I just realized the bird turns it's head to follow the mouse pointer.

Yes, and the animation is made using an interesting technique that I can't immediately figure out.

For example, what is this [0] or this [1].

The code is not obfuscated and seems well documented [2]. Maybe, I'll read it later or have an agent explain it to me.

[0] https://alexwang.ai/images/kea/head-fill.webp

[1] https://alexwang.ai/images/kea/body-feathers.png

[2] https://alexwang.ai/js/kea3d.js

Also, you can ruffle its feathers with the mouse.