When Does a Crossbill's Bill Cross?

A young crossbill starts life with a wedge-shaped beak. As it grows up and starts to feed itself by removing conifer seeds from their tough packaging, the tips of its bill begin to grow rapidly — and then they cross. By the time the bird is a month and a half old, the tips of its bill become fully crossed.

More info and transcript at BirdNote.org. 

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