Inaccessible desert habitats?

It seems likely that if thunderbirds or pterodactyls are still alive today, they would reside in some remote and likely mountainous area. The most likely concentration of surviving flying lizards would have to be Mexico's Sonoran Desert, located south of the American states of Arizona and New Mexico. It's quite possible that pterodactyls still live in this area, undisturbed and unnoticed by civilization. The Mexican state of Sierra Madre, located only 200 miles from the Rio Grande observation site, is one of the least explored areas of North America. Flying reptiles or large birds could still exist in such a region, especially if they are nocturnal.