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How do passerine bills grow back after being worn down?
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Passeriform
Reed warbler (Acrocephalus scirpaceus).
The mandibles of passerine birds consist of bone covered by a horny sheath known as the ramphotheca. This sheath is subject to wear through normal daily activities, such as feeding and preening. To compensate for this loss, the ramphotheca is capable of constant growth, which replaces the material worn away over time.1
In instances where a bird cannot naturally wear down its bill—such as in some captive environments—or if the bill is damaged, this continuous growth can lead to the tip becoming overgrown.1