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Carpal claws in birds
A while ago I posted an inquiry about the occurence of carpal claws in
living birds. I was intrigued by a photograph of such claws on an adult
Thanks to Matt Troutman and others on the list, I was able to track down
some papers on the subject. I was quite surprised by the number of living
bird species exhibiting such a feature, and I thought others on the list
might find this information of general interest.
Published works on this topic were traced back to 1886, but a 1940
article (H.I. Fisher. 1940. The occurence of vestigal claws on the wings
of birds. American Midland Naturalist. Vol. 23:234-243) seems the most
useful in that it uses modern names of taxa to list in tabular form the
order, genus and species of birds found to have carpal claws.
Of the 227 genera (2004 specimens) studied by Fisher, he reported 51
genera (225 species) as having carpal claws on digits I and/or II. He
worked entirely from skins and "alcoholic specimens", but he examined
multiple specimens of most species--apparently in an attempt to determine
if the claws existed in adult as well as immature forms. The age of the
specimens examined is presented as well. He was also careful to
distinquish true claws from the horny "spurs" that are fused to the
metacarpals of some birds. He "usually" found a claw on digit I in the
orders (but not necessarily all genera of) Gaviiformes, Ciconiiformes,
Anseriformes, Falconiformes, Galliformes, Gruiformes, Charadriiformes and
Strigiformes. He found a claw on digit II generally restricted to natal
Anseriformes and natal and adult Opisthocomidae (the only known living
bird born with an unfused carpometacarpus) and occassionally on other
forms. He did not find carpal claws in the Colymbiformes, Columbiformes,
Trogoniformes, Coraciiformes, Picifomres and Passeriformes.
Fisher concluded that the presence or absence of carpal claws could not
be regarded as an ordinal, generic or specific character and that the
"variable, degenerate state" of these claws meant they had little or no
taxonomic significance. Perhaps that's why there seems to be little if
any attention paid to this since the 1940's.