I have dealt with a large number of birds, including magpies, with bill
abnormalities over the last decade or so and the prime reason appears to be
trauma. Some have tried to beat the mouse trap others have simply hit glass
or MVs. Many have presented with other injuries and the bill damage is an
old injury - confirming other correspondents observations that they can
survive.
The bill does not generally re-grow. Bird bills are composed of a number of
outer layers of keratin over partly calcified keratin fibre structure which
has a blood supply virtually to the tip. It is analogous in a rough way to
your finger nail in that the keratin (your nail) grows continuously off a
bone and flesh structure which supply's both reinforcement and a blood
supply to renew the keratin layer. The difference is you finger grows out in
one direction while the bill grows from the inside. This is why bird's bills
often appear flaky at the surface as the older keratin peels or wears away.
Keratin is a protein (the same as feathers and hair) and constitutes most of
the bill's surface it is not a product of calcium consumption beyond some
calcium on the underlying bone fibres. Bill re-growth only occurs when the
underlying fibres and blood vessels have not been badly compromised. Birds
with large bill cracks or where the layers of keratin have been stripped
have reasonable prognosis of full recovery. Those where the bill has been
sheared, bent to the point of stopping blood flow, or punched to the same
effect will not general re-grow their bill - hence the magpies observed. For
some species whose foraging is dependent on a perfectly functioning bill the
injury is a death sentence (many parrots but particularly small ones with
specialised food preferences) while other will cope reasonably well.
Chris Lloyd
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