Eggs and very small nestlings can be preyed on by arboreal mammals, such as sables and ermine, and birds, usually corvids. Any of these small, clever nest predators rely on distraction and stealth to prey on the eagle's nests and are killed if caught by either of the parents. Once it reaches roughly adult size in the fledgling stage, few predators can threaten this species. In one case, a brown bear ("Ursus arctos") was able to access a nest located on a rock formation and ate a fledgling eaglet, though this is believed to be exceptional. Fully grown fledglings in tree nests are probably invulnerable to predation. Excluding the Asian black bear ("Ursus thibetanus"), which has not thus far been recorded as a predator, no other mammalian carnivores are equal to or greater than the eagle's size which can climb trees in the species' range. Due primarily to egg predation and nest collapses, only 45–67% of eggs are successfully reared to adulthood and up to 25% of nestlings may be lost. However, once fully grown, the eagle has no natural predators.