Niddah 51: Ritual Impurity/Kashrut/Tithing of Birds and Fish; When Blessings Must be Recited

In Niddah 50, we learned about some of the laws of pe'a as they overlap with the laws regarding tithes.  Today's daf focuses on the kashrut of birds.  when discussing birds, the rabbis note that contact with the carcass of a non-kosher bird does not impart ritual impurity until at least an olive bulk passes down one's throat.  Further, they consider imparting ritual impurity through one of seven liquids: water, dew, milk, blood, wine, oil or honey must drop onto the owner's clothing without his/her knowledge.

There are several Mishnayot that consider the status of different items: tithing and shearing of certain first-born animals, the kashrut around fish with scales and fins, and the generalization that if a blessing is required following an action, it is also required before the action.  However, it may not be necessary to bless an action after it taken place if the blessing was given beforehand.