1. There is a dispute about which birds one may slaughter on Yom Tov.
2. Rav Chanan bar Ami explains this dispute (#1).
3. Rava explains this dispute differently.
4. One who designated white birds and found black birds on Yom Tov may not use the black birds.
5. The Mishnah says that if one designated two birds and found three, they are all forbidden (on Yom Tov).
A BIT MORE
1. Beis Shamai: One may take only birds that he had held before Yom Tov and designated as ready for Shechitah. Beis Hillel: Holding the birds is unnecessary; it suffices merely to state which birds he plans to take on Yom Tov.
2. He understands that this stringency of Beis Shamai applies only to the oldest batch of pigeons in the coop (that are like mothers to the others). Beis Shamai: Holding the birds he plans to take is necessary, because otherwise he may regret his decision and move these birds unnecessarily on Yom Tov. Beis Hillel: We do not suspect that this will happen as long as he designated them for Shechitah before Yom Tov.
3. He explains that the question is merely whether the birds are fat enough to be slaughtered. The dispute concerns how that is determined to the point that the person will not retract his decision to slaughter certain birds on Yom Tov.
4. Rabah explains that the Mishnah means that even if there were both white and black birds before Yom Tov, and then on Yom Tov he found them but they had switched places within the coop, it is possible that they are not the same birds that he designated before Yom Tov. Rather, the original birds flew the coop and these are new ones (that may not be used since they were not designated before Yom Tov).
5. This is because one of the birds is obviously not designated. There is also the possibility that the others are new birds that switched places with the old birds.