Spanish ibex
Today, more than 1,532 passenger pigeon skins (along with 16 skeletons) are in existence, spread across many institutions all over the world. It has been suggested that the passenger pigeon should be revived when available technology allows it (a concept which has been termed "de-extinction"), using genetic material from such specimens. In 2003, the Pyrenean ibex ("Capra pyrenaica pyrenaica", a subspecies of the Spanish ibex) was the first extinct animal to be cloned back to life; the clone lived for only seven minutes before dying of lung defects. A hindrance to cloning the passenger pigeon is the fact that the DNA of museum specimens has been contaminated and fragmented, due to exposure to heat and oxygen. American geneticist George M. Church has proposed that the passenger pigeon genome can be reconstructed by piecing together DNA fragments from different specimens. The next step would be to splice these genes into the stem cells of rock pigeons (or band-tailed pigeons), which would then be transformed into egg and sperm cells, and placed into the eggs of rock pigeons, resulting in rock pigeons bearing passenger pigeon sperm and eggs. The offspring of these would have passenger pigeon traits, and would be further bred to favor unique features of the extinct species.