Dear Justin,
Thanks for your comments. You're right, it was originally described as a new species by Keulemans, but there is a "dark story" about it, as this young naturalist and wildlife artist visited the Cape Verdes in company of the German ornithologist Heinrich Dohrn, so both discovered the bird in question. Later, once Keulemans returned to home, he quickly published a paper with the species' description without the participation of Dohrn, who published his own article in 1871, 5 years later the original publication of Keulemans. This tale is explained in detail by Hazevoet (1995) in his book The Birds of the Cape Verde Islands (BOU Checklist No. 13, Tring).