The seed-finches are a bit of a taxonomic puzzle. First of all, they are closely related to Sporophila seedeaters and some consider them best all lumped into one genus; the seed-finches being the larger and bigger billed extremes within the group. The other issue is that the few seed-finches that exist are rather similar looking and have been variously treated as subspecies of each other in different arrangements during the past. The Thick-billed Seed-Finch is found in Central America to NW South America. Curiously, it hybridizes with the very different looking Chestnut-bellied Seed-Finch (O. angolensis) in the Magdalena Valley of Colombia; the two have been treated as conspecific in the past and been given the name “Lesser Seed-Finch.” The Thick-billed Seed-Finch has all black males with white bases to the primaries, forming a white “speculum” or “handkerchief” on the closed wing. The females are a rich warm rusty brown. As is typical of the group the bill looks oversized, and in this species it is black. Its nest is lined with horsehair, so thin that light passes through it.