Extra Wing Digit
By Alan Stanford, Ph.D.
14 September 2008
The Journal of Poultry Science published an interesting article1 about how limbs develop in Silkies.
The authors found that, in addition to the extra Silkie toe, there is an extra digit in the wing, the extra wing digit disappears before hatch, and the extra toe resembles the middle toe more than the toe next to it.
The authors state:
"An extra cartilaginous digit was apparent transiently in the wing bud of Japanese Silkie embryos between stages 32, and 36. In contrast, an extra digit of the leg resembled digit 2, in terms of its skeletal elements."
"An extra toe is found in breeds of fowl other than the Silkie, including Dorking, Houdan and Sultan."
The extra toe resembles the middle toe, "Both in size and that it contains three phalanges."
Normal chick embryos often have a transient extra wing digit near "digit 4" and also one in the area of "digit 2" that remains undeveloped.
The introduction to the article presents an interesting bit of Silkie history. "The first unambiguous record of the Silkie fowl is found in a Chinese book, Wu lei
Xiang Gan Zhi, written by Su Dongpo (1037-1101 according to Su Shi ...
in the 11th century. The Silkie fowl was introduced into Japan from China or India early in the 17th century, and the modern breeds of the Silkie fowl are thought to have been established in China and Japan (Roberts, 19972)."
Wada et al.3 compared the mitochondrial DNA of the Japanese Silkie with that of the White Leghorn and found that the similarity between the two breeds was 99.77% (39 nucleotide differences out of a total of 16,784 base pairs).
It turns out the authors are in for a surprise, not all extra wing digits disappear before hatch!
Paula Shepard has several adult chickens with this "extra wing digit." She discovered that whilst bathing her birds. The extra digit appears in her Silkies and her Cochins. Since the two breeds are unrelated in her flock, the extra digit is not as rare as the authors thought.
Paula was nice enough to corral her birds for a photo session and shared the images with us.