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Re: Scales, hair, integumentary structure relationships?

Actually, it's the second and third *metacarpal* (the bones of the
palm of the hand) that fuse.  In all modern birds, including hoatzins
(correct me if I'm wrong), the actual phalanges of the third digit are
very small and encased in the flesh of the manus.

In all* modern birds, including the hoatzin at all growth stages, the third finger contains a single short bone which is probably a fusion product and encased in the same flesh as the second finger. The second and third metacarpals are fused at the proximal and the distal end; the first metacarpal is very short and fused to the proximal ends of the others as well as to the distal carpals.

I was asked yesterday to provide a source that says most birds have claws on the first and/or second fingers. I have come across a few places, such as Feduccia's 1996 book, which cite a "Fischer 1942" paper which is said to report finger claws from all "orders" except six. Granted, the claws (and the bones that carry them) are usually very small and straight.

* Ostriches are reported to sometimes have a claw on the third finger in addition to those of the first two. That probably means a second bone in the third finger. On the other hand, some (!) kiwis lack the third finger entirely, and moas lack wings entirely, but I think this covers all exceptions.