Launched in 1997 the Audubon Birds of America models were 10 sets of 10 watches each, whose dials were inspired by the works of 19th century naturalist JJ Audubon.
The dials in cloisonné enamel depicted different birds found in America and painted by Audubon. The dial work was done by Muriel Senchaud. The 38 mm yellow gold hinged back case housed the caliber 1120 (with Geneva seal) with a skeletonised rotor.
Subsequently Vacheron Constantin used the same principle of enamel dials and same case for specific models whose dials did not represent birds. It is interesting to note that up to now the Audubon series are Vacheron Cosntantin's only modern models with cloissoné enamel dial which do not have an alternative time display!
I don't have scans of all the Birds of Americas series but here are a few:
Subsequent models without bird dials:
A closer look at one of the original Audubon Birds of America
Cloisonné enamel is a method of enamelling which consists of using an extremely thin piece of gold wire (0.05mm thick!) which is bent in to shape that define the image and glued to the dial. The pattern is then filled with enamel powder and baked, each time a new color is added the dial needs to be baked.
In the photo below you can see the gold wire outlining the bird and its wings.