Hoatzin
The crop of the hoatzin is so large that it displaces the flight muscles and keel of the sternum. This is why they fly so poorly. Because of aromatic compounds in the leaves they eat, and the bacterial fermentation, the bird has a disagreeable, manure-like odor. It is only hunted by humans for food in times of great need. If the bird feeds on insects or other animal matter, that is purely accidental.