The bill of a single species of crossbill can curve either to the right or the left. There is no set rule. It is determined while the young are fledging and the beak is beginning to cross. Birds become specifically ?footed? based on the curvature of their bill.



Jeff Kozma



Yakima





From: tweeters-bounces at mailman1.u.washington.edu [mailto:tweeters-bounces at mailman1.u.washington.edu] On Behalf Of Gary Bletsch
Sent: Monday, March 23, 2015 8:59 PM
To: Tweeters Tweeters
Subject: [Tweeters] crossbill question



Dear Tweeters,



It occurred to me today that I did not know the direction of the curvature of the bill of a crossbill. I looked at some pictures on the Internet, and saw some variation. With these images, I figured there might be some photos that had been reversed according to the photographer's whim, perhaps.



The pictures in my copy of Big Sibley show the top mandible going to the bird's right, in both of our species of crossbills.



The pictures in Lars Jonsson's Birds of Europe with North Africa and the Middle East appear to show the top mandible going to the left on a male Red Crossbill, but the top mandible going to the right on the female. This book shows a male Parrot Crossbill with the top mandible going to the right, with the female's top mandible going left. Then it shows the White-winged Crossbill female's top mandible going right. I couldn't tell what Jonsson was showing on the male White-wing, because the bird is depicted with bill open, attacking a cone.



I had always assumed that crossbills had the same sort of chirality as that seen in the various species of flatfishes, where the "handedness" was always the same in a given species, barring rare mutations.



How does this work in crossbills?



Yours truly,



Gary Bletsch