Anatomy Genetics

Alula Thumbs & Wing Claws In Chickens

I do an inordinate amount of scrolling through online chicken groups and reading about chicken health issues. I’m always challenged to learn as much as I can and, in turn, pass that on to my readers. I hoard images, stories and tidbits of information that I pull out when I have enough time to do a bit of a deeper dive and see if I can turn that into a post. Sometimes I hit a dead-end; sometimes the result is a case study or a feature in my avian pathology series, while other times I can write a stand-alone post.

I’ve done a number of articles involving anatomical systems in chickens: respiratory; reproductive; digestive; nervous system and the five senses as well as one about feathers. The focus of this one is a combination of both bone structure and feathers and how, together, they assist in avian mobility and flight.

Most chickens don’t fly, or if they do, it’s for short distances or not very high (unless they’re a bantam or light breed bird). The features at the heart of this lesson is the alula thumb and carpal spur, which have been bred out of most poultry. If you examine chicken wings on your dinner plate or in your flock you might be so lucky as to come across one in person.

We probably don’t think about chickens’ wings being comparable to our arms and hands, but they are. When you look at the image below you can see how human, mammal and bird structures are similar.

The wing bones are akin to our arms. Chickens fly with their forearms and hands. The flight feathers are attached to the ulna and metacarpals. The feathers that attach to their hand and fingers are the primaries and those that attach to the forearm are the secondaries.

Alula (plural: alulae) comes from the Latin for ‘winglet’ and is a small, freely moving first digit on the leading edge of the wings of flying birds. It’s considered the bird’s ‘thumb’ and is covered by three to five small flight feathers, which can stick out and in many bird species is clearly visible. Not so, in chickens who no longer require assistance in flight and therefore most often don’t have them.

Flying birds use the alula thumb to hone their flight angles. Normally the alulae are held flush along the surface of the wing and are difficult to see but when landing or flying at very slow speeds (like hovering) the alulae move forward and upward so they becomes more visible. Much like the slats on airplane wings the alulae are used to increase lift and prevent stalling during slow flight.

A different feature located in the same anatomical area is the carpal spur or wing claw, which looks like a small toenail located at the carpal joint. It’s now a vestigial trait leftover from bird species who used them to climb foliage, slide down tree trunks and as a form of defense when fighting.

Natural selection has significantly decreased the size of the claw over time as they have become unnecessary. In most birds, they have disappeared altogether.

Chicken keepers often jokingly refer to their birds as tiny velociraptors, but in truth birds are the only living descendants of dinosaurs and chickens are considered among the closest living relatives of Tyrannosaurus Rex. They now require neither alulae thumbs or wing claws, but every once in a while those ancient genetics pop up in living birds as reminders of their distant ancestors.

I know this topic is a bit esoteric, but I found it interesting all the same. You probably won’t ever need this information, but if you find yourself a contestant on the gameshow Jeopardy and one of the topics is ‘Weird Facts About Chickens’, then you might just be glad you read this. And if you say ‘alula thumb’ a few times it might sound like it could be a character’s name from a Tennessee Williams’ play.

Credits: Chickenfans; Ron Dudley; Northern Arizona University

2 comments on “Alula Thumbs & Wing Claws In Chickens

  1. Or a good name for a feisty hen!

    Liked by 1 person

  2. I loved this post!! I used to teach children the bat anatomy so they would understand what they were drawing, but I never thought to think about the chicken wing! And your piece left me repeating alula thumb in a southern drawl 🙂 Thanks for a nice way to start my day!

    Liked by 1 person

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