crossbill bill crossing

[Raymond Davis]

I read somewhere years ago that the young crossbills bills cross at about 3 weeks, and they can cross either left or right, with no apparent reason (just like humans left-right handedness?)

Davis.   in Larimer County with only 15 Cassin's Finches.

[Carl Bendorf]

Hi Raymond,

After photographing some Red Crossbills the other day, I noticed the bills of two birds were crossing in different directions (upper mandible curved to the right in one and the left in the other.) To be honest, I'd simply never noticed this before and wondered why. Of course, the answer is on the internet and I found an interesting article by Craig Benkman. Apparently the crossing direction is random and can be about half and half in crossbill populations. Benkman theorizes this may help groups of crossbills to forage more efficiently as different curved bills might be able to reach different areas on conifer cones. Fascinating! By the way, it was Craig's work that led to the "discovery" of Cassia Crossbill as a separate species. Here's a link to the complete article on the bill crossing direction:  Benkman article

Carl Bendorf

Longmont, CO