This incredible bird is unique for several major reasons, least of which is its incredible plumage! I decided to illustrate perhaps the most intriguing aspect of its anatomy…its foregut. This bird is unique for its huge crop, which makes up 25% of its body weight. Normally in birds edibles are digested relatively swiftly in the hindgut, but the hoatzin has this enlarged crop that is basically a big vat of fermentation and microbial breakdown stuffed with stinky leaves. It may retain the leaves for 1-2 days, and the hoatzin spends much of its time stooped over, waiting for the slow release of the nutrients. The crop is in fact so important, evolutionarily speaking, that theflight muscles and associated keel and sternum have both been reduced to give it more space! It is consequently a weak flyer.
While I am on the subject, lets talk about other hoatzin oddities. The chicks when born have a little clawed hand to go foraging among the trees when its parents are away! It is also a way for them to avoid danger as the nests are set up over water, so that when a threat comes the chick drops into the water, and later climbs back to safety when the coast is clear. A vestige of its dinosaur lineage? Interestingly coots also have little single claws on their upper appendages too.
Here is an image composite I created showing what is digital and what is the base painting. I first found some reference images taken at an upward angle. I made a prelim sketch, resized it for the illlustration board I planned to use, transferred the lines, touched up the lines further in graphite, then began painting it all in gouache, laying it down light in the beginning, and working darker and darker. But what this image shows is how much mileage I got out of the digital side of the rendering. As you can see the sternum and crop have many levels of opacity interplay, so sweet with digital. But especially feathering some of the feathers made it a lot softer and downy looking.