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This photo illustrates something I noticed a few years ago. Namely, Crossbills' bills are not invarient. That is, some cross with the upper mandible to the left and some to the right.

The photo below shows one of each. Female and male Red Crossbills from Point Roberts where a flock has overwintered the past few years.

I noticed on a recent TV documentary a comment that almost all Parrots are "left-footed". That is, when holding an item to work with their bills, they use their left foot.

For the Crossbills, my suspicion is that their "handedness" is linked to their eating habits too. It may be that if all the birds had the same side for their upper mandible, some food items would not be taken. That is, a greater population of Crossbills may be sustained for a given food supply when some are "right-billed" and some are "left-billed".
By Rokman
How interesting!. A species of small shorebird from New Zealand has a bill that curves sideways - here is what Wikipedia says about it:

The wrybill or (in Māori) ngutuparore (Anarhynchus frontalis) is a species of plover endemic to New Zealand.[2] It is special since it is the only species of bird in the world with a beak that is bent sideways one way, always to the right (in the Crossbills eg. Loxia pytyopsittacus the tips of the upper and lower mandibles cross because they are bent sideways in opposite directions, sometimes left over right and sometimes the other way).

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