An eagle locked on to a branch and flipped upside down at Great Spirit Bluff on Thursday. It hung there for roughly 32 seconds, looking at its perch mates and flapping its wings every few seconds, seemingly unable to let go of the branch it had fastened on. So what was going on?
Flexor Tendons and Grip
Let’s start by talking about tendons: the tough tissue that connects muscle to bone. Our human flexor tendons run from our forearms to the ends of our fingers across the palm sides of our hands. They help us to bend our fingers by connecting our finger bones to the muscles in our forearms. When we contract our muscles, our tendons and muscles shorten, which pulls our fingers in. When we relax our muscles, they become longer, and our fingers fall away from our palms, curling slightly. If you wrap your left hand around your right arm and make a fist, you’ll feel the muscles in your forearm tighten and contract as your fingers curl inwards. As you relax your fist, you’ll feel your muscles relax back into their non-contracted neutral position.
Knees, ankles, feet, and toes. Image credit: public domain.
Birds also have flexor tendons. Their tendons run from their leg muscles, which are tucked up under their feathers, to their toes, passing behind their legs. As a bird bends its knees and ankles to perch, the distance between its toes and leg muscles lengthens, which pulls its flexor tendons in and curls its toes around the branch. This is known as the perching reflex. A bird has to straighten its knees and ankles to release its toes and fly (or walk) away.
Raptor Rachet Action
A bird’s talon-locking mechanism. The flexor tendon and ligament are red and the tendon sheath is blue. Quinn, T. and J. Baumel. “The digital tendon locking mechanism of the avian foot (Aves).” Zoomorphology 109 (2004): 281-293.
Many species of birds, including bald eagles, have a special adaptation that helps to keep their toes closed. Their flexor tendons have small tubercules that automatically lock or mesh with grooves on their tendon sheaths when they curl their toes. As an eagle tightens its grasp, its flexor ‘pawl’ rachets up, which tightens its grip and prevents its toes from slipping or loosening. Much like a truck tiedown, or any other rachet, they don’t loosen their grip because they can’t. An eagle has to think about releasing its grip, not maintaining it.
Which brings us back to our upside down eagle. Its leg is straight, which should release its toes. But it reflexively racheted its grip around the limb when it fell and can’t quite figure out how to let go. In the 55th second of the video, we see the eagle release its grip as it swings its body down and around and flexes its powerful wings in response to an incoming adult. Although we can’t see its toes, I imagine that it lifted its talons up slightly as it shifted its weight and flexed its wings. This acted as an escapement of sorts, lifting the eagle’s flexor pawl from its sheath rachet and allowing it to fly free!
Things that helped me learn and write about this topic
Just For Fun!
Check out your own muscle and tendon action by lightly holding the wide part of your right forearm with your left hand, wrapping your fingers as far as you can around your arm. Let your right hand hang loose and open. Your forearm should feel soft and relaxed. Now, point your right hand and fingers toward the sky and feel your arm tighten as your muscles contract. Next, point your right hand, palm down, so it is in line with your arm, pointed straight out from your wrist. Make a clenched fist and feel your muscles tighten again!
thang010146 has a youtube channel full of simple animations that clearly illustrate how machines work. I could watch them all day! This video nicely illustrates a linear ratchet, including an escapement at the end.