorptions is considered like pure Isur) to the udder. Also the Rema rules like this, unlike the Rambam, who holds that Chatichah Atzmah Na'asis Nevelah does not apply to Isurim mid'Rabanan such as milk and fowl. R. Efrayim applies Chatichah Atzmah Na'asis Nevelah to all Isurim mid'Rabanan.

Shulchan Aruch (92:3): When a piece becomes forbidden due to Basar v'Chalav, the entire piece becomes Isur. If he cooked it with others, 60 are required to be Mevatel it.

Beis Yosef (DH u'Mah she'Chasav Rabeinu Im): A Mishnah says that Basar v'Chalav forbids b'Mashehu. This refers to meat of a Behemah. Meat and milk of fowl or Chayos are Batel is the majority.

Taz (5): Darchei Moshe asked why the Rashba forbids cooking in a woman's milk due to Mar'is Ayin, but did not forbid meat and milk if one of them is from a Tamei species. I say that Chachamim decreed to forbid cooking when each of them is permitted by itself. If so, all the more so one may not cook milk and fowl, which one Tana forbids mid'Oraisa. Also, sometimes it is not evident that it is fowl, depending on how it is cut. The Maharshal says that there is more concern lest people think that it is permitted, like R. Yosi ha'Glili. The Rema forbids l'Chatchilah. I say that he permits if there is a great need, e.g. for a cure.