A chilensis or "Fuegian" Storm Petrel, currently a subspecies of the Wilson's shows its classic pale underwing patch.
The taxonomy of the Wilson’s Storm-Petrel is in flux. Some have suggested that the Chilean – Patagonian breeding form (the Fuegian Storm-Petrel) should be separated as a good species even! Before considering that, the nomenclature had to be clarified as it was a mess. To some this may seem like arcane and silly, but in the context of taxonomy what name should be applied to a population is important stuff, and you can’t have a messy grey area where that is involved. In any case, I helped Ricardo Palma and New Zealand team to write a short note clarifying the name for the Fuegian Storm-Petrel (Oceanites oceanicus chilensis), which you can download from the link below. I admit that this is not riveting reading to some, but a useful clarification that will become more important if those considering that this is a good species are correct!