Hoatzin (Opisthocomus hoazin).
These are very unusual birds, not only in appearance and behavior but in a scientific manner, as well. There are still many questions as to their evolutionary history and how to, exactly classify them. The several guides I had in Venezuela generally referred to them as 'prehistoric' and left it at that.
Location: Orinoco River Delta, Venezuela
Lens used: Canon 100-400mm f4.5-5.6 IS