Endemic to X means that it's geographical distribution is confined to X. The (original) geographical distribution of basically all bird species is fairly well k

Endemic to X means that it's geographical distribution is confined to X. The (original) geographical distribution of basically all bird species is fairly well known, due to the efforts of 19th and 20th century scientists and their collection of museum specimens. The reference work here is the "Peters check list", completed over the course of several decades in the 20C by various ornithologists. Of course, you have to check when they say "species" whether they're referring to a subspecies "elevated to species status" by dodgy "phylogeography" genetics studies. https://en.m.wikipedia.org/wiki/James_L._Peters https://en.m.wikipedia.org/wiki/James_L._Peters