Chullin 22: Difficulty with Severing Simanim; Immature, Mature Birds

The rabbis speak about the status of a bird whose neck was broken without the flesh being appropriately severed.  They talk of other animals who are lacking legs or are otherwise damaged and thus invalid for slaughter.  It seems that there is some debate about whether or not such animals might be considered valid.  For example, one rabbi recalls the story of a bird whose head was severed.  Both the head and the body were burned as an offering.  

The rabbis speak about the difficulty of sprinkling from the bird once its simanim are mostly severed.  

At the end of our daf, we learn a new Mishna.  It quotes Leviticus (1:14), "He shall bring his offering of doves, or of young pigeons".  The age that is fit for sacrificing doves - mature birds - is unfit for sacrificing pigeons - immature birds.  The age that is fit for sacrificing pigeons is unfit for sacrificing doves.  When the bird's plumage begins to yellow in its middle years, both types of birds are unfit for slaughter.  At that point, birds are considered to be mature.