From the original article:
While your car is standing still this wild goose isn't so wild. He perches sedately upon the radiator cap surveying the world with a glassy eye. But as soon as you start up and shift into high he flattens out his tail, stretches his neck forward and begins to flap his wings as if he were going somewhere, and going there in a hurry.
The article has been converted in to text and scanned as three large images -- everything you should need to make this project yourself.
Here's the article on the Mechanical Flying Goose Decorates Radiator Cap.
[ Thanks deanS! ]