Zevachim 68 - When is a Bird Considered Dead
In bird offerings, sometimes the invalid killing of it makes it "dead by itself (nevelah)," with the result that its flesh transmits ritual impurity by "way of the throat" - that is, when one swallows its meat. At other times, however, the invalidation is not so strict, and although the bird cannot be eaten, it does not transmit impurity. What are the cases?
If an unqualified person performed the melikah (killing the bird with the nail from the back of the neck), it cannot be eaten, but it transmits no impurity. The same result happens when a kohen performed the melikah with his left hand, or at night, or if he slaughtered unconsecrated birds inside the Courtyard.
By contrast, if he performed melikah with a knife rather than his nail, or he performed melikah on an unconsecrated bird inside the Courtyard, or on consecrated birds outside the Courtyard, or if he performed melikah on turtledoves that are too young or on pigeons that are too old, a bird whose wing has atrophies, or whose eye has been blinded or foot cut off - these transmit impurity when swallowed.
Art: Francisco De Goya y Lucientes - Knife Grinder