Wednesday, December 4, 2019

Do birds' knees bend backwards? Bird legs & feet anatomy

Photo of a Greater Yellowlegs with legs bent strongly at the ankle--not the knee!
Backwards bending knee? Nope. That joint is the heel, not the knee! 
Greater Yellowlegs
Photo by Greg Gillson
Even though they may know better, many bird watchers still call the backward facing leg joint on long-legged birds "knees." But that is not correct. Birds walk on their toes. The long lower bone attached to the toes is really the foot. That backward facing joint would be the heel and ankle for you and me.

That long leg bone that disappears forward into the bird's plumage? That is the tibiotarsus, the "drumstick" with tibia and fibula bones and the fat calf muscle on the upper end at the knee. Connecting to the real knee on the end of the drumstick under the feathers is a short stout femur or thigh bone. It connects to the hip near the base of the tail up on the back.

Birds, like cats and dogs, are digitigrade


Digitigrade, as you may remember from high school biology, is when an animal walks on its toes. People are plantigrade, walking with the heel on the ground. The other option used by deer, cows and horses is unguligrade. These animals walk on their toe nails which are hooves. Whether a giraffe, mouse, sparrow, or human, all the leg bones are there, just longer or shorter or sometimes fused together. But the number of toes varies from 1 to 5.

Sketch of chicken showing legs and feet bones
Chicken legs and feet
Sketch by Greg Gillson

Some anatomy


So far in this discussion I haven't been very accurate in my terminology, using rather inaccurate common terms to relate bird anatomy to human anatomy.

Using the sketch I made of the chicken (above) we'll start at the toes and work our way up to the hip with the scientific terms. For this I reference Ornithology in Laboratory and Field (Fourth Edition). 1970. Olin Sewall Pettingill, Jr.

Birds have 4 toes (some birds only 3, Ostrich 2). On a chicken three toes point forward and one is behind. The hind toe is the hallux (toe #1) and is equivalent to the big toe of humans. The hallux can be incumbent, that is, on the same level as the other toes. The hallux of the chicken, though, is elevated, the tip not reaching the ground.

Each toe is made up of phalanges: bones that help the toe bend. Each toe ends with a curved claw. The hallux has 2 phalanges, bending in one place only. The inner toe has 3 phalanges, bending in 2 places. The middle of the forward toes has 4 phalanges, bending in 3 places. The outer toe has 5 phalanges, bending in 4 places. That is quite different from human toes!

The long lower leg bone of a bird is actually the foot, or what would be the instep in humans. It is a single bone, the metatarsus, composed of what would be some ankle bones and 3 foot bones leading to each toe. At the toe joint the three-bones-in-one separate enough to be seen at the knuckle.

Okay, we've reached the drumstick, the "real" lower leg bone. In humans the two lower leg bones are the tibia and fibula. In birds, however, some of the ankle bones (tarsals) are fused with the tibia, forming the main heavy bone called the tibiotarsus. The thin fibula extends only 2/3 of the way down the leg and does not reach the ankle.

There is a small patella, or knee cap. Yes, birds have kneecaps!

The upper leg bone is the femur. It connects to the tibiotarsus at one end and the hip socket at the upper end. This is the thigh of the chicken.

Okay, from now on I expect you to have very interesting dinner conversations every time you have fried chicken!

The length of the various bones vary quite a bit in birds. Long-legged flamingos and short-legged hummingbirds are about as opposite as could be. Yet the bones are all there, if quite different in size and length.

Did I cover everything? I guess there's no need for you to see how this arrangement varies with your dog, whose hind leg is more similar to a bird's leg than your own. That "dogleg" is the heel. The knee is higher up and forward and, like a bird and a human, points forward in the same manner.



Have you ever wondered? Where do birds go in bad weather?



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