How to make birds fly good

Tips and observations of bird flight

by Brendan Body

[disclaimer: this article is just things that I have observed and that I've found useful over the years and are in no way meant to be read as hard and fast rules. If your director/animation supervisor/lead tells you do something that contradicts this, do that.]

Bird flight is notoriously hard to animate convincingly, and there are many reasons for this. The action is very fast, but even in slow motion it is often hard to understand what's happening<br>underneath because the bones are concealed by feathers, plus the movement is completely alien to us, it's not something we can act out or get a feel for. When investigated thoroughly,<br>many things about flight seem odd, even counterintuitive. Over the years I've animated quite a few birds and have slowly gained a rudimentary knowledge of the dynamics and<br>biomechanics of flight, as well as how to apply some of the traditional animation tricks to it. So, here are my tips to make your bird "fly good" and avoid the traps that many fall into. I<br>hope it helps.

I've tried to keep the tips quite concise, but I've added further detail and explanations under the text links that look like this.

Example footage speeds =

* shot at 25 frames per second

* shot at 50 frames per second

* shot at 100 frames per second (best guess)

* shot at 200 frames per second (best guess)

1. Birds don't swim through the air.

Swimming is the closest thing we experience to flying, and it is tempting to animate bird flight like an airborne swim-cycle. Because of the vast difference in density between air and water,<br>this can often lead us astray in a number of ways.

Here is an animated toucan from George and the Jungle (ignore the flying elephant in the background)*.

And here are a couple of up and down wing poses from that movie. As you can see the animator has positioned the wings in the first image up and forward, then they move backward<br>and down, pushing the air down behind, propelling the bird forward. It makes sense, right?

In fact, the exact opposite is true. As the bird's wings move downwards, they are pushed forward and then rotate back as they return to the up position. The reason for this is simple once<br>you understand lift, and if you'd like an explanation click below. As a general rule the slower the bird is flying, the further forward it'll push it's wings to generate extra lift, the faster a bird<br>is flying the less it'll push it wings forward.

Explain please!

2. The all important humerus.

Here is a clip from Harry Potter and the Philosopher's Stone. Harry releases his pet owl, Hedwig, and he climbs up above the castle and into the sky.*

There are a number of issues with this animation, but for the moment lets look at what I think is most jarring about the motion - the up and down ratio of the wings. The wings have been<br>animated to rotate down faster than they move up, at one point I count them moving down twice as fast as they move up (3 frames down, 6 up). This is another mistake that is commonly<br>made and is something real wings just don't do. Again, this sort of makes sense - the bird pushes the air down as hard and fast as he moves up, like he's swimming ... But, of course,<br>birds don't swim through the air.

The bird does push it's wings down as fast as it can, but the wings have a greater surface area and therefore resistance to the air on the way down, where as on the way up the bones in<br>the wing rotate in various ways to reduce the surface area and increase the speed at which they can move them. We'll go on to look at these other rotations later but for now I just want<br>to focus on one rotation. In animation terms this is just the rotation of one bone, the one that is closest to the bird's body; the humerus. And for now we'll look at it's rotation in just one<br>axis, the up and down. I'm going to go into quite a lot of detail on this because I believe that this is one of the most important parts of a flight cycle, all other bones in the<br>wing are effected or initiated by the humerus rotation. I find this is one of the best places to start when animating a bird in flight so it's paramount to get it right.

First we'll look at the time taken for the humerus to rotate up, compared to the time taken to rotate down in flight. Remember that the owl in the Harry Potter movie had more frames of<br>rotating up than down, at it's most extreme point -3 frames down, 6 frames up .

Here is some footage of an eagle owl flying, in the top right I have counted the frames up and down of the humerus. Up frames are in red and down frames are in green (it'll be<br>clearer if you step through the movie).*

Eagle Owl

up   5 5 5 6 5  = adv 5.2

down 9 10 9 9  = adv 9.25

As you can see, the opposite of the Harry Potter owl is true. The wings take longer to rotate down than they do up, in this owl's case, almost double the amount of time taken than to<br>rotate up.

But the eagle owl is a large bird, and the speed...