One of Charles Darwin's famous 14 finches — the island-dwelling bird species that helped inspire the theory of evolution — the medium tree finch is found only on Floreana, one of the nine major islands comprising the Galápagos archipelago. Historic threats to this finch include habitat destruction and fragmentation, but the most pressing threat today is the Philornis downsi, an introduced parasitic fly whose larvae feed on the finch's nestlings. Causing high nestling mortality, lower fledgling success, and reduced nestling growth, the parasite appears to be hitting the medium tree finch the hardest out of all the Galápagos finches, allowing only about 6 percent of the bird's active nests to produce fledglings. More than half the medium tree finch's nestling mortality is attributed to the Philornis downsi.

In response to decades-old listing petitions and a series of lawsuits by the Center, in July 2010 the U.S. Fish and Wildlife Service finally designated the medium tree finch as endangered under the Endangered Species Act. An international listing restricts buying and selling of the bird, increases conservation funding and attention, and will add scrutiny to development projects proposed by U.S. government and multilateral lending agencies that would destroy or alter its habitat.