26 November 2010

Today’s anatomy lesson was inspired by Michelline who asked why she sees only peregrines’ neatly folded talons when they fly.  Where do the rest of their legs go?

The bones in birds’ legs are of nearly equal length and the hinges are opposite like an accordion.  This means birds can fold their legs to lower their bodies straight down to sit on eggs and they can retract their legs to a flat position in flight. 

To illustrate this I’ve highlighted the legs in red and numbered the joints: 

1. From the body to joint #1 is the thigh (femur)
2. Joint #1 to #2 is the shin (tibiotarsus) and calf (fibula)
3. Joint #2 to #3 is the foot (tarsometatarsus)
4. Joint #3 to the end are toes.

.
For most birds, even when standing, the thigh is hidden in the body feathers. Birds’ legs are much longer than we think! See how long a juvenile peregrine’s legs are in this slideshow.

The blue arrows above show how birds fold their legs when they fly.  In step (a) the thigh and shin fold up flat to the body and are hidden in the body feathers.  In step (b) the toes may do one of several things.

• Toes can extend straight backwards as shown by arrow (b), –or–
• Hang down in a slightly open position, –or–
• Retract up to the body just as the thigh and shin were retracted.

.
When you see only a peregrine’s yellow toes in flight it’s because his legs and feet are covered by his body feathers. 

Aeronautical engineers learned from birds.  Watch a jet take off and you’ll see it retract its “legs” under its wings.

(bird skeleton by W. Ramsay Smith and J S Newell, 1889, via Wikimedia Commons, altered to illustrate the leg. Click on the caption to see the original.)